Hi,
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the write side critical section.
There is no built-in debugging mechanism to verify that the lock used
for writer serialization is held and preemption is disabled. Some usage
sites like dma-buf have explicit lockdep checks for the writer-side
lock, but this covers only a small portion of the sequence counter usage
in the kernel.
Add new sequence counter types which allows to associate a lock to the
sequence counter at initialization time. The seqcount API functions are
extended to provide appropriate lockdep assertions depending on the
seqcount/lock type.
For sequence counters with associated locks that do not implicitly
disable preemption, preemption protection is enforced in the sequence
counter write side functions. This removes the need to explicitly add
preempt_disable/enable() around the write side critical sections: the
write_begin/end() functions for these new sequence counter types
automatically do this.
Extend the lockdep API with a macro asserting that preemption is
disabled. Use it to verify that preemption is disabled for all sequence
counters write side critical sections.
If lockdep is disabled, these lock associations and non-preemptibility
checks are compiled out and have neither storage size nor runtime
overhead. If lockdep is enabled, a pointer to the lock is stored in the
seqcount and the write side API functions enable lockdep assertions.
The following seqcount types with associated locks are introduced:
seqcount_spinlock_t
seqcount_raw_spinlock_t
seqcount_rwlock_t
seqcount_mutex_t
seqcount_ww_mutex_t
This lock association is not only useful for debugging purposes, it also
provides a mechanism for PREEMPT_RT to prevent writer starvation. On RT
kernels spinlocks and rwlocks are substituted with sleeping locks and
the code sections protected by these locks become preemptible, which has
the same problem as write side critical section with preemption enabled
on a non-RT kernel. RT utilizes this association by storing the provided
lock pointer and in case that a reader sees an active writer (seqcount
is odd), it does not spin, but blocks on the associated lock similar to
read_seqbegin_or_lock().
By using the lockdep debugging mechanisms added in this patch series, a
number of erroneous seqcount call-sites were discovered across the
kernel. The fixes are included at the beginning of the series.
Thanks,
8<--------------
Ahmed S. Darwish (25):
net: core: device_rename: Use rwsem instead of a seqcount
mm/swap: Don't abuse the seqcount latching API
net: phy: fixed_phy: Remove unused seqcount
block: nr_sects_write(): Disable preemption on seqcount write
u64_stats: Document writer non-preemptibility requirement
dma-buf: Remove custom seqcount lockdep class key
lockdep: Add preemption disabled assertion API
seqlock: lockdep assert non-preemptibility on seqcount_t write
Documentation: locking: Describe seqlock design and usage
seqlock: Add RST directives to kernel-doc code samples and notes
seqlock: Add missing kernel-doc annotations
seqlock: Extend seqcount API with associated locks
dma-buf: Use sequence counter with associated wound/wait mutex
sched: tasks: Use sequence counter with associated spinlock
netfilter: conntrack: Use sequence counter with associated spinlock
netfilter: nft_set_rbtree: Use sequence counter with associated rwlock
xfrm: policy: Use sequence counters with associated lock
timekeeping: Use sequence counter with associated raw spinlock
vfs: Use sequence counter with associated spinlock
raid5: Use sequence counter with associated spinlock
iocost: Use sequence counter with associated spinlock
NFSv4: Use sequence counter with associated spinlock
userfaultfd: Use sequence counter with associated spinlock
kvm/eventfd: Use sequence counter with associated spinlock
hrtimer: Use sequence counter with associated raw spinlock
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 239 +++++
MAINTAINERS | 2 +-
block/blk-iocost.c | 5 +-
block/blk.h | 2 +
drivers/dma-buf/dma-resv.c | 15 +-
.../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 -
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
drivers/net/phy/fixed_phy.c | 25 +-
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 +-
fs/nfs/nfs4_fs.h | 2 +-
fs/nfs/nfs4state.c | 2 +-
fs/userfaultfd.c | 4 +-
include/linux/dcache.h | 2 +-
include/linux/dma-resv.h | 4 +-
include/linux/fs_struct.h | 2 +-
include/linux/hrtimer.h | 2 +-
include/linux/kvm_irqfd.h | 2 +-
include/linux/lockdep.h | 9 +
include/linux/sched.h | 2 +-
include/linux/seqlock.h | 882 +++++++++++++++---
include/linux/seqlock_types_internal.h | 187 ++++
include/linux/u64_stats_sync.h | 38 +-
include/net/netfilter/nf_conntrack.h | 2 +-
init/init_task.c | 3 +-
kernel/fork.c | 2 +-
kernel/locking/lockdep.c | 15 +
kernel/time/hrtimer.c | 13 +-
kernel/time/timekeeping.c | 19 +-
lib/Kconfig.debug | 1 +
mm/swap.c | 57 +-
net/core/dev.c | 30 +-
net/netfilter/nf_conntrack_core.c | 5 +-
net/netfilter/nft_set_rbtree.c | 4 +-
net/xfrm/xfrm_policy.c | 10 +-
virt/kvm/eventfd.c | 2 +-
38 files changed, 1325 insertions(+), 277 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
create mode 100644 include/linux/seqlock_types_internal.h
base-commit: 2ef96a5bb12be62ef75b5828c0aab838ebb29cb8
--
2.20.1
Proper documentation for the design and usage of sequence counters and
sequential locks does not exist. Complete the seqlock.h documentation as
follows:
- Divide all documentation on a seqcount_t vs. seqlock_t basis. The
description for both mechanisms was intermingled, which is incorrect
since the usage constrains for each type are vastly different.
- Add an introductory paragraph describing the internal design of, and
rationale for, sequence counters.
- Document seqcount_t writer non-preemptibility requirement, which was
not previously documented anywhere, and provide a clear rationale.
- Provide template code for seqcount_t and seqlock_t initialization
and reader/writer critical sections.
- Recommend using seqlock_t by default. It implicitly handles the
serialization and non-preemptibility requirements of writers.
At seqlock.h:
- Remove references to brlocks as they've long been removed from the
kernel.
- Remove references to gcc-3.x since the kernel's minimum supported
gcc version is 4.6.
- Remove the severely lacking top comment and reference the newly
introduced Documentation/locking/seqlock.rst file instead.
References: 0f6ed63b1707 ("no need to keep brlock macros anymore...")
References: cafa0010cd51 ("Raise the minimum required gcc version to 4.6")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 181 ++++++++++++++++++++++++++++++
include/linux/seqlock.h | 73 +++++-------
3 files changed, 213 insertions(+), 42 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
index 5d6800a723dc..aad15fc81ccd 100644
--- a/Documentation/locking/index.rst
+++ b/Documentation/locking/index.rst
@@ -14,6 +14,7 @@ locking
mutex-design
rt-mutex-design
rt-mutex
+ seqlock
spinlocks
ww-mutex-design
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
new file mode 100644
index 000000000000..2242ae00e7bf
--- /dev/null
+++ b/Documentation/locking/seqlock.rst
@@ -0,0 +1,181 @@
+======================================
+Sequence counters and sequential locks
+======================================
+
+Introduction
+============
+
+Sequence counters are a reader-writer consistency mechanism with
+lockless readers (read-only retry loops), and no writer starvation. They
+are used for data that's rarely written to (e.g. system time), where the
+reader wants a consistent set of information and is willing to retry if
+that information changes.
+
+A data set is consistent when the sequence count at the beginning of the
+read side critical section is even and the same sequence count value is
+read again at the end of the critical section. The data in the set must
+be copied out inside the read side critical section. If the sequence
+count has changed between the start and the end of the critical section,
+the reader must retry.
+
+Writers increment the sequence count at the start and the end of their
+critical section. After starting the critical section the sequence count
+is odd and indicates to the readers that an update is in progress. At
+the end of the write side critical section the sequence count becomes
+even again which lets readers make progress.
+
+A sequence counter write side critical section must never be preempted
+or interrupted by read side sections. Otherwise the reader will spin for
+the entire scheduler tick due to the odd sequence count value and the
+interrupted writer. If that reader belongs to a real-time scheduling
+class, it can spin forever and the kernel will livelock.
+
+.. _seqcount_t:
+
+Sequence counters (:c:type:`seqcount_t`)
+========================================
+
+This is the the raw counting mechanism, which does not protect against
+multiple writers. Write side critical sections must thus be serialized
+by an external lock.
+
+If the write serialization primitive is not implicitly disabling
+preemption, preemption must be explicitly disabled before entering the
+write side section. If the sequence counter read section can be invoked
+from hardirq or softirq contexts, interrupts or bottom halves must be
+respectively disabled before entering the write side section.
+
+If it's desired to automatically handle the sequence counter
+requirements of writer serialization and non-preemptibility, use a
+:ref:`sequential lock <seqlock_t>` instead.
+
+Initialization:
+
+.. code-block:: c
+
+ /* dynamic */
+ seqcount_t foo_seqcount;
+ seqcount_init(&foo_seqcount);
+
+ /* static */
+ static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_ZERO(foo.seq),
+ } foo;
+
+Write path:
+
+.. code-block:: c
+
+ /* Serialized context with disabled preemption */
+
+ write_seqcount_begin(&foo_seqcount);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_seqcount_end(&foo_seqcount);
+
+Read path:
+
+.. code-block:: c
+
+ do {
+ seq = read_seqcount_begin(&foo_seqcount);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqcount_retry(&foo_seqcount, seq));
+
+.. _seqlock_t:
+
+Sequential locks (:c:type:`seqlock_t`)
+======================================
+
+This contains the :ref:`sequence counting mechanism <seqcount_t>`
+earlier discussed, plus an embedded spinlock for writer serialization
+and non-preemptibility.
+
+If the read side section can be invoked from hardirq or softirq context,
+use the write side function variants which respectively disable
+interrupts or bottom halves.
+
+Initialization:
+
+.. code-block:: c
+
+ /* dynamic */
+ seqlock_t foo_seqlock;
+ seqlock_init(&foo_seqlock);
+
+ /* static */
+ static DEFINE_SEQLOCK(foo_seqlock);
+
+ /* C99 struct init */
+ struct {
+ .seql = __SEQLOCK_UNLOCKED(foo.seql)
+ } foo;
+
+Write path:
+
+.. code-block:: c
+
+ write_seqlock(&foo_seqlock);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_sequnlock(&foo_seqlock);
+
+Read path, three categories:
+
+1. Normal Sequence readers which never block a writer but they must
+ retry if a writer is in progress by detecting change in the sequence
+ number. Writers do not wait for a sequence reader.
+
+ .. code-block:: c
+
+ do {
+ seq = read_seqbegin(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqretry(&foo_seqlock, seq));
+
+2. Locking readers which will wait if a writer or another locking reader
+ is in progress. A locking reader in progress will also block a writer
+ from entering its critical section. This read lock is
+ exclusive. Unlike rwlock_t, only one locking reader can acquire it.
+
+ .. code-block:: c
+
+ read_seqlock_excl(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ read_sequnlock_excl(&foo_seqlock);
+
+3. Conditional lockless reader (as in 1), or locking reader (as in 2),
+ according to a passed marker. This is used to avoid lockless readers
+ starvation (too much retry loops) in case of a sharp spike in write
+ activity. First, a lockless read is tried (even marker passed). If
+ that trial fails (odd sequence counter is returned, which is used as
+ the next iteration marker), the lockless read is transformed to a
+ full locking read and no retry loop is necessary.
+
+ .. code-block:: c
+
+ /* marker; even initialization */
+ int seq = 0;
+ do {
+ read_seqbegin_or_lock(&foo_seqlock, &seq);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (need_seqretry(&foo_seqlock, seq));
+ done_seqretry(&foo_seqlock, seq);
+
+API documentation
+=================
+
+.. kernel-doc:: include/linux/seqlock.h
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d35be7709403..2a4af746b1da 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
*
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
+ * See Documentation/locking/seqlock.rst for full description.
*
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
- *
- *
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/
#include <linux/spinlock.h>
@@ -40,11 +19,23 @@
#include <asm/processor.h>
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * The raw counting mechanism without any writer protection. Write side
+ * critical sections must be serialized and readers on the same CPU
+ * (e.g. through preemption or interrupts) must be excluded.
+ *
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_t) instead.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
+
typedef struct seqcount {
unsigned sequence;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -221,8 +212,6 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_retry(s, start);
}
-
-
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
s->sequence++;
@@ -367,11 +356,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-/*
- * Sequence counter only version assumes that callers are using their
- * own locking and preemption is disabled.
- */
-
static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
@@ -419,15 +403,20 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
s->sequence+=2;
}
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/sched.h | 2 +-
init/init_task.c | 3 ++-
kernel/fork.c | 2 +-
3 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..a9ce6fbeb735 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1046,7 +1046,7 @@ struct task_struct {
/* Protected by ->alloc_lock: */
nodemask_t mems_allowed;
/* Seqence number to catch updates: */
- seqcount_t mems_allowed_seq;
+ seqcount_spinlock_t mems_allowed_seq;
int cpuset_mem_spread_rotor;
int cpuset_slab_spread_rotor;
#endif
diff --git a/init/init_task.c b/init/init_task.c
index bd403ed3e418..94bf4aea8293 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -142,7 +142,8 @@ struct task_struct init_task
.rcu_tasks_idle_cpu = -1,
#endif
#ifdef CONFIG_CPUSETS
- .mems_allowed_seq = SEQCNT_ZERO(init_task.mems_allowed_seq),
+ .mems_allowed_seq = SEQCNT_SPINLOCK_ZERO(init_task.mems_allowed_seq,
+ &init_task.alloc_lock),
#endif
#ifdef CONFIG_RT_MUTEXES
.pi_waiters = RB_ROOT_CACHED,
diff --git a/kernel/fork.c b/kernel/fork.c
index 8c700f881d92..a0fde1f17e0a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2019,7 +2019,7 @@ static __latent_entropy struct task_struct *copy_process(
#ifdef CONFIG_CPUSETS
p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
- seqcount_init(&p->mems_allowed_seq);
+ seqcount_spinlock_init(&p->mems_allowed_seq, &p->alloc_lock);
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
p->irq_events = 0;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
The dma-buf reservation subsystem uses plain sequence counters to manage
updates to reservations. Writer serialization is accomplished through a
wound/wait mutex.
Acquiring a wound/wait mutex does not disable preemption, so this needs
to be done manually before and after the write side critical section.
Use the newly-added seqcount_ww_mutex_t instead:
- It associates the ww_mutex with the sequence count, which enables
lockdep to validate that the write side critical section is properly
serialized.
- It removes the need to explicitly add preempt_disable/enable()
around the write side critical section because the write_begin/end()
functions for this new data type automatically do this.
If lockdep is disabled this ww_mutex lock association is compiled out
and has neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
drivers/dma-buf/dma-resv.c | 8 +-------
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 --
include/linux/dma-resv.h | 2 +-
3 files changed, 2 insertions(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 590ce7ad60a0..3aba2b2bfc48 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -128,7 +128,7 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
- seqcount_init(&obj->seq);
+ seqcount_ww_mutex_init(&obj->seq, &obj->lock);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
@@ -259,7 +259,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
fobj = dma_resv_get_list(obj);
count = fobj->shared_count;
- preempt_disable();
write_seqcount_begin(&obj->seq);
for (i = 0; i < count; ++i) {
@@ -281,7 +280,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
smp_store_mb(fobj->shared_count, count);
write_seqcount_end(&obj->seq);
- preempt_enable();
dma_fence_put(old);
}
EXPORT_SYMBOL(dma_resv_add_shared_fence);
@@ -308,14 +306,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
if (fence)
dma_fence_get(fence);
- preempt_disable();
write_seqcount_begin(&obj->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(obj->fence_excl, fence);
if (old)
old->shared_count = 0;
write_seqcount_end(&obj->seq);
- preempt_enable();
/* inplace update, no shared fences */
while (i--)
@@ -393,13 +389,11 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
src_list = dma_resv_get_list(dst);
old = dma_resv_get_excl(dst);
- preempt_disable();
write_seqcount_begin(&dst->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(dst->fence_excl, new);
RCU_INIT_POINTER(dst->fence, dst_list);
write_seqcount_end(&dst->seq);
- preempt_enable();
dma_resv_list_free(src_list);
dma_fence_put(old);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
index 9dff792c9290..87fd32aae8f9 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
@@ -258,11 +258,9 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
new->shared_count = k;
/* Install the new fence list, seqcount provides the barriers */
- preempt_disable();
write_seqcount_begin(&resv->seq);
RCU_INIT_POINTER(resv->fence, new);
write_seqcount_end(&resv->seq);
- preempt_enable();
/* Drop the references to the removed fences or move them to ef_list */
for (i = j, k = 0; i < old->shared_count; ++i) {
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index a6538ae7d93f..d44a77e8a7e3 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -69,7 +69,7 @@ struct dma_resv_list {
*/
struct dma_resv {
struct ww_mutex lock;
- seqcount_t seq;
+ seqcount_ww_mutex_t seq;
struct dma_fence __rcu *fence_excl;
struct dma_resv_list __rcu *fence;
--
2.20.1
A small number of the the exported seqlock.h functions are kernel-doc
annotated.
Since seqlock.h is now included by the kernel's RST documentation, add
kernel-doc annotations for all of the remaining functions.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 414 +++++++++++++++++++++++++++++++++++-----
1 file changed, 361 insertions(+), 53 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index dfec0c9c19c4..dd55555ff607 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -57,6 +57,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
+/**
+ * seqcount_init() - runtime initializer for seqcount_t
+ * @s: Pointer to the &typedef seqcount_t instance
+ */
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
@@ -80,13 +84,17 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
# define seqcount_lockdep_reader_access(x)
#endif
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
+/**
+ * SEQCNT_ZERO() - static initializer for seqcount_t
+ * @name: Name of the &typedef seqcount_t instance
+ */
+#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * __read_seqcount_begin() - begin a seq-read critical section (without barrier)
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -110,9 +118,9 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount() - Read the raw seqcount
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
* raw_read_seqcount opens a read critical section of the given
* seqcount without any lockdep checking and without checking or
@@ -126,13 +134,13 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount_begin() - start seq-read critical section w/o lockdep
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
* raw_read_seqcount_begin opens a read critical section of the given
* seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
+ * section is tested by calling read_seqcount_retry().
*/
static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
@@ -142,13 +150,13 @@ static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
}
/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * read_seqcount_begin() - begin a seq-read critical section
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * read_seqcount_begin opens a read critical section of the given
+ * seqcount_t. Validity of the critical section is tested by calling
+ * read_seqcount_retry().
*/
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
@@ -157,8 +165,8 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
+ * raw_seqcount_begin() - begin a seq-read critical section
+ * @s: Pointer to &typedef seqcount_t
* Returns: count to be passed to read_seqcount_retry
*
* raw_seqcount_begin opens a read critical section of the given seqcount.
@@ -178,8 +186,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
}
/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
+ * __read_seqcount_retry() - end a seq-read critical section (without barrier)
+ * @s: Pointer to &typedef seqcount_t
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -197,8 +205,8 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
}
/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
+ * read_seqcount_retry() - end a seq-read critical section
+ * @s: Pointer to &typedef seqcount_t
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -225,8 +233,8 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
}
/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_barrier() - do a seq write barrier
+ * @s: Pointer to &typedef seqcount_t
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
@@ -267,6 +275,21 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
s->sequence++;
}
+/**
+ * raw_read_seqcount_latch() - pick even or odd seqcount latch data copy
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * Use seqcount latching to switch between two storage places with
+ * sequence protection to allow interruptible, preemptible, writer
+ * sections.
+ *
+ * Check raw_write_seqcount_latch() for more details and a full reader
+ * and writer usage example.
+ *
+ * Return: sequence counter. Use the lowest bit as index for picking
+ * which data copy to read. Full counter must then be passed to
+ * read_seqcount_retry().
+ */
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
@@ -275,8 +298,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
}
/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_latch() - redirect readers to even/odd copy
+ * @s: Pointer to &typedef seqcount_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -336,8 +359,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes necessary smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq);
*
* return entry;
* }
@@ -391,11 +414,26 @@ static inline void __write_seqcount_begin(seqcount_t *s)
__write_seqcount_begin_nested(s, 0);
}
+/**
+ * write_seqcount_begin() - start a seqcount write-side critical section
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * write_seqcount_begin opens a write-side critical section of the given
+ * seqcount. Seqcount write-side critical sections must be externally
+ * serialized and non-preemptible.
+ */
static inline void write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
+/**
+ * write_seqcount_end() - end a seqcount write-side critical section
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * write_seqcount_end closes a write-side critical section of the given
+ * seqcount.
+ */
static inline void write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
@@ -403,8 +441,8 @@ static inline void write_seqcount_end(seqcount_t *s)
}
/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
+ * write_seqcount_invalidate() - invalidate in-progress read-side seq operations
+ * @s: Pointer to &typedef seqcount_t
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
@@ -435,32 +473,68 @@ typedef struct {
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+/**
+ * seqlock_init() - dynamic initializer for seqlock_t
+ * @sl: Pointer to the seqlock_t instance
+ */
+#define seqlock_init(sl) \
do { \
- seqcount_init(&(x)->seqcount); \
- spin_lock_init(&(x)->lock); \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+/**
+ * DEFINE_SEQLOCK() - Define a statically-allocated seqlock_t
+ * @sl: Name of the &typedef seqlock_t instance
+ */
+#define DEFINE_SEQLOCK(sl) \
+ seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
-/*
- * Read side functions for starting and finalizing a read side section.
+/**
+ * read_seqbegin() - start a seqlock_t read-side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqbegin opens a read side critical section of the given
+ * seqlock_t. Validity of the critical section is tested by checking
+ * read_seqretry().
+ *
+ * Return: count to be passed to read_seqretry()
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
return read_seqcount_begin(&sl->seqcount);
}
+/**
+ * read_seqretry() - end a seqlock_t read side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ * @start: count, from read_seqbegin()
+ *
+ * read_seqretry closes a read side critical section of the given
+ * seqlock_t. If the read side critical section was invalid, it must be
+ * ignored and retried.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
return read_seqcount_retry(&sl->seqcount, start);
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * write_seqlock() - start a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_seqlock opens a write side critical section of the given
+ * seqlock_t. It also acquires the spinlock embedded inside the
+ * sequential lock. All seqlock_t write side critical sections are thus
+ * automatically serialized and non-preemptible.
+ *
+ * If the seqlock_t read side section can be invoked from a hardirq or
+ * softirq context, the ``_irqsave`` and ``_bh`` variants of this
+ * function must be respectively used instead.
+ *
+ * The opened write side section must be closed with write_sequnlock().
*/
static inline void write_seqlock(seqlock_t *sl)
{
@@ -468,30 +542,74 @@ static inline void write_seqlock(seqlock_t *sl)
__write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock() - end a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock closes the (serialized and non-preemptible) write
+ * side critical section of the given seqlock_t.
+ */
static inline void write_sequnlock(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
+/**
+ * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_seqlock_bh is a write_seqlock() variant that disables softirqs
+ * before opening the serialized seqlock_t write side critical section.
+ * Use it only if the read side section, or other writers, can be
+ * invoked from a softirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_bh().
+ */
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
__write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock_bh closes the serialized, softirqs-disabled,
+ * seqlock_t write side critical section. It enables softirqs if they
+ * were already enabled before calling the paired write_seqlock_bh().
+ */
static inline void write_sequnlock_bh(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
+/**
+ * write_seqlock_irq() - start a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_seqlock_irq is a write_seqlock() variant where hardirqs are
+ * disabled before opening the serialized and non-preemptible seqlock_t
+ * write side critical section.
+ */
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
__write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_irq() - end a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock_irq closes the serialized and non-interruptible write
+ * side critical section of the given seqlock_t. It enables local
+ * interrupts afterwards.
+ *
+ * The write critical section must've been opened with write_seqlock_irq().
+ */
static inline void write_sequnlock_irq(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
@@ -507,9 +625,36 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * write_seqlock_irqsave() - start a non-interruptible seqlock_t write section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to write_sequnlock_irqrestore().
+ *
+ * write_seqlock_irqsave is a write_seqlock() variant where the caller's
+ * local interrupts state is saved, then local interrupts are disabled,
+ * before opening the serialized and non-preemptible seqlock_t write
+ * side critical section.
+ *
+ * Use this only if the read side section can be invoked from a hardirq
+ * context.
+ *
+ * The opened write section must be closed with write_sequnlock_irqrestore().
+ */
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
+/**
+ * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
+ *
+ * write_sequnlock_irq closes the serialized and non-interruptible write
+ * side critical section of the given seqlock_t. It then restores the
+ * caller's local interrupts saved state.
+ *
+ * The write section must've been opened with write_seqlock_irqsave().
+ */
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
@@ -517,30 +662,61 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * read_seqlock_excl() - begin a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqlock_excl opens a locking reader critical section for the
+ * given seqlock_t. A locking reader exclusively locks out other writers
+ * and other locking readers, but doesn't update the sequence number.
+ *
+ * Locking readers act like a normal spin_lock()/spin_unlock().
+ *
+ * The opened read side section must be closed with read_sequnlock_excl().
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
+/**
+ * read_sequnlock_excl() - end a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_sequnlock_excl closes a locking reader critical section. The
+ * read section must've been opened with read_seqlock_excl().
+ */
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
+ * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq : Marker and return parameter. If the passed value is even, the
+ * reader will become a *lockless* seqlock_t sequence counter reader as
+ * in read_seqbegin(). If the passed value is odd, the reader will
+ * become a fully locking reader, as in read_seqlock_excl(). In the
+ * first call to read_seqbegin_or_lock(), the caller **must** initialize
+ * and pass an even value in @seq so a lockless read is optimistically
+ * tried first.
*
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
+ * read_seqbegin_or_lock optimistically tries a lockless seqlock_t
+ * sequence counter read first. If an odd counter is found, the lockless
+ * read trial has failed, and the reader transforms to a full seqlock_t
+ * locking reader as in read_seqlock_excl(). This is typically used to
+ * avoid lockless seqlock_t readers starvation (too much retry loops) in
+ * the case of a sharp spike in write activity.
+ *
+ * The opened read section must be closed with done_seqretry(). Check
+ * Documentation/locking/seqlock.rst for template example code.
+ *
+ * Return: The read critical section status is returned through @seq,
+ * which is overloaded as a return parameter. This value must be passed
+ * to need_seqretry() to check the validity of the tried seqlock_t read
+ * section. If the read section must be retried, the returned value must
+ * also be passed to the next iteration of read_seqbegin_or_lock().
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
@@ -550,32 +726,98 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
read_seqlock_excl(lock);
}
+/**
+ * need_seqretry() - validate seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * need_seqretry checks if the seqlock_t read-side critical section
+ * started with read_seqbegin_or_lock() is valid. If it was not, the
+ * caller must retry the read-side section.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
+/**
+ * done_seqretry() - end seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * done_seqretry finishes the seqlock_t read side critical section
+ * started by read_seqbegin_or_lock(). Before finishing the critical
+ * section, the validity of the read side section must've been already
+ * verified with need_seqretry().
+ */
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
+/**
+ * read_seqlock_excl_bh() - start a locking reader seqlock_t section
+ * with softirqs disabled
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqlock_excl_bh is a variant of read_seqlock_excl() that saves
+ * softirqs state, then disables softirqs, before starting the locking
+ * reader read side section. Only use this variant if the seqlock_t
+ * write side section, *or other read sections*, can be invoked from a
+ * softirq context
+ *
+ * The opened section must be closed with read_sequnlock_excl_bh().
+ */
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
+/**
+ * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_sequnlock_excl_bh ends the softirq-disabled seqlock_t locking
+ * reader read side section. It restores the softirqs state saved by
+ * read_seqlock_excl_bh() afterwards.
+ */
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
+/**
+ * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqlock_excl_irq is a variant of read_seqlock_excl() that
+ * disables interrupts before starting the locking reader read side
+ * section. Only use this variant if the seqlock_t write side section,
+ * *or other read sections*, can be invoked from a hardirq context
+ *
+ * The opened read section must be closed with read_sequnlock_excl_irq().
+ */
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
+/**
+ * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_sequnlock_excl_irq ends the interrupts-disabled seqlock_t
+ * locking reader read side critical section. It enables local
+ * interrupts afterwards.
+ *
+ * The read section must've been started with read_seqlock_excl_irq().
+ */
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
@@ -589,15 +831,68 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
+ * locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to read_sequnlock_excl_irqrestore().
+ *
+ * read_seqlock_excl_irqsave is a read_seqlock_excl() variant which
+ * saves the caller's local interrupts state, then disables local
+ * interrupts, before opening the seqlock_t locking reader critical
+ * section.
+ *
+ * Use this only if the seqlock_t write side critical section, or other
+ * read side sections, can be invoked from a hardirq context.
+ *
+ * The opened locking reader critical section must be closed with
+ * read_sequnlock_excl_irqrestore().
+ */
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+/**
+ * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from
+ * read_seqlock_excl_irqsave()
+ *
+ * read_sequnlock_excl_irqrestore closes the non-interruptible seqlock_t
+ * locking reader section. It then restores the caller's local
+ * interrupts saved state.
+ *
+ * The read section must've been opened with read_seqlock_excl_irqsave().
+ */
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
spin_unlock_irqrestore(&sl->lock, flags);
}
+/**
+ * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
+ * a non-interruptible locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
+ *
+ * read_seqbegin_or_lock_irqsave is a variant of read_seqbegin_or_lock()
+ * which saves the local interrupts state, then disables local
+ * interrupts, before opening a seqlock_t *locking reader* critical
+ * section.
+ *
+ * The opened section must be closed with done_seqretry_irqrestore().
+ *
+ * Return:
+ *
+ * 1. The saved local interrupts state in case of a locking reader, to
+ * be passed to done_seqretry_irqrestore().
+ *
+ * 2. The read critical section status, returned through @seq which is
+ * overloaded as a return parameter. Check read_seqbegin_or_lock()
+ * for more info.
+ */
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
@@ -611,6 +906,19 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
return flags;
}
+/**
+ * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
+ * non-interruptible locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Count, from read_seqbegin_or_lock_irqsave()
+ * @flags: Caller's saved local interrupt state in case of a locking
+ * reader, also from read_seqbegin_or_lock_irqsave()
+ *
+ * done_seqretry_irqrestore is a variant of done_seqretry() which
+ * restores the callers saved local interrupts state in case of a
+ * locking reader. Check done_seqretry() for more information. The read
+ * section must've been opened with read_seqbegin_or_lock_irqsave().
+ */
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index ba00e9877f02..69f31c675b58 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6929,7 +6929,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
} else
goto abort;
spin_lock_init(&conf->device_lock);
- seqcount_init(&conf->gen_lock);
+ seqcount_spinlock_init(&conf->gen_lock, &conf->device_lock);
mutex_init(&conf->cache_size_mutex);
init_waitqueue_head(&conf->wait_for_quiescent);
init_waitqueue_head(&conf->wait_for_stripe);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index f90e0704bed9..a2c9e9e9f5ac 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -589,7 +589,7 @@ struct r5conf {
int prev_chunk_sectors;
int prev_algo;
short generation; /* increments with every reshape */
- seqcount_t gen_lock; /* lock against generation changes */
+ seqcount_spinlock_t gen_lock; /* lock against generation changes */
unsigned long reshape_checkpoint; /* Time we last updated
* metadata */
long long min_offset_diff; /* minimum difference between
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
kernel/time/timekeeping.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 9ebaab13339d..24e91a1e2acd 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -39,18 +39,19 @@ enum timekeeping_adv_mode {
TK_ADV_FREQ
};
+static DEFINE_RAW_SPINLOCK(timekeeper_lock);
+
/*
* The most important data for readout fits into a single 64 byte
* cache line.
*/
static struct {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct timekeeper timekeeper;
} tk_core ____cacheline_aligned = {
- .seq = SEQCNT_ZERO(tk_core.seq),
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_core.seq, &timekeeper_lock),
};
-static DEFINE_RAW_SPINLOCK(timekeeper_lock);
static struct timekeeper shadow_timekeeper;
/**
@@ -63,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct tk_read_base base[2];
};
@@ -80,11 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -157,7 +160,7 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
* tk_clock_read - atomic clocksource read() helper
*
* This helper is necessary to use in the read paths because, while the
- * seqlock ensures we don't return a bad value while structures are updated,
+ * seqcount ensures we don't return a bad value while structures are updated,
* it doesn't protect from potential crashes. There is the possibility that
* the tkr's clocksource may change between the read reference, and the
* clock reference passed to the read function. This can cause crashes if
@@ -222,10 +225,10 @@ static inline u64 timekeeping_get_delta(const struct tk_read_base *tkr)
unsigned int seq;
/*
- * Since we're called holding a seqlock, the data may shift
+ * Since we're called holding a seqcount, the data may shift
* under us while we're doing the calculation. This can cause
* false positives, since we'd note a problem but throw the
- * results away. So nest another seqlock here to atomically
+ * results away. So nest another seqcount here to atomically
* grab the points we are checking with.
*/
do {
@@ -486,7 +489,7 @@ EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
*
* To keep it NMI safe since we're accessing from tracing, we're not using a
* separate timekeeper with updates to monotonic clock and boot offset
- * protected with seqlocks. This has the following minor side effects:
+ * protected with seqcounts. This has the following minor side effects:
*
* (1) Its possible that a timestamp be taken after the boot offset is updated
* but before the timekeeper is updated. If this happens, the new boot offset
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
A plain seqcount_t does not contain the information of which lock must
be held when entering a write side critical section.
Use the new seqcount_spinlock_t and seqcount_mutex_t data types instead,
which allow to associate a lock with the sequence counter. This enables
lockdep to verify that the lock used for writer serialization is held
when the write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/xfrm/xfrm_policy.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 297b2fdb3c29..aae78a7aecd7 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -122,7 +122,7 @@ struct xfrm_pol_inexact_bin {
/* list containing '*:*' policies */
struct hlist_head hhead;
- seqcount_t count;
+ seqcount_spinlock_t count;
/* tree sorted by daddr/prefix */
struct rb_root root_d;
@@ -155,7 +155,7 @@ static struct xfrm_policy_afinfo const __rcu *xfrm_policy_afinfo[AF_INET6 + 1]
__read_mostly;
static struct kmem_cache *xfrm_dst_cache __ro_after_init;
-static __read_mostly seqcount_t xfrm_policy_hash_generation;
+static __read_mostly seqcount_mutex_t xfrm_policy_hash_generation;
static struct rhashtable xfrm_policy_inexact_table;
static const struct rhashtable_params xfrm_pol_inexact_params;
@@ -719,7 +719,7 @@ xfrm_policy_inexact_alloc_bin(const struct xfrm_policy *pol, u8 dir)
INIT_HLIST_HEAD(&bin->hhead);
bin->root_d = RB_ROOT;
bin->root_s = RB_ROOT;
- seqcount_init(&bin->count);
+ seqcount_spinlock_init(&bin->count, &net->xfrm.xfrm_policy_lock);
prev = rhashtable_lookup_get_insert_key(&xfrm_policy_inexact_table,
&bin->k, &bin->head,
@@ -1911,7 +1911,7 @@ static int xfrm_policy_match(const struct xfrm_policy *pol,
static struct xfrm_pol_inexact_node *
xfrm_policy_lookup_inexact_addr(const struct rb_root *r,
- seqcount_t *count,
+ seqcount_spinlock_t *count,
const xfrm_address_t *addr, u16 family)
{
const struct rb_node *parent;
@@ -4158,7 +4158,7 @@ void __init xfrm_init(void)
{
register_pernet_subsys(&xfrm_net_ops);
xfrm_dev_init();
- seqcount_init(&xfrm_policy_hash_generation);
+ seqcount_mutex_init(&xfrm_policy_hash_generation, &hash_resize_mutex);
xfrm_input_init();
#ifdef CONFIG_INET_ESPINTCP
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/kvm_irqfd.h | 2 +-
virt/kvm/eventfd.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dc1da020305b..dac047abdba7 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
wait_queue_entry_t wait;
/* Update side is protected by irqfds.lock */
struct kvm_kernel_irq_routing_entry irq_entry;
- seqcount_t irq_entry_sc;
+ seqcount_spinlock_t irq_entry_sc;
/* Used for level IRQ fast-path */
int gsi;
struct work_struct inject;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 67b6fc153e9c..8694a2920ea9 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
INIT_LIST_HEAD(&irqfd->list);
INIT_WORK(&irqfd->inject, irqfd_inject);
INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
- seqcount_init(&irqfd->irq_entry_sc);
+ seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
f = fdget(args->fd);
if (!f.file) {
--
2.20.1
Preemption must be disabled before entering a sequence count write side
critical section. Failing to do so, the seqcount read side can preempt
the write side section and spin for the entire scheduler tick. If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.
Assert through lockdep that preemption is disabled for seqcount writers.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 30 ++++++++++++++++++++++++------
1 file changed, 24 insertions(+), 6 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 0491d963d47e..d35be7709403 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -369,14 +369,32 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
/*
* Sequence counter only version assumes that callers are using their
- * own mutexing.
+ * own locking and preemption is disabled.
*/
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
+static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ lockdep_assert_preemption_disabled();
+ __write_seqcount_begin_nested(s, subclass);
+}
+
+/*
+ * write_seqcount_begin() without lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption
+ * is already disabled. For example, seqlock_t write functions.
+ */
+static inline void __write_seqcount_begin(seqcount_t *s)
+{
+ __write_seqcount_begin_nested(s, 0);
+}
+
static inline void write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
@@ -446,7 +464,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
static inline void write_sequnlock(seqlock_t *sl)
@@ -458,7 +476,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
static inline void write_sequnlock_bh(seqlock_t *sl)
@@ -470,7 +488,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
static inline void write_sequnlock_irq(seqlock_t *sl)
@@ -484,7 +502,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
return flags;
}
--
2.20.1
Commit 3c3b177a9369 ("reservation: add support for read-only access
using rcu") introduced a sequence counter to manage updates to
reservations. Back then, the reservation object initializer
reservation_object_init() was always inlined.
Having the sequence counter initialization inlined meant that each of
the call sites would have a different lockdep class key, which would've
broken lockdep's deadlock detection. The aforementioned commit thus
introduced, and exported, a custom seqcount lockdep class key and name.
The commit 8735f16803f00 ("dma-buf: cleanup reservation_object_init...")
transformed the reservation object initializer to a normal non-inlined C
function. seqcount_init(), which automatically defines the seqcount
lockdep class key and must be called non-inlined, can now be safely used.
Remove the seqcount custom lockdep class key, name, and export. Use
seqcount_init() inside the dma reservation object initializer.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
---
drivers/dma-buf/dma-resv.c | 9 +--------
include/linux/dma-resv.h | 2 --
2 files changed, 1 insertion(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 4264e64788c4..590ce7ad60a0 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -50,12 +50,6 @@
DEFINE_WD_CLASS(reservation_ww_class);
EXPORT_SYMBOL(reservation_ww_class);
-struct lock_class_key reservation_seqcount_class;
-EXPORT_SYMBOL(reservation_seqcount_class);
-
-const char reservation_seqcount_string[] = "reservation_seqcount";
-EXPORT_SYMBOL(reservation_seqcount_string);
-
/**
* dma_resv_list_alloc - allocate fence list
* @shared_max: number of fences we need space for
@@ -134,9 +128,8 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
+ seqcount_init(&obj->seq);
- __seqcount_init(&obj->seq, reservation_seqcount_string,
- &reservation_seqcount_class);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
}
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index ee50d10f052b..a6538ae7d93f 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -46,8 +46,6 @@
#include <linux/rcupdate.h>
extern struct ww_class reservation_ww_class;
-extern struct lock_class_key reservation_seqcount_class;
-extern const char reservation_seqcount_string[];
/**
* struct dma_resv_list - a list of shared fences
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/net/netfilter/nf_conntrack.h | 2 +-
net/netfilter/nf_conntrack_core.c | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 9f551f3b69c6..333fd54aec30 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -286,7 +286,7 @@ int nf_conntrack_hash_resize(unsigned int hashsize);
extern struct hlist_nulls_head *nf_conntrack_hash;
extern unsigned int nf_conntrack_htable_size;
-extern seqcount_t nf_conntrack_generation;
+extern seqcount_spinlock_t nf_conntrack_generation;
extern unsigned int nf_conntrack_max;
/* must be called with rcu read lock held */
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index c4582eb71766..48a839377da2 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -180,7 +180,7 @@ EXPORT_SYMBOL_GPL(nf_conntrack_htable_size);
unsigned int nf_conntrack_max __read_mostly;
EXPORT_SYMBOL_GPL(nf_conntrack_max);
-seqcount_t nf_conntrack_generation __read_mostly;
+seqcount_spinlock_t nf_conntrack_generation __read_mostly;
static unsigned int nf_conntrack_hash_rnd __read_mostly;
static u32 hash_conntrack_raw(const struct nf_conntrack_tuple *tuple,
@@ -2512,7 +2512,8 @@ int nf_conntrack_init_start(void)
/* struct nf_ct_ext uses u8 to store offsets/size */
BUILD_BUG_ON(total_extension_size() > 255u);
- seqcount_init(&nf_conntrack_generation);
+ seqcount_spinlock_init(&nf_conntrack_generation,
+ &nf_conntrack_locks_all_lock);
for (i = 0; i < CONNTRACK_LOCKS; i++)
spin_lock_init(&nf_conntrack_locks[i]);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the write side critical section.
There is no built-in debugging mechanism to verify that the lock used
for writer serialization is held and preemption is disabled. Some usage
sites like dma-buf have explicit lockdep checks for the writer-side
lock, but this covers only a small portion of the sequence counter usage
in the kernel.
Add new sequence counter types which allows to associate a lock to the
sequence counter at initialization time. The seqcount API functions are
extended to provide appropriate lockdep assertions depending on the
seqcount/lock type.
For sequence counters with associated locks that do not implicitly
disable preemption, preemption protection is enforced in the sequence
counter write side functions. This removes the need to explicitly add
preempt_disable/enable() around the write side critical sections: the
write_begin/end() functions for these new sequence counter types
automatically do this.
Introduce the following seqcount types with associated locks:
seqcount_spinlock_t
seqcount_raw_spinlock_t
seqcount_rwlock_t
seqcount_mutex_t
seqcount_ww_mutex_t
Extend the seqcount read and write functions to branch out to the
specific seqcount_LOCKTYPE_t implementation at compile-time. This avoids
kernel API explosion per each new seqcount_LOCKTYPE_t added. Add such
compile-time type detection logic into a new, internal, seqlock header.
Document the proper seqcount_LOCKTYPE_t usage, and rationale, at
Documentation/locking/seqlock.rst.
If lockdep is disabled, this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/seqlock.rst | 64 ++++-
MAINTAINERS | 2 +-
include/linux/seqlock.h | 355 +++++++++++++++++++++----
include/linux/seqlock_types_internal.h | 187 +++++++++++++
4 files changed, 549 insertions(+), 59 deletions(-)
create mode 100644 include/linux/seqlock_types_internal.h
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index 2242ae00e7bf..e6f8e4be7db8 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -45,9 +45,11 @@ write side section. If the sequence counter read section can be invoked
from hardirq or softirq contexts, interrupts or bottom halves must be
respectively disabled before entering the write side section.
-If it's desired to automatically handle the sequence counter
-requirements of writer serialization and non-preemptibility, use a
-:ref:`sequential lock <seqlock_t>` instead.
+If the write serialization mechanism is one of the common kernel locking
+primitives, use :ref:`sequence counters with associated locks
+<seqcount_locktype_t>` instead. If it's desired to automatically handle
+the sequence counter writer serialization and non-preemptibility
+requirements, use a :ref:`sequential lock <seqlock_t>`.
Initialization:
@@ -67,6 +69,7 @@ Initialization:
Write path:
+.. _seqcount_write_ops:
.. code-block:: c
/* Serialized context with disabled preemption */
@@ -79,6 +82,7 @@ Write path:
Read path:
+.. _seqcount_read_ops:
.. code-block:: c
do {
@@ -88,6 +92,60 @@ Read path:
} while (read_seqcount_retry(&foo_seqcount, seq));
+.. _seqcount_locktype_t:
+
+Sequence counters with associated locks (:c:type:`seqcount_LOCKTYPE_t`)
+-----------------------------------------------------------------------
+
+As :ref:`earlier discussed <seqcount_t>`, seqcount write side critical
+sections must be serialized and non-preemptible. This variant of
+sequence counters associate the lock used for writer serialization at
+the seqcount initialization time. This enables lockdep to validate that
+the write side critical section is properly serialized.
+
+This lock association is a NOOP if lockdep is disabled and has neither
+storage nor runtime overhead. If lockdep is enabled, the lock pointer is
+stored in struct seqcount and lockdep's "lock is held" assertions are
+injected at the beginning of the write side critical section to validate
+that it is properly protected.
+
+For lock types which do not implicitly disable preemption, preemption
+protection is enforced in the write side function.
+
+The following seqcounts with associated locks are defined:
+
+ - :c:type:`seqcount_spinlock_t`
+ - :c:type:`seqcount_raw_spinlock_t`
+ - :c:type:`seqcount_rwlock_t`
+ - :c:type:`seqcount_mutex_t`
+ - :c:type:`seqcount_ww_mutex_t`
+
+The plain seqcount read and write APIs branch out to the specific
+seqcount_LOCKTYPE_t implementation at compile-time. This avoids kernel
+API explosion per each new seqcount LOCKTYPE.
+
+Initialization (replace "LOCKTYPE" with one of the supported locks):
+
+.. code-block:: c
+
+ /* dynamic */
+ seqcount_LOCKTYPE_t foo_seqcount;
+ seqcount_LOCKTYPE_init(&foo_seqcount, &lock);
+
+ /* static */
+ static seqcount_LOCKTYPE_t foo_seqcount =
+ SEQCNT_LOCKTYPE_ZERO(foo_seqcount, &lock);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_LOCKTYPE_ZERO(foo.seq, &lock),
+ } foo;
+
+Write path: same as in :ref:`plain seqcount_t <seqcount_write_ops>`,
+while running from a context with the associated LOCKTYPE lock acquired.
+
+Read path: same as in :ref:`plain seqcount_t <seqcount_read_ops>`.
+
.. _seqlock_t:
Sequential locks (:c:type:`seqlock_t`)
diff --git a/MAINTAINERS b/MAINTAINERS
index 091ec22c1a23..f3ae546009ee 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9925,7 +9925,7 @@ F: include/linux/lockdep.h
F: include/linux/mutex*.h
F: include/linux/rwlock*.h
F: include/linux/rwsem*.h
-F: include/linux/seqlock.h
+F: include/linux/seqlock*.h
F: include/linux/spinlock*.h
F: kernel/locking/
F: lib/locking*.[ch]
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index dd55555ff607..eca464ecf012 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -90,11 +90,10 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*/
#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
-
/**
* __read_seqcount_begin() - begin a seq-read critical section (without barrier)
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -104,7 +103,9 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+#define __read_seqcount_begin(s) do___read_seqcount_begin(s)
+
+static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
{
unsigned ret;
@@ -119,14 +120,16 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
/**
* raw_read_seqcount() - Read the raw seqcount
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount opens a read critical section of the given
* seqcount without any lockdep checking and without checking or
* masking the LSB. Calling code is responsible for handling that.
*/
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
+#define raw_read_seqcount(s) do_raw_read_seqcount(s)
+
+static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -135,38 +138,42 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
/**
* raw_read_seqcount_begin() - start seq-read critical section w/o lockdep
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount_begin opens a read critical section of the given
* seqcount, but without any lockdep checking. Validity of the critical
* section is tested by calling read_seqcount_retry().
*/
-static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+#define raw_read_seqcount_begin(s) do_raw_read_seqcount_begin(s)
+
+static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
+ unsigned ret = __read_seqcount_t_begin(s);
smp_rmb();
return ret;
}
/**
* read_seqcount_begin() - begin a seq-read critical section
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* read_seqcount_begin opens a read critical section of the given
* seqcount_t. Validity of the critical section is tested by calling
* read_seqcount_retry().
*/
-static inline unsigned read_seqcount_begin(const seqcount_t *s)
+#define read_seqcount_begin(s) do_read_seqcount_begin(s)
+
+static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
{
seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
+ return raw_read_seqcount_t_begin(s);
}
/**
* raw_seqcount_begin() - begin a seq-read critical section
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* raw_seqcount_begin opens a read critical section of the given seqcount.
@@ -178,7 +185,9 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
* read_seqcount_retry() instead of stabilizing at the beginning of the
* critical section.
*/
-static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+#define raw_seqcount_begin(s) do_raw_seqcount_begin(s)
+
+static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -187,7 +196,7 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
/**
* __read_seqcount_retry() - end a seq-read critical section (without barrier)
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -199,14 +208,16 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define __read_seqcount_retry(s, start) do___read_seqcount_retry(s, start)
+
+static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
return unlikely(s->sequence != start);
}
/**
* read_seqcount_retry() - end a seq-read critical section
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -214,19 +225,25 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
* If the critical section was invalid, it must be ignored (and typically
* retried).
*/
-static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define read_seqcount_retry(s, start) do_read_seqcount_retry(s, start)
+
+static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return __read_seqcount_t_retry(s, start);
}
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+#define raw_write_seqcount_begin(s) do_raw_write_seqcount_begin(s)
+
+static inline void raw_write_seqcount_t_begin(seqcount_t *s)
{
s->sequence++;
smp_wmb();
}
-static inline void raw_write_seqcount_end(seqcount_t *s)
+#define raw_write_seqcount_end(s) do_raw_write_seqcount_end(s)
+
+static inline void raw_write_seqcount_t_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
@@ -234,7 +251,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
/**
* raw_write_seqcount_barrier() - do a seq write barrier
- * @s: Pointer to &typedef seqcount_t
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
@@ -268,7 +285,9 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* X = false;
* }
*/
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
+#define raw_write_seqcount_barrier(s) do_raw_write_seqcount_barrier(s)
+
+static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
{
s->sequence++;
smp_wmb();
@@ -277,7 +296,7 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
/**
* raw_read_seqcount_latch() - pick even or odd seqcount latch data copy
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* Use seqcount latching to switch between two storage places with
* sequence protection to allow interruptible, preemptible, writer
@@ -290,7 +309,9 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
* which data copy to read. Full counter must then be passed to
* read_seqcount_retry().
*/
-static inline int raw_read_seqcount_latch(seqcount_t *s)
+#define raw_read_seqcount_latch(s) do_raw_read_seqcount_latch(s)
+
+static inline int raw_read_seqcount_t_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
@@ -299,7 +320,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
/**
* raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -384,34 +405,39 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
-static inline void raw_write_seqcount_latch(seqcount_t *s)
+#define raw_write_seqcount_latch(s) do_raw_write_seqcount_latch(s)
+
+static inline void raw_write_seqcount_t_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
smp_wmb(); /* increment "sequence" before following stores */
}
-static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+#define write_seqcount_begin_nested(s, subclass) \
+ do_write_seqcount_begin_nested(s, subclass)
+
+static inline void __write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
- raw_write_seqcount_begin(s);
+ raw_write_seqcount_t_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
lockdep_assert_preemption_disabled();
- __write_seqcount_begin_nested(s, subclass);
+ __write_seqcount_t_begin_nested(s, subclass);
}
/*
- * write_seqcount_begin() without lockdep non-preemptibility checks.
+ * write_seqcount_t_begin() without lockdep non-preemptibility check.
*
* Use for internal seqlock.h code where it's known that preemption
- * is already disabled. For example, seqlock_t write functions.
+ * is already disabled. For example, seqlock_t write side functions.
*/
-static inline void __write_seqcount_begin(seqcount_t *s)
+static inline void __write_seqcount_t_begin(seqcount_t *s)
{
- __write_seqcount_begin_nested(s, 0);
+ __write_seqcount_t_begin_nested(s, 0);
}
/**
@@ -422,9 +448,11 @@ static inline void __write_seqcount_begin(seqcount_t *s)
* seqcount. Seqcount write-side critical sections must be externally
* serialized and non-preemptible.
*/
-static inline void write_seqcount_begin(seqcount_t *s)
+#define write_seqcount_begin(s) do_write_seqcount_begin(s)
+
+static inline void write_seqcount_t_begin(seqcount_t *s)
{
- write_seqcount_begin_nested(s, 0);
+ write_seqcount_t_begin_nested(s, 0);
}
/**
@@ -434,25 +462,242 @@ static inline void write_seqcount_begin(seqcount_t *s)
* write_seqcount_end closes a write-side critical section of the given
* seqcount.
*/
-static inline void write_seqcount_end(seqcount_t *s)
+#define write_seqcount_end(s) do_write_seqcount_end(s)
+
+static inline void write_seqcount_t_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
- raw_write_seqcount_end(s);
+ raw_write_seqcount_t_end(s);
}
/**
* write_seqcount_invalidate() - invalidate in-progress read-side seq operations
- * @s: Pointer to &typedef seqcount_t
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
*/
-static inline void write_seqcount_invalidate(seqcount_t *s)
+#define write_seqcount_invalidate(s) do_write_seqcount_invalidate(s)
+
+static inline void write_seqcount_t_invalidate(seqcount_t *s)
{
smp_wmb();
s->sequence+=2;
}
+/*
+ * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ *
+ * A sequence counter which associates the lock used for writer
+ * serialization at initialization time. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ *
+ * For assicated locks which do not implicitly disable preemption,
+ * preemption protection is enforced in the write side function.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
+/**
+ * typedef seqcount_spinlock_t - sequence count with spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * spinlock. The spinlock is associated to the sequence count in the
+ * static initializer or init function. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_spinlock {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ spinlock_t *lock;
+#endif
+} seqcount_spinlock_t;
+
+#ifdef CONFIG_LOCKDEP
+
+#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+ .lock = (assoc_lock), \
+}
+
+/* Define as macro due to static lockdep key @ seqcount_init() */
+#define seqcount_locktype_init(s, assoc_lock) \
+do { \
+ seqcount_init(&(s)->seqcount); \
+ (s)->lock = (assoc_lock); \
+} while (0)
+
+#else /* !CONFIG_LOCKDEP */
+
+#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+}
+
+#define seqcount_locktype_init(s, assoc_lock) \
+do { \
+ seqcount_init(&(s)->seqcount); \
+} while (0)
+
+#endif
+
+/**
+ * SEQCNT_SPINLOCK_ZERO - static initializer for seqcount_spinlock_t
+ * @name: Name of the &typedef seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define SEQCNT_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_spinlock_init - runtime initializer for seqcount_spinlock_t
+ * @s: Pointer to the &typedef seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define seqcount_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_raw_spinlock_t - sequence count with raw spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated raw spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * raw spinlock. The raw spinlock is associated to the sequence count in
+ * the static initializer or init function. This enables lockdep to
+ * validate that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_raw_spinlock {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ raw_spinlock_t *lock;
+#endif
+} seqcount_raw_spinlock_t;
+
+/**
+ * SEQCNT_RAW_SPINLOCK_ZERO - static initializer for seqcount_raw_spinlock_t
+ * @name: Name of the &typedef seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_raw_spinlock_init - runtime initializer for seqcount_raw_spinlock_t
+ * @s: Pointer to the &typedef seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define seqcount_raw_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_rwlock_t - sequence count with rwlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated rwlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * rwlock. The rwlock is associated to the sequence count in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ */
+typedef struct seqcount_rwlock {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ rwlock_t *lock;
+#endif
+} seqcount_rwlock_t;
+
+/**
+ * SEQCNT_RWLOCK_ZERO - static initializer for seqcount_rwlock_t
+ * @name: Name of the &typedef seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define SEQCNT_RWLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_rwlock_init - runtime initializer for seqcount_rwlock_t
+ * @s: Pointer to the &typedef seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define seqcount_rwlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_mutex_t - sequence count with mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * mutex. The mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_mutex_t.
+ */
+typedef struct seqcount_mutex {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ struct mutex *lock;
+#endif
+} seqcount_mutex_t;
+
+/**
+ * SEQCNT_MUTEX_ZERO - static initializer for seqcount_mutex_t
+ * @name: Name of the &typedef seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define SEQCNT_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_mutex_init - runtime initializer for seqcount_mutex_t
+ * @s: Pointer to the &typedef seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define seqcount_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_ww_mutex_t - sequence count with ww_mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated ww_mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * ww_mutex. The ww_mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_ww_mutex_t.
+ */
+typedef struct seqcount_ww_mutex {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ struct ww_mutex *lock;
+#endif
+} seqcount_ww_mutex_t;
+
+/**
+ * SEQCNT_WW_MUTEX_ZERO - static initializer for seqcount_ww_mutex_t
+ * @name: Name of the &typedef seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define SEQCNT_WW_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_ww_mutex_init - runtime initializer for seqcount_ww_mutex_t
+ * @s: Pointer to the &typedef seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define seqcount_ww_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+#include <linux/seqlock_types_internal.h>
+
/*
* Sequential locks (seqlock_t)
*
@@ -475,7 +720,7 @@ typedef struct {
/**
* seqlock_init() - dynamic initializer for seqlock_t
- * @sl: Pointer to the seqlock_t instance
+ * @sl: Pointer to the &typedef seqlock_t instance
*/
#define seqlock_init(sl) \
do { \
@@ -502,7 +747,7 @@ typedef struct {
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- return read_seqcount_begin(&sl->seqcount);
+ return read_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -518,7 +763,7 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
*/
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
- return read_seqcount_retry(&sl->seqcount, start);
+ return read_seqcount_t_retry(&sl->seqcount, start);
}
/**
@@ -539,7 +784,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -551,7 +796,7 @@ static inline void write_seqlock(seqlock_t *sl)
*/
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
@@ -569,7 +814,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -582,7 +827,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
*/
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
@@ -597,7 +842,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -612,7 +857,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
*/
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -621,7 +866,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
return flags;
}
@@ -658,7 +903,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
diff --git a/include/linux/seqlock_types_internal.h b/include/linux/seqlock_types_internal.h
new file mode 100644
index 000000000000..de635f4c7297
--- /dev/null
+++ b/include/linux/seqlock_types_internal.h
@@ -0,0 +1,187 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_SEQLOCK_TYPES_INTERNAL_H
+#define __LINUX_SEQLOCK_TYPES_INTERNAL_H
+
+/*
+ * Sequence counters with associated locks
+ *
+ * Copyright (C) 2020 Linutronix GmbH
+ */
+
+#ifndef __LINUX_SEQLOCK_H
+#error This is an INTERNAL header; it must only be included by seqlock.h
+#endif
+
+#include <linux/mutex.h>
+#include <linux/rwlock.h>
+#include <linux/spinlock.h>
+#include <linux/ww_mutex.h>
+
+/*
+ * @s: pointer to seqcount_t or any of the seqcount_locktype_t variants
+ */
+#define __to_seqcount_t(s) \
+({ \
+ seqcount_t *seq; \
+ \
+ if (__same_type(*(s), seqcount_t)) \
+ seq = (seqcount_t *)(s); \
+ else if (__same_type(*(s), seqcount_spinlock_t)) \
+ seq = &((seqcount_spinlock_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_raw_spinlock_t)) \
+ seq = &((seqcount_raw_spinlock_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_rwlock_t)) \
+ seq = &((seqcount_rwlock_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_mutex_t)) \
+ seq = &((seqcount_mutex_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_ww_mutex_t)) \
+ seq = &((seqcount_ww_mutex_t *)(s))->seqcount; \
+ else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+ \
+ seq; \
+})
+
+/*
+ * seqcount_LOCKTYPE_t -- write APIs
+ *
+ * For associated lock types which do not implicitly disable preemption,
+ * enforce preemption protection in the write side functions.
+ *
+ * Never use lockdep for the raw write variants.
+ */
+
+#define __associated_lock_is_preemptible(s) \
+({ \
+ bool ret; \
+ \
+ if (__same_type(*(s), seqcount_t) || \
+ __same_type(*(s), seqcount_spinlock_t) || \
+ __same_type(*(s), seqcount_raw_spinlock_t) || \
+ __same_type(*(s), seqcount_rwlock_t)) { \
+ ret = false; \
+ } else if (__same_type(*(s), seqcount_mutex_t) || \
+ __same_type(*(s), seqcount_ww_mutex_t)) { \
+ ret = true; \
+ } else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+ \
+ ret; \
+})
+
+#ifdef CONFIG_LOCKDEP
+
+#define __assert_associated_lock_held(s) \
+do { \
+ if (__same_type(*(s), seqcount_t)) \
+ break; \
+ \
+ if (__same_type(*(s), seqcount_spinlock_t)) \
+ lockdep_assert_held(((seqcount_spinlock_t *)(s))->lock);\
+ else if (__same_type(*(s), seqcount_raw_spinlock_t)) \
+ lockdep_assert_held(((seqcount_raw_spinlock_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_rwlock_t)) \
+ lockdep_assert_held_write(((seqcount_rwlock_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_mutex_t)) \
+ lockdep_assert_held(((seqcount_mutex_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_ww_mutex_t)) \
+ lockdep_assert_held(&((seqcount_ww_mutex_t *)(s))->lock->base); \
+ else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+} while (0)
+
+#else
+
+#define __assert_associated_lock_held(s) \
+do { \
+ (void) __to_seqcount_t(s); \
+} while (0)
+
+#endif /* CONFIG_LOCKDEP */
+
+#define do_raw_write_seqcount_begin(s) \
+do { \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ raw_write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+#define do_raw_write_seqcount_end(s) \
+do { \
+ raw_write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+#define do_write_seqcount_begin_nested(s, subclass) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin_nested(__to_seqcount_t(s), subclass); \
+} while (0)
+
+#define do_write_seqcount_begin(s) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+#define do_write_seqcount_end(s) \
+do { \
+ write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+#define do_write_seqcount_invalidate(s) \
+ write_seqcount_t_invalidate(__to_seqcount_t(s))
+
+#define do_raw_write_seqcount_barrier(s) \
+ raw_write_seqcount_t_barrier(__to_seqcount_t(s))
+
+/*
+ * Latch sequence counters write side critical sections don't need to
+ * run with preemption disabled. Check @raw_write_seqcount_latch().
+ */
+#define do_raw_write_seqcount_latch(s) \
+ raw_write_seqcount_t_latch(__to_seqcount_t(s))
+
+/*
+ * seqcount_LOCKTYPE_t -- read APIs
+ */
+
+#define do___read_seqcount_begin(s) \
+ __read_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_raw_read_seqcount(s) \
+ raw_read_seqcount_t(__to_seqcount_t(s))
+
+#define do_raw_seqcount_begin(s) \
+ raw_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_raw_read_seqcount_begin(s) \
+ raw_read_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_read_seqcount_begin(s) \
+ read_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_raw_read_seqcount_latch(s) \
+ raw_read_seqcount_t_latch(__to_seqcount_t(s))
+
+#define do___read_seqcount_retry(s, start) \
+ __read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+#define do_read_seqcount_retry(s, start) \
+ read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+#endif /* __LINUX_SEQLOCK_TYPES_INTERNAL_H */
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 ++--
include/linux/dcache.h | 2 +-
include/linux/fs_struct.h | 2 +-
4 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index b280e07e162b..e5f365d8fd67 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1727,7 +1727,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
dentry->d_lockref.count = 1;
dentry->d_flags = 0;
spin_lock_init(&dentry->d_lock);
- seqcount_init(&dentry->d_seq);
+ seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
dentry->d_inode = NULL;
dentry->d_parent = dentry;
dentry->d_sb = sb;
diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index ca639ed967b7..04b3f5b9c629 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -117,7 +117,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
- seqcount_init(&fs->seq);
+ seqcount_spinlock_init(&fs->seq, &fs->lock);
fs->umask = old->umask;
spin_lock(&old->lock);
@@ -163,6 +163,6 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
- .seq = SEQCNT_ZERO(init_fs.seq),
+ .seq = SEQCNT_SPINLOCK_ZERO(init_fs.seq, &init_fs.lock),
.umask = 0022,
};
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index c1488cc84fd9..235563da356d 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -89,7 +89,7 @@ extern struct dentry_stat_t dentry_stat;
struct dentry {
/* RCU lookup touched fields */
unsigned int d_flags; /* protected by d_lock */
- seqcount_t d_seq; /* per dentry seqlock */
+ seqcount_spinlock_t d_seq; /* per dentry seqlock */
struct hlist_bl_node d_hash; /* lookup hash list */
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index cf1015abfbf2..783b48dedb72 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -9,7 +9,7 @@
struct fs_struct {
int users;
spinlock_t lock;
- seqcount_t seq;
+ seqcount_spinlock_t seq;
int umask;
int in_exec;
struct path root, pwd;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_rwlock_t data type, which allows to associate a
rwlock with the sequence counter. This enables lockdep to verify that
the rwlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/netfilter/nft_set_rbtree.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/netfilter/nft_set_rbtree.c b/net/netfilter/nft_set_rbtree.c
index 3ffef454d469..f50d986d43c5 100644
--- a/net/netfilter/nft_set_rbtree.c
+++ b/net/netfilter/nft_set_rbtree.c
@@ -18,7 +18,7 @@
struct nft_rbtree {
struct rb_root root;
rwlock_t lock;
- seqcount_t count;
+ seqcount_rwlock_t count;
struct delayed_work gc_work;
};
@@ -505,7 +505,7 @@ static int nft_rbtree_init(const struct nft_set *set,
struct nft_rbtree *priv = nft_set_priv(set);
rwlock_init(&priv->lock);
- seqcount_init(&priv->count);
+ seqcount_rwlock_init(&priv->count, &priv->lock);
priv->root = RB_ROOT;
INIT_DEFERRABLE_WORK(&priv->gc_work, nft_rbtree_gc);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/userfaultfd.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e39fdec8a0b0..dd3aab31c50f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -61,7 +61,7 @@ struct userfaultfd_ctx {
/* waitqueue head for events */
wait_queue_head_t event_wqh;
/* a refile sequence protected by fault_pending_wqh lock */
- struct seqcount refile_seq;
+ seqcount_spinlock_t refile_seq;
/* pseudo fd refcounting */
refcount_t refcount;
/* userfaultfd syscall flags */
@@ -1998,7 +1998,7 @@ static void init_once_userfaultfd_ctx(void *mem)
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->event_wqh);
init_waitqueue_head(&ctx->fd_wqh);
- seqcount_init(&ctx->refile_seq);
+ seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
}
SYSCALL_DEFINE1(userfaultfd, int, flags)
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/hrtimer.h | 2 +-
kernel/time/hrtimer.c | 13 ++++++++++---
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 15c8ac313678..25993b86ac5c 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -159,7 +159,7 @@ struct hrtimer_clock_base {
struct hrtimer_cpu_base *cpu_base;
unsigned int index;
clockid_t clockid;
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct hrtimer *running;
struct timerqueue_head active;
ktime_t (*get_time)(void);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index d89da1c7e005..c4038511d5c9 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -135,7 +135,11 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
* timer->base->cpu_base
*/
static struct hrtimer_cpu_base migration_cpu_base = {
- .clock_base = { { .cpu_base = &migration_cpu_base, }, },
+ .clock_base = { {
+ .cpu_base = &migration_cpu_base,
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(migration_cpu_base.seq,
+ &migration_cpu_base.lock),
+ }, },
};
#define migration_base migration_cpu_base.clock_base[0]
@@ -1998,8 +2002,11 @@ int hrtimers_prepare_cpu(unsigned int cpu)
int i;
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
- cpu_base->clock_base[i].cpu_base = cpu_base;
- timerqueue_init_head(&cpu_base->clock_base[i].active);
+ struct hrtimer_clock_base *clock_b = &cpu_base->clock_base[i];
+
+ clock_b->cpu_base = cpu_base;
+ seqcount_raw_spinlock_init(&clock_b->seq, &cpu_base->lock);
+ timerqueue_init_head(&clock_b->active);
}
cpu_base->cpu = cpu;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
block/blk-iocost.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 7c1fe605d0d6..8029a9e8fa55 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -405,7 +405,7 @@ struct ioc {
enum ioc_running running;
atomic64_t vtime_rate;
- seqcount_t period_seqcount;
+ seqcount_spinlock_t period_seqcount;
u32 period_at; /* wallclock starttime */
u64 period_at_vtime; /* vtime starttime */
@@ -872,7 +872,6 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
{
- lockdep_assert_held(&ioc->lock);
WARN_ON_ONCE(ioc->running != IOC_RUNNING);
write_seqcount_begin(&ioc->period_seqcount);
@@ -1958,7 +1957,7 @@ static int blk_iocost_init(struct request_queue *q)
ioc->running = IOC_IDLE;
atomic64_set(&ioc->vtime_rate, VTIME_PER_USEC);
- seqcount_init(&ioc->period_seqcount);
+ seqcount_spinlock_init(&ioc->period_seqcount, &ioc->lock);
ioc->period_at = ktime_to_us(ktime_get());
atomic64_set(&ioc->cur_period, 0);
atomic_set(&ioc->hweight_gen, 0);
--
2.20.1
Am 19.05.20 um 23:45 schrieb Ahmed S. Darwish:
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. If the serialization primitive is
> not disabling preemption implicitly, preemption has to be explicitly
> disabled before entering the sequence counter write side critical
> section.
>
> The dma-buf reservation subsystem uses plain sequence counters to manage
> updates to reservations. Writer serialization is accomplished through a
> wound/wait mutex.
>
> Acquiring a wound/wait mutex does not disable preemption, so this needs
> to be done manually before and after the write side critical section.
>
> Use the newly-added seqcount_ww_mutex_t instead:
>
> - It associates the ww_mutex with the sequence count, which enables
> lockdep to validate that the write side critical section is properly
> serialized.
>
> - It removes the need to explicitly add preempt_disable/enable()
> around the write side critical section because the write_begin/end()
> functions for this new data type automatically do this.
>
> If lockdep is disabled this ww_mutex lock association is compiled out
> and has neither storage size nor runtime overhead.
Mhm, is the dma_resv object the only user of this new seqcount_ww_mutex
variant ?
If yes we are trying to get rid of this sequence counter for quite some
time, so I would rather invest the additional time to finish this.
Regards,
Christian.
>
> Signed-off-by: Ahmed S. Darwish <[email protected]>
> ---
> drivers/dma-buf/dma-resv.c | 8 +-------
> drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 --
> include/linux/dma-resv.h | 2 +-
> 3 files changed, 2 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 590ce7ad60a0..3aba2b2bfc48 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -128,7 +128,7 @@ subsys_initcall(dma_resv_lockdep);
> void dma_resv_init(struct dma_resv *obj)
> {
> ww_mutex_init(&obj->lock, &reservation_ww_class);
> - seqcount_init(&obj->seq);
> + seqcount_ww_mutex_init(&obj->seq, &obj->lock);
>
> RCU_INIT_POINTER(obj->fence, NULL);
> RCU_INIT_POINTER(obj->fence_excl, NULL);
> @@ -259,7 +259,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
> fobj = dma_resv_get_list(obj);
> count = fobj->shared_count;
>
> - preempt_disable();
> write_seqcount_begin(&obj->seq);
>
> for (i = 0; i < count; ++i) {
> @@ -281,7 +280,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
> smp_store_mb(fobj->shared_count, count);
>
> write_seqcount_end(&obj->seq);
> - preempt_enable();
> dma_fence_put(old);
> }
> EXPORT_SYMBOL(dma_resv_add_shared_fence);
> @@ -308,14 +306,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
> if (fence)
> dma_fence_get(fence);
>
> - preempt_disable();
> write_seqcount_begin(&obj->seq);
> /* write_seqcount_begin provides the necessary memory barrier */
> RCU_INIT_POINTER(obj->fence_excl, fence);
> if (old)
> old->shared_count = 0;
> write_seqcount_end(&obj->seq);
> - preempt_enable();
>
> /* inplace update, no shared fences */
> while (i--)
> @@ -393,13 +389,11 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
> src_list = dma_resv_get_list(dst);
> old = dma_resv_get_excl(dst);
>
> - preempt_disable();
> write_seqcount_begin(&dst->seq);
> /* write_seqcount_begin provides the necessary memory barrier */
> RCU_INIT_POINTER(dst->fence_excl, new);
> RCU_INIT_POINTER(dst->fence, dst_list);
> write_seqcount_end(&dst->seq);
> - preempt_enable();
>
> dma_resv_list_free(src_list);
> dma_fence_put(old);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> index 9dff792c9290..87fd32aae8f9 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> @@ -258,11 +258,9 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
> new->shared_count = k;
>
> /* Install the new fence list, seqcount provides the barriers */
> - preempt_disable();
> write_seqcount_begin(&resv->seq);
> RCU_INIT_POINTER(resv->fence, new);
> write_seqcount_end(&resv->seq);
> - preempt_enable();
>
> /* Drop the references to the removed fences or move them to ef_list */
> for (i = j, k = 0; i < old->shared_count; ++i) {
> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
> index a6538ae7d93f..d44a77e8a7e3 100644
> --- a/include/linux/dma-resv.h
> +++ b/include/linux/dma-resv.h
> @@ -69,7 +69,7 @@ struct dma_resv_list {
> */
> struct dma_resv {
> struct ww_mutex lock;
> - seqcount_t seq;
> + seqcount_ww_mutex_t seq;
>
> struct dma_fence __rcu *fence_excl;
> struct dma_resv_list __rcu *fence;
On Wed, May 20, 2020, Christian K?nig wrote:
> Am 19.05.20 um 23:45 schrieb Ahmed S. Darwish:
> > A sequence counter write side critical section must be protected by some
> > form of locking to serialize writers. If the serialization primitive is
> > not disabling preemption implicitly, preemption has to be explicitly
> > disabled before entering the sequence counter write side critical
> > section.
> >
> > The dma-buf reservation subsystem uses plain sequence counters to manage
> > updates to reservations. Writer serialization is accomplished through a
> > wound/wait mutex.
> >
> > Acquiring a wound/wait mutex does not disable preemption, so this needs
> > to be done manually before and after the write side critical section.
> >
> > Use the newly-added seqcount_ww_mutex_t instead:
> >
> > - It associates the ww_mutex with the sequence count, which enables
> > lockdep to validate that the write side critical section is properly
> > serialized.
> >
> > - It removes the need to explicitly add preempt_disable/enable()
> > around the write side critical section because the write_begin/end()
> > functions for this new data type automatically do this.
> >
> > If lockdep is disabled this ww_mutex lock association is compiled out
> > and has neither storage size nor runtime overhead.
>
> Mhm, is the dma_resv object the only user of this new seqcount_ww_mutex
> variant ?
>
> If yes we are trying to get rid of this sequence counter for quite some
> time, so I would rather invest the additional time to finish this.
>
In this patch series, each extra "seqcount with associated lock" data
type costs us, exactly:
- 1 typedef definition, seqcount_ww_mutex_t
- 1 static initializer, SEQCNT_WW_MUTEX_ZERO()
- 1 runtime initializer, seqcount_ww_mutex_init()
Definitions for the typedef and the 2 initializers above are
template-code one liners.
The logic which automatically disables preemption upon entering a
seqcount_ww_mutex_t write side critical section is also already shared
with seqcount_mutex_t and any future, preemptible, associated lock.
So, yes, dma-resv is the only user of seqcount_ww_mutex.
But even in that case, given the one liner template code nature of
seqcount_ww_mutex_t logic, it does not make sense to block the dma_resv
and amdgpu change until at some point in the future the sequence counter
is completely removed.
**If and when** the sequence counter gets removed, please just remove
the seqcount_ww_mutex_t data type with it. It will be extremely simple.
> Regards,
> Christian.
>
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
Am 21.05.20 um 02:09 schrieb Ahmed S. Darwish:
> On Wed, May 20, 2020, Christian König wrote:
>> Am 19.05.20 um 23:45 schrieb Ahmed S. Darwish:
>>> A sequence counter write side critical section must be protected by some
>>> form of locking to serialize writers. If the serialization primitive is
>>> not disabling preemption implicitly, preemption has to be explicitly
>>> disabled before entering the sequence counter write side critical
>>> section.
>>>
>>> The dma-buf reservation subsystem uses plain sequence counters to manage
>>> updates to reservations. Writer serialization is accomplished through a
>>> wound/wait mutex.
>>>
>>> Acquiring a wound/wait mutex does not disable preemption, so this needs
>>> to be done manually before and after the write side critical section.
>>>
>>> Use the newly-added seqcount_ww_mutex_t instead:
>>>
>>> - It associates the ww_mutex with the sequence count, which enables
>>> lockdep to validate that the write side critical section is properly
>>> serialized.
>>>
>>> - It removes the need to explicitly add preempt_disable/enable()
>>> around the write side critical section because the write_begin/end()
>>> functions for this new data type automatically do this.
>>>
>>> If lockdep is disabled this ww_mutex lock association is compiled out
>>> and has neither storage size nor runtime overhead.
>> Mhm, is the dma_resv object the only user of this new seqcount_ww_mutex
>> variant ?
>>
>> If yes we are trying to get rid of this sequence counter for quite some
>> time, so I would rather invest the additional time to finish this.
>>
> In this patch series, each extra "seqcount with associated lock" data
> type costs us, exactly:
>
> - 1 typedef definition, seqcount_ww_mutex_t
> - 1 static initializer, SEQCNT_WW_MUTEX_ZERO()
> - 1 runtime initializer, seqcount_ww_mutex_init()
>
> Definitions for the typedef and the 2 initializers above are
> template-code one liners.
In this case I'm perfectly fine with this.
>
> The logic which automatically disables preemption upon entering a
> seqcount_ww_mutex_t write side critical section is also already shared
> with seqcount_mutex_t and any future, preemptible, associated lock.
>
> So, yes, dma-resv is the only user of seqcount_ww_mutex.
>
> But even in that case, given the one liner template code nature of
> seqcount_ww_mutex_t logic, it does not make sense to block the dma_resv
> and amdgpu change until at some point in the future the sequence counter
> is completely removed.
>
> **If and when** the sequence counter gets removed, please just remove
> the seqcount_ww_mutex_t data type with it. It will be extremely simple.
Completely agree, just wanted to prevent that we now add a lot of code
which gets removed again ~3 month from now.
Regards,
Christian.
>
>> Regards,
>> Christian.
>>
> Thanks,
>
> --
> Ahmed S. Darwish
> Linutronix GmbH
On Tue, May 19, 2020 at 11:45:31PM +0200, Ahmed S. Darwish wrote:
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index d35be7709403..2a4af746b1da 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -1,36 +1,15 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #ifndef __LINUX_SEQLOCK_H
> #define __LINUX_SEQLOCK_H
> +
> /*
> - * Reader/writer consistent mechanism without starving writers. This type of
> - * lock for data where the reader wants a consistent set of information
> - * and is willing to retry if the information changes. There are two types
> - * of readers:
> - * 1. Sequence readers which never block a writer but they may have to retry
> - * if a writer is in progress by detecting change in sequence number.
> - * Writers do not wait for a sequence reader.
> - * 2. Locking readers which will wait if a writer or another locking reader
> - * is in progress. A locking reader in progress will also block a writer
> - * from going forward. Unlike the regular rwlock, the read lock here is
> - * exclusive so that only one locking reader can get it.
> + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
> + * lockless readers (read-only retry loops), and no writer starvation.
> *
> - * This is not as cache friendly as brlock. Also, this may not work well
> - * for data that contains pointers, because any writer could
> - * invalidate a pointer that a reader was following.
> + * See Documentation/locking/seqlock.rst for full description.
So I really really hate that... I _much_ prefer code comments to crappy
documents.
On Fri, 22 May 2020 20:01:45 +0200
Peter Zijlstra <[email protected]> wrote:
> On Tue, May 19, 2020 at 11:45:31PM +0200, Ahmed S. Darwish wrote:
> > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > index d35be7709403..2a4af746b1da 100644
> > --- a/include/linux/seqlock.h
> > +++ b/include/linux/seqlock.h
> > @@ -1,36 +1,15 @@
> > /* SPDX-License-Identifier: GPL-2.0 */
> > #ifndef __LINUX_SEQLOCK_H
> > #define __LINUX_SEQLOCK_H
> > +
> > /*
> > - * Reader/writer consistent mechanism without starving writers. This type of
> > - * lock for data where the reader wants a consistent set of information
> > - * and is willing to retry if the information changes. There are two types
> > - * of readers:
> > - * 1. Sequence readers which never block a writer but they may have to retry
> > - * if a writer is in progress by detecting change in sequence number.
> > - * Writers do not wait for a sequence reader.
> > - * 2. Locking readers which will wait if a writer or another locking reader
> > - * is in progress. A locking reader in progress will also block a writer
> > - * from going forward. Unlike the regular rwlock, the read lock here is
> > - * exclusive so that only one locking reader can get it.
> > + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
> > + * lockless readers (read-only retry loops), and no writer starvation.
> > *
> > - * This is not as cache friendly as brlock. Also, this may not work well
> > - * for data that contains pointers, because any writer could
> > - * invalidate a pointer that a reader was following.
> > + * See Documentation/locking/seqlock.rst for full description.
>
> So I really really hate that... I _much_ prefer code comments to crappy
> documents.
Agreed. Comments are much less likely to bitrot than documents. The
farther away the documentation is from the code, the quicker it becomes
stale.
It's fine to add "See Documentation/..." but please don't *ever* remove
comments that's next to the actual code.
-- Steve
Steven Rostedt <[email protected]> wrote:
> Peter Zijlstra <[email protected]> wrote:
> > On Tue, May 19, 2020 at 11:45:31PM +0200, Ahmed S. Darwish wrote:
> > > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > > index d35be7709403..2a4af746b1da 100644
> > > --- a/include/linux/seqlock.h
> > > +++ b/include/linux/seqlock.h
> > > @@ -1,36 +1,15 @@
> > > /* SPDX-License-Identifier: GPL-2.0 */
> > > #ifndef __LINUX_SEQLOCK_H
> > > #define __LINUX_SEQLOCK_H
> > > +
> > > /*
> > > - * Reader/writer consistent mechanism without starving writers. This type of
> > > - * lock for data where the reader wants a consistent set of information
> > > - * and is willing to retry if the information changes. There are two types
> > > - * of readers:
> > > - * 1. Sequence readers which never block a writer but they may have to retry
> > > - * if a writer is in progress by detecting change in sequence number.
> > > - * Writers do not wait for a sequence reader.
> > > - * 2. Locking readers which will wait if a writer or another locking reader
> > > - * is in progress. A locking reader in progress will also block a writer
> > > - * from going forward. Unlike the regular rwlock, the read lock here is
> > > - * exclusive so that only one locking reader can get it.
> > > + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
> > > + * lockless readers (read-only retry loops), and no writer starvation.
> > > *
> > > - * This is not as cache friendly as brlock. Also, this may not work well
> > > - * for data that contains pointers, because any writer could
> > > - * invalidate a pointer that a reader was following.
> > > + * See Documentation/locking/seqlock.rst for full description.
> >
> > So I really really hate that... I _much_ prefer code comments to crappy
> > documents.
>
> Agreed. Comments are much less likely to bitrot than documents. The
> farther away the documentation is from the code, the quicker it becomes
> stale.
>
> It's fine to add "See Documentation/..." but please don't *ever* remove
> comments that's next to the actual code.
>
This patch was unfairly cut at the hunk above :)
If you follow the rest of it, you see that the documentation has just
moved 3 lines below:
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * The raw counting mechanism without any writer protection. Write side
+ * critical sections must be serialized and readers on the same CPU
+ * (e.g. through preemption or interrupts) must be excluded.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
+
typedef struct seqcount {
and:
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
This was done because, as said in the commit log, documentation of
seqcount_t and seqlock_t was originally intermingled. This is incorrect
and confusing since the usage constrains for each type are vastly
different.
Then, the brlock comment:
This is not as cache friendly as brlock. Also, this may not work
well for data that contains pointers, because any writer could
invalidate a pointer that a reader was following.
was removed not because it's moved to Documentation/locking/seqlock.rst,
but because it's obsolete: 0f6ed63b1707 ("no need to keep brlock macros
anymore...").
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
Ahmed S. Darwish <[email protected]> wrote:
> > Steven Rostedt <[email protected]> wrote:
> > > Peter Zijlstra <[email protected]> wrote:
...
> > >
> > > So I really really hate that... I _much_ prefer code comments to crappy
> > > documents.
> >
> > Agreed. Comments are much less likely to bitrot than documents. The
> > farther away the documentation is from the code, the quicker it becomes
> > stale.
> >
> > It's fine to add "See Documentation/..." but please don't *ever* remove
> > comments that's next to the actual code.
...
>
> Then, the brlock comment:
>
> This is not as cache friendly as brlock. Also, this may not work
> well for data that contains pointers, because any writer could
> invalidate a pointer that a reader was following.
>
> was removed not because it's moved to Documentation/locking/seqlock.rst,
> but because it's obsolete: 0f6ed63b1707 ("no need to keep brlock macros
> anymore...").
>
Hmm, the part about not including pointers is only mentiond in the RST
file though, and not at seqlock.h.
Anyway, ACK, I'll beef up the comments at seqlock.h and make sure they
are self-contained.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
Hi,
This is v2 of the seqlock patch series:
[PATCH v1 00/25] seqlock: Extend seqcount API with associated locks
https://lore.kernel.org/lkml/[email protected]
Patches 1=>3 of this v2 series add documentation for the existing
seqlock.h datatypes and APIs. Hopefully they can hit v5.8 -rc2 or -rc3.
Changelog-v2
============
1. Drop, for now, the seqlock v1 patches #7 and #8. These patches added
lockdep non-preemptibility checks to seqcount_t write paths, but they
now depend on on-going work by Peter:
[PATCH v3 0/5] lockdep: Change IRQ state tracking to use per-cpu variables
https://lkml.kernel.org/r/[email protected]
[PATCH 00/14] x86/entry: disallow #DB more and x86/entry lockdep/nmi
https://lkml.kernel.org/r/[email protected]
Once Peter's work get merged, I'll send the non-preemptibility checks as
a separate series.
2. Drop the v1 seqcount_t call-sites bugfixes. I've already posted them
in an isolated series. They got merged into their respective trees, and
will hit v5.8-rc1 soon:
[PATCH v2 0/6] seqlock: seqcount_t call sites bugfixes
https://lore.kernel.org/lkml/[email protected]
3. Patch #1: Add a small paragraph explaining that seqcount_t/seqlock_t
cannot be used if the protected data contains pointers. A similar
paragraph already existed in seqlock.h, but got mistakenly dropped.
4. Patch #2: Don't add RST directives inside kernel-doc comments. Peter
doesn't like them :) I've kept the indentation though, and found a
minimal way for Sphinx to properly render these code samples without too
much disruption.
5. Patch #3: Brush up the introduced kernel-doc comments. Make them more
consistent overall, and more concise.
Thanks,
8<--------------
Ahmed S. Darwish (18):
Documentation: locking: Describe seqlock design and usage
seqlock: Properly format kernel-doc code samples
seqlock: Add missing kernel-doc annotations
seqlock: Extend seqcount API with associated locks
dma-buf: Remove custom seqcount lockdep class key
dma-buf: Use sequence counter with associated wound/wait mutex
sched: tasks: Use sequence counter with associated spinlock
netfilter: conntrack: Use sequence counter with associated spinlock
netfilter: nft_set_rbtree: Use sequence counter with associated rwlock
xfrm: policy: Use sequence counters with associated lock
timekeeping: Use sequence counter with associated raw spinlock
vfs: Use sequence counter with associated spinlock
raid5: Use sequence counter with associated spinlock
iocost: Use sequence counter with associated spinlock
NFSv4: Use sequence counter with associated spinlock
userfaultfd: Use sequence counter with associated spinlock
kvm/eventfd: Use sequence counter with associated spinlock
hrtimer: Use sequence counter with associated raw spinlock
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 242 +++++
MAINTAINERS | 2 +-
block/blk-iocost.c | 5 +-
drivers/dma-buf/dma-resv.c | 15 +-
.../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 -
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 +-
fs/nfs/nfs4_fs.h | 2 +-
fs/nfs/nfs4state.c | 2 +-
fs/userfaultfd.c | 4 +-
include/linux/dcache.h | 2 +-
include/linux/dma-resv.h | 4 +-
include/linux/fs_struct.h | 2 +-
include/linux/hrtimer.h | 2 +-
include/linux/kvm_irqfd.h | 2 +-
include/linux/sched.h | 2 +-
include/linux/seqlock.h | 855 ++++++++++++++----
include/linux/seqlock_types_internal.h | 187 ++++
include/net/netfilter/nf_conntrack.h | 2 +-
init/init_task.c | 3 +-
kernel/fork.c | 2 +-
kernel/time/hrtimer.c | 13 +-
kernel/time/timekeeping.c | 19 +-
net/netfilter/nf_conntrack_core.c | 5 +-
net/netfilter/nft_set_rbtree.c | 4 +-
net/xfrm/xfrm_policy.c | 10 +-
virt/kvm/eventfd.c | 2 +-
30 files changed, 1175 insertions(+), 226 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
create mode 100644 include/linux/seqlock_types_internal.h
base-commit: 3d77e6a8804abcc0504c904bd6e5cdf3a5cf8162
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/net/netfilter/nf_conntrack.h | 2 +-
net/netfilter/nf_conntrack_core.c | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 90690e37a56f..ea4e2010b246 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -286,7 +286,7 @@ int nf_conntrack_hash_resize(unsigned int hashsize);
extern struct hlist_nulls_head *nf_conntrack_hash;
extern unsigned int nf_conntrack_htable_size;
-extern seqcount_t nf_conntrack_generation;
+extern seqcount_spinlock_t nf_conntrack_generation;
extern unsigned int nf_conntrack_max;
/* must be called with rcu read lock held */
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index bb72ca5f3999..1f9518569195 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -180,7 +180,7 @@ EXPORT_SYMBOL_GPL(nf_conntrack_htable_size);
unsigned int nf_conntrack_max __read_mostly;
EXPORT_SYMBOL_GPL(nf_conntrack_max);
-seqcount_t nf_conntrack_generation __read_mostly;
+seqcount_spinlock_t nf_conntrack_generation __read_mostly;
static unsigned int nf_conntrack_hash_rnd __read_mostly;
static u32 hash_conntrack_raw(const struct nf_conntrack_tuple *tuple,
@@ -2589,7 +2589,8 @@ int nf_conntrack_init_start(void)
/* struct nf_ct_ext uses u8 to store offsets/size */
BUILD_BUG_ON(total_extension_size() > 255u);
- seqcount_init(&nf_conntrack_generation);
+ seqcount_spinlock_init(&nf_conntrack_generation,
+ &nf_conntrack_locks_all_lock);
for (i = 0; i < CONNTRACK_LOCKS; i++)
spin_lock_init(&nf_conntrack_locks[i]);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 ++--
include/linux/dcache.h | 2 +-
include/linux/fs_struct.h | 2 +-
4 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index b280e07e162b..e5f365d8fd67 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1727,7 +1727,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
dentry->d_lockref.count = 1;
dentry->d_flags = 0;
spin_lock_init(&dentry->d_lock);
- seqcount_init(&dentry->d_seq);
+ seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
dentry->d_inode = NULL;
dentry->d_parent = dentry;
dentry->d_sb = sb;
diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index ca639ed967b7..04b3f5b9c629 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -117,7 +117,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
- seqcount_init(&fs->seq);
+ seqcount_spinlock_init(&fs->seq, &fs->lock);
fs->umask = old->umask;
spin_lock(&old->lock);
@@ -163,6 +163,6 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
- .seq = SEQCNT_ZERO(init_fs.seq),
+ .seq = SEQCNT_SPINLOCK_ZERO(init_fs.seq, &init_fs.lock),
.umask = 0022,
};
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index c1488cc84fd9..235563da356d 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -89,7 +89,7 @@ extern struct dentry_stat_t dentry_stat;
struct dentry {
/* RCU lookup touched fields */
unsigned int d_flags; /* protected by d_lock */
- seqcount_t d_seq; /* per dentry seqlock */
+ seqcount_spinlock_t d_seq; /* per dentry seqlock */
struct hlist_bl_node d_hash; /* lookup hash list */
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index cf1015abfbf2..783b48dedb72 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -9,7 +9,7 @@
struct fs_struct {
int users;
spinlock_t lock;
- seqcount_t seq;
+ seqcount_spinlock_t seq;
int umask;
int in_exec;
struct path root, pwd;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
block/blk-iocost.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 7c1fe605d0d6..8029a9e8fa55 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -405,7 +405,7 @@ struct ioc {
enum ioc_running running;
atomic64_t vtime_rate;
- seqcount_t period_seqcount;
+ seqcount_spinlock_t period_seqcount;
u32 period_at; /* wallclock starttime */
u64 period_at_vtime; /* vtime starttime */
@@ -872,7 +872,6 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
{
- lockdep_assert_held(&ioc->lock);
WARN_ON_ONCE(ioc->running != IOC_RUNNING);
write_seqcount_begin(&ioc->period_seqcount);
@@ -1958,7 +1957,7 @@ static int blk_iocost_init(struct request_queue *q)
ioc->running = IOC_IDLE;
atomic64_set(&ioc->vtime_rate, VTIME_PER_USEC);
- seqcount_init(&ioc->period_seqcount);
+ seqcount_spinlock_init(&ioc->period_seqcount, &ioc->lock);
ioc->period_at = ktime_to_us(ktime_get());
atomic64_set(&ioc->cur_period, 0);
atomic_set(&ioc->hweight_gen, 0);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/kvm_irqfd.h | 2 +-
virt/kvm/eventfd.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dc1da020305b..dac047abdba7 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
wait_queue_entry_t wait;
/* Update side is protected by irqfds.lock */
struct kvm_kernel_irq_routing_entry irq_entry;
- seqcount_t irq_entry_sc;
+ seqcount_spinlock_t irq_entry_sc;
/* Used for level IRQ fast-path */
int gsi;
struct work_struct inject;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 67b6fc153e9c..8694a2920ea9 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
INIT_LIST_HEAD(&irqfd->list);
INIT_WORK(&irqfd->inject, irqfd_inject);
INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
- seqcount_init(&irqfd->irq_entry_sc);
+ seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
f = fdget(args->fd);
if (!f.file) {
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/hrtimer.h | 2 +-
kernel/time/hrtimer.c | 13 ++++++++++---
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 15c8ac313678..25993b86ac5c 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -159,7 +159,7 @@ struct hrtimer_clock_base {
struct hrtimer_cpu_base *cpu_base;
unsigned int index;
clockid_t clockid;
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct hrtimer *running;
struct timerqueue_head active;
ktime_t (*get_time)(void);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index d89da1c7e005..c4038511d5c9 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -135,7 +135,11 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
* timer->base->cpu_base
*/
static struct hrtimer_cpu_base migration_cpu_base = {
- .clock_base = { { .cpu_base = &migration_cpu_base, }, },
+ .clock_base = { {
+ .cpu_base = &migration_cpu_base,
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(migration_cpu_base.seq,
+ &migration_cpu_base.lock),
+ }, },
};
#define migration_base migration_cpu_base.clock_base[0]
@@ -1998,8 +2002,11 @@ int hrtimers_prepare_cpu(unsigned int cpu)
int i;
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
- cpu_base->clock_base[i].cpu_base = cpu_base;
- timerqueue_init_head(&cpu_base->clock_base[i].active);
+ struct hrtimer_clock_base *clock_b = &cpu_base->clock_base[i];
+
+ clock_b->cpu_base = cpu_base;
+ seqcount_raw_spinlock_init(&clock_b->seq, &cpu_base->lock);
+ timerqueue_init_head(&clock_b->active);
}
cpu_base->cpu = cpu;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index ba00e9877f02..69f31c675b58 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6929,7 +6929,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
} else
goto abort;
spin_lock_init(&conf->device_lock);
- seqcount_init(&conf->gen_lock);
+ seqcount_spinlock_init(&conf->gen_lock, &conf->device_lock);
mutex_init(&conf->cache_size_mutex);
init_waitqueue_head(&conf->wait_for_quiescent);
init_waitqueue_head(&conf->wait_for_stripe);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index f90e0704bed9..a2c9e9e9f5ac 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -589,7 +589,7 @@ struct r5conf {
int prev_chunk_sectors;
int prev_algo;
short generation; /* increments with every reshape */
- seqcount_t gen_lock; /* lock against generation changes */
+ seqcount_spinlock_t gen_lock; /* lock against generation changes */
unsigned long reshape_checkpoint; /* Time we last updated
* metadata */
long long min_offset_diff; /* minimum difference between
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
kernel/time/timekeeping.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 9ebaab13339d..24e91a1e2acd 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -39,18 +39,19 @@ enum timekeeping_adv_mode {
TK_ADV_FREQ
};
+static DEFINE_RAW_SPINLOCK(timekeeper_lock);
+
/*
* The most important data for readout fits into a single 64 byte
* cache line.
*/
static struct {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct timekeeper timekeeper;
} tk_core ____cacheline_aligned = {
- .seq = SEQCNT_ZERO(tk_core.seq),
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_core.seq, &timekeeper_lock),
};
-static DEFINE_RAW_SPINLOCK(timekeeper_lock);
static struct timekeeper shadow_timekeeper;
/**
@@ -63,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct tk_read_base base[2];
};
@@ -80,11 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -157,7 +160,7 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
* tk_clock_read - atomic clocksource read() helper
*
* This helper is necessary to use in the read paths because, while the
- * seqlock ensures we don't return a bad value while structures are updated,
+ * seqcount ensures we don't return a bad value while structures are updated,
* it doesn't protect from potential crashes. There is the possibility that
* the tkr's clocksource may change between the read reference, and the
* clock reference passed to the read function. This can cause crashes if
@@ -222,10 +225,10 @@ static inline u64 timekeeping_get_delta(const struct tk_read_base *tkr)
unsigned int seq;
/*
- * Since we're called holding a seqlock, the data may shift
+ * Since we're called holding a seqcount, the data may shift
* under us while we're doing the calculation. This can cause
* false positives, since we'd note a problem but throw the
- * results away. So nest another seqlock here to atomically
+ * results away. So nest another seqcount here to atomically
* grab the points we are checking with.
*/
do {
@@ -486,7 +489,7 @@ EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
*
* To keep it NMI safe since we're accessing from tracing, we're not using a
* separate timekeeper with updates to monotonic clock and boot offset
- * protected with seqlocks. This has the following minor side effects:
+ * protected with seqcounts. This has the following minor side effects:
*
* (1) Its possible that a timestamp be taken after the boot offset is updated
* but before the timekeeper is updated. If this happens, the new boot offset
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
A plain seqcount_t does not contain the information of which lock must
be held when entering a write side critical section.
Use the new seqcount_spinlock_t and seqcount_mutex_t data types instead,
which allow to associate a lock with the sequence counter. This enables
lockdep to verify that the lock used for writer serialization is held
when the write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/xfrm/xfrm_policy.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 564aa6492e7c..732a940468b0 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -122,7 +122,7 @@ struct xfrm_pol_inexact_bin {
/* list containing '*:*' policies */
struct hlist_head hhead;
- seqcount_t count;
+ seqcount_spinlock_t count;
/* tree sorted by daddr/prefix */
struct rb_root root_d;
@@ -155,7 +155,7 @@ static struct xfrm_policy_afinfo const __rcu *xfrm_policy_afinfo[AF_INET6 + 1]
__read_mostly;
static struct kmem_cache *xfrm_dst_cache __ro_after_init;
-static __read_mostly seqcount_t xfrm_policy_hash_generation;
+static __read_mostly seqcount_mutex_t xfrm_policy_hash_generation;
static struct rhashtable xfrm_policy_inexact_table;
static const struct rhashtable_params xfrm_pol_inexact_params;
@@ -719,7 +719,7 @@ xfrm_policy_inexact_alloc_bin(const struct xfrm_policy *pol, u8 dir)
INIT_HLIST_HEAD(&bin->hhead);
bin->root_d = RB_ROOT;
bin->root_s = RB_ROOT;
- seqcount_init(&bin->count);
+ seqcount_spinlock_init(&bin->count, &net->xfrm.xfrm_policy_lock);
prev = rhashtable_lookup_get_insert_key(&xfrm_policy_inexact_table,
&bin->k, &bin->head,
@@ -1906,7 +1906,7 @@ static int xfrm_policy_match(const struct xfrm_policy *pol,
static struct xfrm_pol_inexact_node *
xfrm_policy_lookup_inexact_addr(const struct rb_root *r,
- seqcount_t *count,
+ seqcount_spinlock_t *count,
const xfrm_address_t *addr, u16 family)
{
const struct rb_node *parent;
@@ -4153,7 +4153,7 @@ void __init xfrm_init(void)
{
register_pernet_subsys(&xfrm_net_ops);
xfrm_dev_init();
- seqcount_init(&xfrm_policy_hash_generation);
+ seqcount_mutex_init(&xfrm_policy_hash_generation, &hash_resize_mutex);
xfrm_input_init();
#ifdef CONFIG_INET_ESPINTCP
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/userfaultfd.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e39fdec8a0b0..dd3aab31c50f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -61,7 +61,7 @@ struct userfaultfd_ctx {
/* waitqueue head for events */
wait_queue_head_t event_wqh;
/* a refile sequence protected by fault_pending_wqh lock */
- struct seqcount refile_seq;
+ seqcount_spinlock_t refile_seq;
/* pseudo fd refcounting */
refcount_t refcount;
/* userfaultfd syscall flags */
@@ -1998,7 +1998,7 @@ static void init_once_userfaultfd_ctx(void *mem)
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->event_wqh);
init_waitqueue_head(&ctx->fd_wqh);
- seqcount_init(&ctx->refile_seq);
+ seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
}
SYSCALL_DEFINE1(userfaultfd, int, flags)
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_rwlock_t data type, which allows to associate a
rwlock with the sequence counter. This enables lockdep to verify that
the rwlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/netfilter/nft_set_rbtree.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/netfilter/nft_set_rbtree.c b/net/netfilter/nft_set_rbtree.c
index 62f416bc0579..9f58261ee4c7 100644
--- a/net/netfilter/nft_set_rbtree.c
+++ b/net/netfilter/nft_set_rbtree.c
@@ -18,7 +18,7 @@
struct nft_rbtree {
struct rb_root root;
rwlock_t lock;
- seqcount_t count;
+ seqcount_rwlock_t count;
struct delayed_work gc_work;
};
@@ -516,7 +516,7 @@ static int nft_rbtree_init(const struct nft_set *set,
struct nft_rbtree *priv = nft_set_priv(set);
rwlock_init(&priv->lock);
- seqcount_init(&priv->count);
+ seqcount_rwlock_init(&priv->count, &priv->lock);
priv->root = RB_ROOT;
INIT_DEFERRABLE_WORK(&priv->gc_work, nft_rbtree_gc);
--
2.20.1
On 08/06/20 02:57, Ahmed S. Darwish wrote:
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. A plain seqcount_t does not
> contain the information of which lock must be held when entering a write
> side critical section.
>
> Use the new seqcount_spinlock_t data type, which allows to associate a
> spinlock with the sequence counter. This enables lockdep to verify that
> the spinlock used for writer serialization is held when the write side
> critical section is entered.
>
> If lockdep is disabled this lock association is compiled out and has
> neither storage size nor runtime overhead.
>
> Signed-off-by: Ahmed S. Darwish <[email protected]>
> ---
> include/linux/kvm_irqfd.h | 2 +-
> virt/kvm/eventfd.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
> index dc1da020305b..dac047abdba7 100644
> --- a/include/linux/kvm_irqfd.h
> +++ b/include/linux/kvm_irqfd.h
> @@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
> wait_queue_entry_t wait;
> /* Update side is protected by irqfds.lock */
> struct kvm_kernel_irq_routing_entry irq_entry;
> - seqcount_t irq_entry_sc;
> + seqcount_spinlock_t irq_entry_sc;
> /* Used for level IRQ fast-path */
> int gsi;
> struct work_struct inject;
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 67b6fc153e9c..8694a2920ea9 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
> INIT_LIST_HEAD(&irqfd->list);
> INIT_WORK(&irqfd->inject, irqfd_inject);
> INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
> - seqcount_init(&irqfd->irq_entry_sc);
> + seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
>
> f = fdget(args->fd);
> if (!f.file) {
>
Acked-by: Paolo Bonzini <[email protected]>
Hi,
This is v3 of the seqlock patch series:
[PATCH v1 00/25] seqlock: Extend seqcount API with associated locks
https://lore.kernel.org/lkml/[email protected]
[PATCH v2 00/18]
https://lore.kernel.org/lkml/[email protected]
It's based over:
git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/core
to get Peter's lockdep irqstate tracking series below, which untangles
mainline seqlock.h<=>sched.h 'current->' task_struct circular dependency:
https://lkml.kernel.org/r/linuxppc-dev/[email protected]
Changelog-v3:
- Re-add lockdep non-preemptibility checks on seqcount_t write paths.
They were removed from v2 due to the circular dependencies mentioned.
- Slight rebase over the new v5.8-rc1 KCSAN seqlock.h changes
- Collect seqcount_t call-sites acked-by tags
Thanks,
8<--------------
Ahmed S. Darwish (20):
Documentation: locking: Describe seqlock design and usage
seqlock: Properly format kernel-doc code samples
seqlock: Add missing kernel-doc annotations
lockdep: Add preemption enabled/disabled assertion APIs
seqlock: lockdep assert non-preemptibility on seqcount_t write
seqlock: Extend seqcount API with associated locks
dma-buf: Remove custom seqcount lockdep class key
dma-buf: Use sequence counter with associated wound/wait mutex
sched: tasks: Use sequence counter with associated spinlock
netfilter: conntrack: Use sequence counter with associated spinlock
netfilter: nft_set_rbtree: Use sequence counter with associated rwlock
xfrm: policy: Use sequence counters with associated lock
timekeeping: Use sequence counter with associated raw spinlock
vfs: Use sequence counter with associated spinlock
raid5: Use sequence counter with associated spinlock
iocost: Use sequence counter with associated spinlock
NFSv4: Use sequence counter with associated spinlock
userfaultfd: Use sequence counter with associated spinlock
kvm/eventfd: Use sequence counter with associated spinlock
hrtimer: Use sequence counter with associated raw spinlock
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 242 +++++
MAINTAINERS | 2 +-
block/blk-iocost.c | 5 +-
drivers/dma-buf/dma-resv.c | 15 +-
.../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 -
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 +-
fs/nfs/nfs4_fs.h | 2 +-
fs/nfs/nfs4state.c | 2 +-
fs/userfaultfd.c | 4 +-
include/linux/dcache.h | 2 +-
include/linux/dma-resv.h | 4 +-
include/linux/fs_struct.h | 2 +-
include/linux/hrtimer.h | 2 +-
include/linux/kvm_irqfd.h | 2 +-
include/linux/lockdep.h | 18 +
include/linux/sched.h | 2 +-
include/linux/seqlock.h | 872 ++++++++++++++----
include/linux/seqlock_types_internal.h | 186 ++++
include/net/netfilter/nf_conntrack.h | 2 +-
init/init_task.c | 3 +-
kernel/fork.c | 2 +-
kernel/time/hrtimer.c | 13 +-
kernel/time/timekeeping.c | 19 +-
lib/Kconfig.debug | 1 +
net/netfilter/nf_conntrack_core.c | 5 +-
net/netfilter/nft_set_rbtree.c | 4 +-
net/xfrm/xfrm_policy.c | 10 +-
virt/kvm/eventfd.c | 2 +-
32 files changed, 1211 insertions(+), 225 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
create mode 100644 include/linux/seqlock_types_internal.h
base-commit: 997e89fa345e9006f311cf9f9c8fd9f7d96c240f
--
2.20.1
Proper documentation for the design and usage of sequence counters and
sequential locks does not exist. Complete the seqlock.h documentation as
follows:
- Divide all documentation on a seqcount_t vs. seqlock_t basis. The
description for both mechanisms was intermingled, which is incorrect
since the usage constrains for each type are vastly different.
- Add an introductory paragraph describing the internal design of, and
rationale for, sequence counters.
- Document seqcount_t writer non-preemptibility requirement, which was
not previously documented anywhere, and provide a clear rationale.
- Provide template code for seqcount_t and seqlock_t initialization
and reader/writer critical sections.
- Recommend using seqlock_t by default. It implicitly handles the
serialization and non-preemptibility requirements of writers.
At seqlock.h:
- Remove references to brlocks as they've long been removed from the
kernel.
- Remove references to gcc-3.x since the kernel's minimum supported
gcc version is 4.6.
References: 0f6ed63b1707 ("no need to keep brlock macros anymore...")
References: cafa0010cd51 ("Raise the minimum required gcc version to 4.6")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 184 ++++++++++++++++++++++++++++++
include/linux/seqlock.h | 77 ++++++-------
3 files changed, 221 insertions(+), 41 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
index d785878cad65..7003bd5aeff4 100644
--- a/Documentation/locking/index.rst
+++ b/Documentation/locking/index.rst
@@ -14,6 +14,7 @@ locking
mutex-design
rt-mutex-design
rt-mutex
+ seqlock
spinlocks
ww-mutex-design
preempt-locking
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
new file mode 100644
index 000000000000..c9916efe038e
--- /dev/null
+++ b/Documentation/locking/seqlock.rst
@@ -0,0 +1,184 @@
+======================================
+Sequence counters and sequential locks
+======================================
+
+Introduction
+============
+
+Sequence counters are a reader-writer consistency mechanism with
+lockless readers (read-only retry loops), and no writer starvation. They
+are used for data that's rarely written to (e.g. system time), where the
+reader wants a consistent set of information and is willing to retry if
+that information changes.
+
+A data set is consistent when the sequence count at the beginning of the
+read side critical section is even and the same sequence count value is
+read again at the end of the critical section. The data in the set must
+be copied out inside the read side critical section. If the sequence
+count has changed between the start and the end of the critical section,
+the reader must retry.
+
+Writers increment the sequence count at the start and the end of their
+critical section. After starting the critical section the sequence count
+is odd and indicates to the readers that an update is in progress. At
+the end of the write side critical section the sequence count becomes
+even again which lets readers make progress.
+
+A sequence counter write side critical section must never be preempted
+or interrupted by read side sections. Otherwise the reader will spin for
+the entire scheduler tick due to the odd sequence count value and the
+interrupted writer. If that reader belongs to a real-time scheduling
+class, it can spin forever and the kernel will livelock.
+
+This mechanism cannot be used if the protected data contains pointers,
+as the writer can invalidate a pointer that the reader is following.
+
+.. _seqcount_t:
+
+Sequence counters (:c:type:`seqcount_t`)
+========================================
+
+This is the the raw counting mechanism, which does not protect against
+multiple writers. Write side critical sections must thus be serialized
+by an external lock.
+
+If the write serialization primitive is not implicitly disabling
+preemption, preemption must be explicitly disabled before entering the
+write side section. If the read section can be invoked from hardirq or
+softirq contexts, interrupts or bottom halves must also be respectively
+disabled before entering the write section.
+
+If it's desired to automatically handle the sequence counter
+requirements of writer serialization and non-preemptibility, use a
+:ref:`sequential lock <seqlock_t>` instead.
+
+Initialization:
+
+.. code-block:: c
+
+ /* dynamic */
+ seqcount_t foo_seqcount;
+ seqcount_init(&foo_seqcount);
+
+ /* static */
+ static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_ZERO(foo.seq),
+ } foo;
+
+Write path:
+
+.. code-block:: c
+
+ /* Serialized context with disabled preemption */
+
+ write_seqcount_begin(&foo_seqcount);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_seqcount_end(&foo_seqcount);
+
+Read path:
+
+.. code-block:: c
+
+ do {
+ seq = read_seqcount_begin(&foo_seqcount);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqcount_retry(&foo_seqcount, seq));
+
+.. _seqlock_t:
+
+Sequential locks (:c:type:`seqlock_t`)
+======================================
+
+This contains the :ref:`sequence counting mechanism <seqcount_t>`
+earlier discussed, plus an embedded spinlock for writer serialization
+and non-preemptibility.
+
+If the read side section can be invoked from hardirq or softirq context,
+use the write side function variants which disable interrupts or bottom
+halves respectively.
+
+Initialization:
+
+.. code-block:: c
+
+ /* dynamic */
+ seqlock_t foo_seqlock;
+ seqlock_init(&foo_seqlock);
+
+ /* static */
+ static DEFINE_SEQLOCK(foo_seqlock);
+
+ /* C99 struct init */
+ struct {
+ .seql = __SEQLOCK_UNLOCKED(foo.seql)
+ } foo;
+
+Write path:
+
+.. code-block:: c
+
+ write_seqlock(&foo_seqlock);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_sequnlock(&foo_seqlock);
+
+Read path, three categories:
+
+1. Normal Sequence readers which never block a writer but they must
+ retry if a writer is in progress by detecting change in the sequence
+ number. Writers do not wait for a sequence reader.
+
+ .. code-block:: c
+
+ do {
+ seq = read_seqbegin(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqretry(&foo_seqlock, seq));
+
+2. Locking readers which will wait if a writer or another locking reader
+ is in progress. A locking reader in progress will also block a writer
+ from entering its critical section. This read lock is
+ exclusive. Unlike rwlock_t, only one locking reader can acquire it.
+
+ .. code-block:: c
+
+ read_seqlock_excl(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ read_sequnlock_excl(&foo_seqlock);
+
+3. Conditional lockless reader (as in 1), or locking reader (as in 2),
+ according to a passed marker. This is used to avoid lockless readers
+ starvation (too much retry loops) in case of a sharp spike in write
+ activity. First, a lockless read is tried (even marker passed). If
+ that trial fails (odd sequence counter is returned, which is used as
+ the next iteration marker), the lockless read is transformed to a
+ full locking read and no retry loop is necessary.
+
+ .. code-block:: c
+
+ /* marker; even initialization */
+ int seq = 0;
+ do {
+ read_seqbegin_or_lock(&foo_seqlock, &seq);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (need_seqretry(&foo_seqlock, seq));
+ done_seqretry(&foo_seqlock, seq);
+
+API documentation
+=================
+
+.. kernel-doc:: include/linux/seqlock.h
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8b97204f35a7..e54ff48e87f8 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
*
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
+ * See Documentation/locking/seqlock.rst for full description.
*
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
- *
- *
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/
#include <linux/spinlock.h>
@@ -41,8 +20,8 @@
#include <asm/processor.h>
/*
- * The seqlock interface does not prescribe a precise sequence of read
- * begin/retry/end. For readers, typically there is a call to
+ * The seqlock seqcount_t interface does not prescribe a precise sequence of
+ * read begin/retry/end. For readers, typically there is a call to
* read_seqcount_begin() and read_seqcount_retry(), however, there are more
* esoteric cases which do not follow this pattern.
*
@@ -56,10 +35,24 @@
#define KCSAN_SEQLOCK_REGION_MAX 1000
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * This is the raw counting mechanism, without any writer protection.
+ *
+ * Write side critical sections must be serialized and non-preemptible.
+ *
+ * If readers can be invoked from hardirq or softirq contexts,
+ * interrupts or bottom halves must also be respectively disabled before
+ * entering the write section.
+ *
+ * This mechanism can't be used if the protected data contains pointers,
+ * as the writer can invalidate a pointer that a reader is following.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
typedef struct seqcount {
unsigned sequence;
@@ -398,10 +391,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-/*
- * Sequence counter only version assumes that callers are using their
- * own mutexing.
- */
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
@@ -434,15 +423,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * For more info, see:
+ * - Comments on top of seqcount_t
+ * - Documentation/locking/seqlock.rst
+ */
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
--
2.20.1
Align the code samples and note sections inside kernel-doc comments with
tabs. This way they can be properly parsed and rendered by Sphinx. It
also makes the code samples easier to read from text editors.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 82 +++++++++++++++++++++--------------------
1 file changed, 43 insertions(+), 39 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index e54ff48e87f8..d3bba59eb4df 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -256,7 +256,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
- * collapse the two back-to-back wmb()s.
+ * collapse the two back-to-back wmb()s::
*
* Note that writes surrounding the barrier should be declared atomic (e.g.
* via WRITE_ONCE): a) to ensure the writes become visible to other threads
@@ -325,64 +325,68 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* Very simply put: we first modify one copy and then the other. This ensures
* there is always one copy in a stable state, ready to give us an answer.
*
- * The basic form is a data structure like:
+ * The basic form is a data structure like::
*
- * struct latch_struct {
- * seqcount_t seq;
- * struct data_struct data[2];
- * };
+ * struct latch_struct {
+ * seqcount_t seq;
+ * struct data_struct data[2];
+ * };
*
* Where a modification, which is assumed to be externally serialized, does the
- * following:
+ * following::
*
- * void latch_modify(struct latch_struct *latch, ...)
- * {
- * smp_wmb(); <- Ensure that the last data[1] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * void latch_modify(struct latch_struct *latch, ...)
+ * {
+ * smp_wmb(); // Ensure that the last data[1] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[0], ...);
+ * modify(latch->data[0], ...);
*
- * smp_wmb(); <- Ensure that the data[0] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * smp_wmb(); // Ensure that the data[0] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[1], ...);
- * }
+ * modify(latch->data[1], ...);
+ * }
*
- * The query will have a form like:
+ * The query will have a form like::
*
- * struct entry *latch_query(struct latch_struct *latch, ...)
- * {
- * struct entry *entry;
- * unsigned seq, idx;
+ * struct entry *latch_query(struct latch_struct *latch, ...)
+ * {
+ * struct entry *entry;
+ * unsigned seq, idx;
*
- * do {
- * seq = raw_read_seqcount_latch(&latch->seq);
+ * do {
+ * seq = raw_read_seqcount_latch(&latch->seq);
*
- * idx = seq & 0x01;
- * entry = data_query(latch->data[idx], ...);
+ * idx = seq & 0x01;
+ * entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * smp_rmb();
+ * } while (seq != latch->seq);
*
- * return entry;
- * }
+ * return entry;
+ * }
*
* So during the modification, queries are first redirected to data[1]. Then we
* modify data[0]. When that is complete, we redirect queries back to data[0]
* and we can modify data[1].
*
- * NOTE: The non-requirement for atomic modifications does _NOT_ include
- * the publishing of new entries in the case where data is a dynamic
- * data structure.
+ * NOTE:
*
- * An iteration might start in data[0] and get suspended long enough
- * to miss an entire modification sequence, once it resumes it might
- * observe the new entry.
+ * The non-requirement for atomic modifications does _NOT_ include
+ * the publishing of new entries in the case where data is a dynamic
+ * data structure.
*
- * NOTE: When data is a dynamic data structure; one should use regular RCU
- * patterns to manage the lifetimes of the objects within.
+ * An iteration might start in data[0] and get suspended long enough
+ * to miss an entire modification sequence, once it resumes it might
+ * observe the new entry.
+ *
+ * NOTE:
+ *
+ * When data is a dynamic data structure; one should use regular RCU
+ * patterns to manage the lifetimes of the objects within.
*/
static inline void raw_write_seqcount_latch(seqcount_t *s)
{
--
2.20.1
A small number of the the exported seqlock.h functions are kernel-doc
annotated.
Since seqlock.h is now included by the kernel's RST documentation, add
kernel-doc annotations for all of the remaining functions.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 398 ++++++++++++++++++++++++++++++++++------
1 file changed, 339 insertions(+), 59 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d3bba59eb4df..057f7326a877 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -75,6 +75,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
+/**
+ * seqcount_init() - runtime initializer for seqcount_t
+ * @s: Pointer to the &typedef seqcount_t instance
+ */
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
@@ -98,13 +102,17 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
# define seqcount_lockdep_reader_access(x)
#endif
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
+/**
+ * SEQCNT_ZERO() - static initializer for seqcount_t
+ * @name: Name of the &typedef seqcount_t instance
+ */
+#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * __read_seqcount_begin() - begin a seqcount_t read section (without barrier)
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -129,13 +137,14 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount() - Read the seqcount_t raw counter value
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
* raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
+ * seqcount_t, without any lockdep checks and without checking or
+ * masking the sequence counter LSB. Calling code is responsible for
+ * handling that.
*/
static inline unsigned raw_read_seqcount(const seqcount_t *s)
{
@@ -146,13 +155,13 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount_begin() - start a seqcount_t read section w/o lockdep
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
* raw_read_seqcount_begin opens a read critical section of the given
- * seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
+ * seqcount_t, but without any lockdep checking. Validity of the read
+ * section must be checked with read_seqcount_retry().
*/
static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
@@ -162,13 +171,13 @@ static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
}
/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * read_seqcount_begin() - start a seqcount_t read critical section
+ * @s: Pointer to &typedef seqcount_t
+ * Returns: count to be passed to read_seqcount_retry()
*
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * read_seqcount_begin opens a read critical section of the given
+ * seqcount_t. Validity of the read section must be checked with
+ * read_seqcount_retry().
*/
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
@@ -177,11 +186,11 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
+ * raw_seqcount_begin() - begin a seq-read critical section
+ * @s: Pointer to &typedef seqcount_t
* Returns: count to be passed to read_seqcount_retry
*
- * raw_seqcount_begin opens a read critical section of the given seqcount.
+ * raw_seqcount_begin opens a read critical section of the given seqcount_t.
* Validity of the critical section is tested by checking read_seqcount_retry
* function.
*
@@ -199,8 +208,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
}
/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
+ * __read_seqcount_retry() - end a seq-read critical section (without barrier)
+ * @s: Pointer to &typedef seqcount_t
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -219,12 +228,12 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
}
/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
+ * read_seqcount_retry() - end a seq-read critical section
+ * @s: Pointer to &typedef seqcount_t
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
- * read_seqcount_retry closes a read critical section of the given seqcount.
+ * read_seqcount_retry closes a read critical section of given seqcount_t.
* If the critical section was invalid, it must be ignored (and typically
* retried).
*/
@@ -251,8 +260,8 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
}
/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_barrier() - do a seq write barrier
+ * @s: Pointer to &typedef seqcount_t
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
@@ -300,6 +309,21 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * raw_read_seqcount_latch() - pick even or odd seqcount_t latch data copy
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * Use seqcount latching to switch between two storage places with
+ * sequence protection to allow interruptible, preemptible, writer
+ * sections.
+ *
+ * Check raw_write_seqcount_latch() for more details and a full reader
+ * and writer usage example.
+ *
+ * Return: sequence counter. Use the lowest bit as index for picking
+ * which data copy to read. Full counter must then be checked with
+ * read_seqcount_retry().
+ */
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
@@ -308,8 +332,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
}
/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_latch() - redirect readers to even/odd copy
+ * @s: Pointer to &typedef seqcount_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -363,8 +387,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes necessary smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq);
*
* return entry;
* }
@@ -401,11 +425,27 @@ static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
+/**
+ * write_seqcount_begin() - start a seqcount_t write-side critical section
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * write_seqcount_begin opens a write-side critical section of the given
+ * seqcount_t. Sequence counter write-side critical sections must be
+ * serialized and non-preemptible. If readers can be invoked from
+ * hardirq or softirq contexts, interrupts or bottom halves must be
+ * respectively disabled.
+ */
static inline void write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
+/**
+ * write_seqcount_end() - end a seqcount_t write-side critical section
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * The write section must've been opened with write_seqcount_begin().
+ */
static inline void write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
@@ -413,8 +453,8 @@ static inline void write_seqcount_end(seqcount_t *s)
}
/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
+ * write_seqcount_invalidate() - invalidate in-progress read-side seq operations
+ * @s: Pointer to &typedef seqcount_t
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
@@ -448,17 +488,32 @@ typedef struct {
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+/**
+ * seqlock_init() - dynamic initializer for seqlock_t
+ * @sl: Pointer to the &typedef seqlock_t instance
+ */
+#define seqlock_init(sl) \
do { \
- seqcount_init(&(x)->seqcount); \
- spin_lock_init(&(x)->lock); \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+/**
+ * DEFINE_SEQLOCK() - Define a statically-allocated seqlock_t
+ * @sl: Name of the &typedef seqlock_t instance
+ */
+#define DEFINE_SEQLOCK(sl) \
+ seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
-/*
- * Read side functions for starting and finalizing a read side section.
+/**
+ * read_seqbegin() - start a seqlock_t read-side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqbegin opens a read side critical section of the given
+ * seqlock_t. Validity of the critical section is tested by checking
+ * read_seqretry().
+ *
+ * Return: count to be passed to read_seqretry()
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
@@ -469,6 +524,17 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
return ret;
}
+/**
+ * read_seqretry() - end and validate a seqlock_t read side section
+ * @sl: Pointer to &typedef seqlock_t
+ * @start: count, from read_seqbegin()
+ *
+ * read_seqretry closes the given seqlock_t read side critical section,
+ * and checks its validity. If the read section was invalid, it must be
+ * ignored and retried.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
/*
@@ -480,10 +546,19 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
return read_seqcount_retry(&sl->seqcount, start);
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * write_seqlock() - start a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_seqlock opens a write side critical section of the given
+ * seqlock_t. It also acquires the spinlock_t embedded inside the
+ * sequential lock. All the seqlock_t write side critical sections are
+ * thus automatically serialized and non-preemptible.
+ *
+ * Use the ``_irqsave`` and ``_bh`` variants instead if the read side
+ * can be invoked from a hardirq or softirq context.
+ *
+ * The opened write side section must be closed with write_sequnlock().
*/
static inline void write_seqlock(seqlock_t *sl)
{
@@ -491,30 +566,68 @@ static inline void write_seqlock(seqlock_t *sl)
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock() - end a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock closes the (serialized and non-preemptible) write
+ * side critical section of given seqlock_t.
+ */
static inline void write_sequnlock(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
+/**
+ * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of write_seqlock(). Use only if the read side section
+ * can be invoked from a softirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_bh().
+ */
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock_bh closes the serialized, non-preemptible,
+ * softirqs-disabled, seqlock_t write side critical section opened with
+ * write_seqlock_bh().
+ */
static inline void write_sequnlock_bh(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
+/**
+ * write_seqlock_irq() - start a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * This is the ``_irq`` variant of write_seqlock(). Use only if the read
+ * section of given seqlock_t can be invoked from a hardirq context.
+ */
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_irq() - end a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of write_sequnlock(). The write side section of
+ * given seqlock_t must've been opened with write_seqlock_irq().
+ */
static inline void write_sequnlock_irq(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
@@ -530,9 +643,28 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * write_seqlock_irqsave() - start a non-interruptible seqlock_t write section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to write_sequnlock_irqrestore().
+ *
+ * ``_irqsave`` variant of write_seqlock(). Use if the read section of
+ * given seqlock_t can be invoked from a hardirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_irqrestore().
+ */
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
+/**
+ * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
+ *
+ * ``_irqrestore`` variant of write_sequnlock(). The write section of
+ * given seqlock_t must've been opened with write_seqlock_irqsave().
+ */
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
@@ -540,30 +672,64 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * read_seqlock_excl() - begin a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqlock_excl opens a locking reader critical section for the
+ * given seqlock_t. A locking reader exclusively locks out other writers
+ * and other *locking* readers, but doesn't update the sequence number.
+ *
+ * Locking readers act like a normal spin_lock()/spin_unlock().
+ *
+ * The opened read side section must be closed with read_sequnlock_excl().
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
+/**
+ * read_sequnlock_excl() - end a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_sequnlock_excl closes the locking reader critical section opened
+ * with read_seqlock_excl().
+ */
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
- *
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
+ * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq : Marker and return parameter. If the passed value is even, the
+ * reader will become a *lockless* seqlock_t sequence counter reader as
+ * in read_seqbegin(). If the passed value is odd, the reader will
+ * become a fully locking reader, as in read_seqlock_excl(). In the
+ * first call to read_seqbegin_or_lock(), the caller **must** initialize
+ * and pass an even value to @seq so a lockless read is optimistically
+ * tried first.
+ *
+ * read_seqbegin_or_lock is an API designed to optimistically try a
+ * normal lockless seqlock_t read section first, as in read_seqbegin().
+ * If an odd counter is found, the normal lockless read trial has
+ * failed, and the next reader iteration transforms to a full seqlock_t
+ * locking reader as in read_seqlock_excl().
+ *
+ * This is typically used to avoid lockless seqlock_t readers starvation
+ * (too much retry loops) in the case of a sharp spike in write
+ * activity.
+ *
+ * The opened read section must be closed with done_seqretry(). Check
+ * Documentation/locking/seqlock.rst for template example code.
+ *
+ * Return: The encountered sequence counter value, returned through the
+ * @seq parameter, which is overloaded as a return parameter. The
+ * returned value must be checked with need_seqretry(). If the read
+ * section must be retried, the returned value must also be passed to
+ * the @seq parameter of the next read_seqbegin_or_lock() iteration.
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
@@ -573,32 +739,90 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
read_seqlock_excl(lock);
}
+/**
+ * need_seqretry() - validate seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * need_seqretry checks if the seqlock_t read-side critical section
+ * started with read_seqbegin_or_lock() is valid. If it was not, the
+ * caller must retry the read-side section.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
+/**
+ * done_seqretry() - end seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * done_seqretry finishes the seqlock_t read side critical section
+ * started by read_seqbegin_or_lock(). The read section must've been
+ * already validated with need_seqretry().
+ */
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
+/**
+ * read_seqlock_excl_bh() - start a locking reader seqlock_t section
+ * with softirqs disabled
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of read_seqlock_excl(). Use this variant if the
+ * seqlock_t write side section, *or other read sections*, can be
+ * invoked from a softirq context
+ *
+ * The opened section must be closed with read_sequnlock_excl_bh().
+ */
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
+/**
+ * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of read_sequnlock_excl(). The closed section must've
+ * been opened with read_seqlock_excl_bh().
+ */
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
+/**
+ * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of read_seqlock_excl(). Use this only if the
+ * seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from a hardirq context.
+ *
+ * The opened read section must be closed with read_sequnlock_excl_irq().
+ */
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
+/**
+ * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of read_sequnlock_excl(). The closed section must've
+ * been opened with read_seqlock_excl_irq().
+ */
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
@@ -612,15 +836,59 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
+ * locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to read_sequnlock_excl_irqrestore().
+ *
+ * ``_irqsave`` variant of read_seqlock_excl(). Use this only if the
+ * seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from a hardirq context.
+ *
+ * Opened section must be closed with read_sequnlock_excl_irqrestore().
+ */
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+/**
+ * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from
+ * read_seqlock_excl_irqsave()
+ *
+ * ``_irqrestore`` variant of read_sequnlock_excl(). The closed section
+ * must've been opened with read_seqlock_excl_irqsave().
+ */
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
spin_unlock_irqrestore(&sl->lock, flags);
}
+/**
+ * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
+ * a non-interruptible locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
+ *
+ * This is the ``_irqsave`` variant of read_seqbegin_or_lock(). Use if
+ * the seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from hardirq context.
+ *
+ * The validity of the read section must be checked with need_seqretry().
+ * The opened section must be closed with done_seqretry_irqrestore().
+ *
+ * Return:
+ *
+ * 1. The saved local interrupts state in case of a locking reader, to be
+ * passed to done_seqretry_irqrestore().
+ *
+ * 2. The encountered sequence counter value, returned through @seq which
+ * is overloaded as a return parameter. Check read_seqbegin_or_lock().
+ */
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
@@ -634,6 +902,18 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
return flags;
}
+/**
+ * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
+ * non-interruptible locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Count, from read_seqbegin_or_lock_irqsave()
+ * @flags: Caller's saved local interrupt state in case of a locking
+ * reader, also from read_seqbegin_or_lock_irqsave()
+ *
+ * This is the ``_irqrestore`` variant of done_seqretry(). The read
+ * section must've been opened with read_seqbegin_or_lock_irqsave(), and
+ * validated with need_seqretry().
+ */
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
--
2.20.1
Preemption must be disabled before entering a sequence count write side
critical section. Failing to do so, the seqcount read side can preempt
the write side section and spin for the entire scheduler tick. If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.
Assert through lockdep that preemption is disabled for seqcount writers.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 27 ++++++++++++++++++++++-----
1 file changed, 22 insertions(+), 5 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 057f7326a877..679c440b17fe 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -419,12 +419,29 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
+static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ lockdep_assert_preemption_disabled();
+ __write_seqcount_begin_nested(s, subclass);
+}
+
+/*
+ * write_seqcount_begin() without lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption is
+ * already disabled. For example, seqlock_t write side functions.
+ */
+static inline void __write_seqcount_begin(seqcount_t *s)
+{
+ __write_seqcount_begin_nested(s, 0);
+}
+
/**
* write_seqcount_begin() - start a seqcount_t write-side critical section
* @s: Pointer to &typedef seqcount_t
@@ -563,7 +580,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -591,7 +608,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -618,7 +635,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -639,7 +656,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
return flags;
}
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the write side critical section.
There is no built-in debugging mechanism to verify that the lock used
for writer serialization is held and preemption is disabled. Some usage
sites like dma-buf have explicit lockdep checks for the writer-side
lock, but this covers only a small portion of the sequence counter usage
in the kernel.
Add new sequence counter types which allows to associate a lock to the
sequence counter at initialization time. The seqcount API functions are
extended to provide appropriate lockdep assertions depending on the
seqcount/lock type.
For sequence counters with associated locks that do not implicitly
disable preemption, preemption protection is enforced in the sequence
counter write side functions. This removes the need to explicitly add
preempt_disable/enable() around the write side critical sections: the
write_begin/end() functions for these new sequence counter types
automatically do this.
Introduce the following seqcount types with associated locks:
seqcount_spinlock_t
seqcount_raw_spinlock_t
seqcount_rwlock_t
seqcount_mutex_t
seqcount_ww_mutex_t
Extend the seqcount read and write functions to branch out to the
specific seqcount_LOCKTYPE_t implementation at compile-time. This avoids
kernel API explosion per each new seqcount_LOCKTYPE_t added. Add such
compile-time type detection logic into a new, internal, seqlock header.
Document the proper seqcount_LOCKTYPE_t usage, and rationale, at
Documentation/locking/seqlock.rst.
If lockdep is disabled, this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/seqlock.rst | 64 ++++-
MAINTAINERS | 2 +-
include/linux/seqlock.h | 372 ++++++++++++++++++++-----
include/linux/seqlock_types_internal.h | 186 +++++++++++++
4 files changed, 558 insertions(+), 66 deletions(-)
create mode 100644 include/linux/seqlock_types_internal.h
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index c9916efe038e..2d526dc95408 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -48,9 +48,11 @@ write side section. If the read section can be invoked from hardirq or
softirq contexts, interrupts or bottom halves must also be respectively
disabled before entering the write section.
-If it's desired to automatically handle the sequence counter
-requirements of writer serialization and non-preemptibility, use a
-:ref:`sequential lock <seqlock_t>` instead.
+If the write serialization mechanism is one of the common kernel locking
+primitives, use :ref:`sequence counters with associated locks
+<seqcount_locktype_t>` instead. If it's desired to automatically handle
+the sequence counter writer serialization and non-preemptibility
+requirements, use a :ref:`sequential lock <seqlock_t>`.
Initialization:
@@ -70,6 +72,7 @@ Initialization:
Write path:
+.. _seqcount_write_ops:
.. code-block:: c
/* Serialized context with disabled preemption */
@@ -82,6 +85,7 @@ Write path:
Read path:
+.. _seqcount_read_ops:
.. code-block:: c
do {
@@ -91,6 +95,60 @@ Read path:
} while (read_seqcount_retry(&foo_seqcount, seq));
+.. _seqcount_locktype_t:
+
+Sequence counters with associated locks (:c:type:`seqcount_LOCKTYPE_t`)
+-----------------------------------------------------------------------
+
+As :ref:`earlier discussed <seqcount_t>`, seqcount write side critical
+sections must be serialized and non-preemptible. This variant of
+sequence counters associate the lock used for writer serialization at
+the seqcount initialization time. This enables lockdep to validate that
+the write side critical section is properly serialized.
+
+This lock association is a NOOP if lockdep is disabled and has neither
+storage nor runtime overhead. If lockdep is enabled, the lock pointer is
+stored in struct seqcount and lockdep's "lock is held" assertions are
+injected at the beginning of the write side critical section to validate
+that it is properly protected.
+
+For lock types which do not implicitly disable preemption, preemption
+protection is enforced in the write side function.
+
+The following seqcounts with associated locks are defined:
+
+ - :c:type:`seqcount_spinlock_t`
+ - :c:type:`seqcount_raw_spinlock_t`
+ - :c:type:`seqcount_rwlock_t`
+ - :c:type:`seqcount_mutex_t`
+ - :c:type:`seqcount_ww_mutex_t`
+
+The plain seqcount read and write APIs branch out to the specific
+seqcount_LOCKTYPE_t implementation at compile-time. This avoids kernel
+API explosion per each new seqcount LOCKTYPE.
+
+Initialization (replace "LOCKTYPE" with one of the supported locks):
+
+.. code-block:: c
+
+ /* dynamic */
+ seqcount_LOCKTYPE_t foo_seqcount;
+ seqcount_LOCKTYPE_init(&foo_seqcount, &lock);
+
+ /* static */
+ static seqcount_LOCKTYPE_t foo_seqcount =
+ SEQCNT_LOCKTYPE_ZERO(foo_seqcount, &lock);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_LOCKTYPE_ZERO(foo.seq, &lock),
+ } foo;
+
+Write path: same as in :ref:`plain seqcount_t <seqcount_write_ops>`,
+while running from a context with the associated LOCKTYPE lock acquired.
+
+Read path: same as in :ref:`plain seqcount_t <seqcount_read_ops>`.
+
.. _seqlock_t:
Sequential locks (:c:type:`seqlock_t`)
diff --git a/MAINTAINERS b/MAINTAINERS
index 68f21d46614c..84a859bc8b4c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10077,7 +10077,7 @@ F: include/linux/lockdep.h
F: include/linux/mutex*.h
F: include/linux/rwlock*.h
F: include/linux/rwsem*.h
-F: include/linux/seqlock.h
+F: include/linux/seqlock*.h
F: include/linux/spinlock*.h
F: kernel/locking/
F: lib/locking*.[ch]
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 679c440b17fe..fb0fe5b7c4f4 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -48,6 +48,10 @@
* This mechanism can't be used if the protected data contains pointers,
* as the writer can invalidate a pointer that a reader is following.
*
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_t) instead.
+ *
* If it's desired to automatically handle the sequence counter writer
* serialization and non-preemptibility requirements, use a sequential
* lock (seqlock_t) instead.
@@ -108,11 +112,223 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*/
#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
+/*
+ * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ *
+ * A sequence counter which associates the lock used for writer
+ * serialization at initialization time. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ *
+ * For associated locks which do not implicitly disable preemption,
+ * preemption protection is enforced in the write side function.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
/**
- * __read_seqcount_begin() - begin a seqcount_t read section (without barrier)
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * typedef seqcount_spinlock_t - sequence count with spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * spinlock. The spinlock is associated to the sequence count in the
+ * static initializer or init function. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_spinlock {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ spinlock_t *lock;
+#endif
+} seqcount_spinlock_t;
+
+#ifdef CONFIG_LOCKDEP
+
+#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+ .lock = (assoc_lock), \
+}
+
+/* Define as macro due to static lockdep key @ seqcount_init() */
+#define seqcount_locktype_init(s, assoc_lock) \
+do { \
+ seqcount_init(&(s)->seqcount); \
+ (s)->lock = (assoc_lock); \
+} while (0)
+
+#else /* !CONFIG_LOCKDEP */
+
+#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+}
+
+#define seqcount_locktype_init(s, assoc_lock) \
+do { \
+ seqcount_init(&(s)->seqcount); \
+} while (0)
+
+#endif
+
+/**
+ * SEQCNT_SPINLOCK_ZERO - static initializer for seqcount_spinlock_t
+ * @name: Name of the &typedef seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define SEQCNT_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_spinlock_init - runtime initializer for seqcount_spinlock_t
+ * @s: Pointer to the &typedef seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define seqcount_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_raw_spinlock_t - sequence count with raw spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated raw spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * raw spinlock. The raw spinlock is associated to the sequence count in
+ * the static initializer or init function. This enables lockdep to
+ * validate that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_raw_spinlock {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ raw_spinlock_t *lock;
+#endif
+} seqcount_raw_spinlock_t;
+
+/**
+ * SEQCNT_RAW_SPINLOCK_ZERO - static initializer for seqcount_raw_spinlock_t
+ * @name: Name of the &typedef seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_raw_spinlock_init - runtime initializer for seqcount_raw_spinlock_t
+ * @s: Pointer to the &typedef seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define seqcount_raw_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_rwlock_t - sequence count with rwlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated rwlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * rwlock. The rwlock is associated to the sequence count in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ */
+typedef struct seqcount_rwlock {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ rwlock_t *lock;
+#endif
+} seqcount_rwlock_t;
+
+/**
+ * SEQCNT_RWLOCK_ZERO - static initializer for seqcount_rwlock_t
+ * @name: Name of the &typedef seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define SEQCNT_RWLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_rwlock_init - runtime initializer for seqcount_rwlock_t
+ * @s: Pointer to the &typedef seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define seqcount_rwlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_mutex_t - sequence count with mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * mutex. The mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_mutex_t.
+ */
+typedef struct seqcount_mutex {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ struct mutex *lock;
+#endif
+} seqcount_mutex_t;
+
+/**
+ * SEQCNT_MUTEX_ZERO - static initializer for seqcount_mutex_t
+ * @name: Name of the &typedef seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define SEQCNT_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_mutex_init - runtime initializer for seqcount_mutex_t
+ * @s: Pointer to the &typedef seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define seqcount_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_ww_mutex_t - sequence count with ww_mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated ww_mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * ww_mutex. The ww_mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_ww_mutex_t.
+ */
+typedef struct seqcount_ww_mutex {
+ seqcount_t seqcount;
+#ifdef CONFIG_LOCKDEP
+ struct ww_mutex *lock;
+#endif
+} seqcount_ww_mutex_t;
+
+/**
+ * SEQCNT_WW_MUTEX_ZERO - static initializer for seqcount_ww_mutex_t
+ * @name: Name of the &typedef seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define SEQCNT_WW_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_ww_mutex_init - runtime initializer for seqcount_ww_mutex_t
+ * @s: Pointer to the &typedef seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define seqcount_ww_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+#include <linux/seqlock_types_internal.h>
+
+/**
+ * __read_seqcount_begin() - begin a seqcount read section (without barrier)
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -122,7 +338,9 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+#define __read_seqcount_begin(s) do___read_seqcount_begin(s)
+
+static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
{
unsigned ret;
@@ -137,16 +355,18 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount() - Read the seqcount_t raw counter value
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * raw_read_seqcount() - Read the seqcount raw counter value
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount opens a read critical section of the given
* seqcount_t, without any lockdep checks and without checking or
* masking the sequence counter LSB. Calling code is responsible for
* handling that.
*/
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
+#define raw_read_seqcount(s) do_raw_read_seqcount(s)
+
+static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -155,39 +375,43 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_read_seqcount_begin() - start a seqcount_t read section w/o lockdep
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * raw_read_seqcount_begin() - start a seqcount read section w/o lockdep
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount_begin opens a read critical section of the given
* seqcount_t, but without any lockdep checking. Validity of the read
* section must be checked with read_seqcount_retry().
*/
-static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+#define raw_read_seqcount_begin(s) do_raw_read_seqcount_begin(s)
+
+static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
+ unsigned ret = __read_seqcount_t_begin(s);
smp_rmb();
return ret;
}
/**
- * read_seqcount_begin() - start a seqcount_t read critical section
- * @s: Pointer to &typedef seqcount_t
- * Returns: count to be passed to read_seqcount_retry()
+ * read_seqcount_begin() - start a seqcount read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ * Returns: count to be passed to read_seqcount_retry
*
* read_seqcount_begin opens a read critical section of the given
* seqcount_t. Validity of the read section must be checked with
* read_seqcount_retry().
*/
-static inline unsigned read_seqcount_begin(const seqcount_t *s)
+#define read_seqcount_begin(s) do_read_seqcount_begin(s)
+
+static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
{
seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
+ return raw_read_seqcount_t_begin(s);
}
/**
* raw_seqcount_begin() - begin a seq-read critical section
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* raw_seqcount_begin opens a read critical section of the given seqcount_t.
@@ -199,7 +423,9 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
* read_seqcount_retry() instead of stabilizing at the beginning of the
* critical section.
*/
-static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+#define raw_seqcount_begin(s) do_raw_seqcount_begin(s)
+
+static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -209,7 +435,7 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
/**
* __read_seqcount_retry() - end a seq-read critical section (without barrier)
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -221,7 +447,9 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define __read_seqcount_retry(s, start) do___read_seqcount_retry(s, start)
+
+static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
kcsan_atomic_next(0);
return unlikely(READ_ONCE(s->sequence) != start);
@@ -229,7 +457,7 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
/**
* read_seqcount_retry() - end a seq-read critical section
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -237,22 +465,28 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
* If the critical section was invalid, it must be ignored (and typically
* retried).
*/
-static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define read_seqcount_retry(s, start) do_read_seqcount_retry(s, start)
+
+static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return __read_seqcount_t_retry(s, start);
}
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+#define raw_write_seqcount_begin(s) do_raw_write_seqcount_begin(s)
+
+static inline void raw_write_seqcount_t_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
}
-static inline void raw_write_seqcount_end(seqcount_t *s)
+#define raw_write_seqcount_end(s) do_raw_write_seqcount_end(s)
+
+static inline void raw_write_seqcount_t_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
@@ -261,7 +495,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
/**
* raw_write_seqcount_barrier() - do a seq write barrier
- * @s: Pointer to &typedef seqcount_t
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
@@ -300,7 +534,9 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* WRITE_ONCE(X, false);
* }
*/
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
+#define raw_write_seqcount_barrier(s) do_raw_write_seqcount_barrier(s)
+
+static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -310,8 +546,8 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
}
/**
- * raw_read_seqcount_latch() - pick even or odd seqcount_t latch data copy
- * @s: Pointer to &typedef seqcount_t
+ * raw_read_seqcount_latch() - pick even or odd seqcount latch data copy
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* Use seqcount latching to switch between two storage places with
* sequence protection to allow interruptible, preemptible, writer
@@ -324,7 +560,9 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
* which data copy to read. Full counter must then be checked with
* read_seqcount_retry().
*/
-static inline int raw_read_seqcount_latch(seqcount_t *s)
+#define raw_read_seqcount_latch(s) do_raw_read_seqcount_latch(s)
+
+static inline int raw_read_seqcount_t_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
@@ -333,7 +571,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
/**
* raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to &typedef seqcount_t
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -412,49 +650,54 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
-static inline void raw_write_seqcount_latch(seqcount_t *s)
+#define raw_write_seqcount_latch(s) do_raw_write_seqcount_latch(s)
+
+static inline void raw_write_seqcount_t_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
smp_wmb(); /* increment "sequence" before following stores */
}
-static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+static inline void __write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
- raw_write_seqcount_begin(s);
+ raw_write_seqcount_t_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+#define write_seqcount_begin_nested(s, subclass) \
+ do_write_seqcount_begin_nested(s, subclass)
+
+static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
lockdep_assert_preemption_disabled();
- __write_seqcount_begin_nested(s, subclass);
+ __write_seqcount_t_begin_nested(s, subclass);
}
/*
- * write_seqcount_begin() without lockdep non-preemptibility checks.
+ * write_seqcount_t_begin() without lockdep non-preemptibility checks.
*
* Use for internal seqlock.h code where it's known that preemption is
* already disabled. For example, seqlock_t write side functions.
*/
-static inline void __write_seqcount_begin(seqcount_t *s)
+static inline void __write_seqcount_t_begin(seqcount_t *s)
{
- __write_seqcount_begin_nested(s, 0);
+ __write_seqcount_t_begin_nested(s, 0);
}
/**
- * write_seqcount_begin() - start a seqcount_t write-side critical section
- * @s: Pointer to &typedef seqcount_t
+ * write_seqcount_begin() - start a seqcount write-side critical section
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* write_seqcount_begin opens a write-side critical section of the given
- * seqcount_t. Sequence counter write-side critical sections must be
- * serialized and non-preemptible. If readers can be invoked from
- * hardirq or softirq contexts, interrupts or bottom halves must be
- * respectively disabled.
+ * seqcount. Seqcount write-side critical sections must be externally
+ * serialized and non-preemptible.
*/
-static inline void write_seqcount_begin(seqcount_t *s)
+#define write_seqcount_begin(s) do_write_seqcount_begin(s)
+
+static inline void write_seqcount_t_begin(seqcount_t *s)
{
- write_seqcount_begin_nested(s, 0);
+ write_seqcount_t_begin_nested(s, 0);
}
/**
@@ -463,7 +706,9 @@ static inline void write_seqcount_begin(seqcount_t *s)
*
* The write section must've been opened with write_seqcount_begin().
*/
-static inline void write_seqcount_end(seqcount_t *s)
+#define write_seqcount_end(s) do_write_seqcount_end(s)
+
+static inline void write_seqcount_t_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
raw_write_seqcount_end(s);
@@ -471,12 +716,14 @@ static inline void write_seqcount_end(seqcount_t *s)
/**
* write_seqcount_invalidate() - invalidate in-progress read-side seq operations
- * @s: Pointer to &typedef seqcount_t
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
*/
-static inline void write_seqcount_invalidate(seqcount_t *s)
+#define write_seqcount_invalidate(s) do_write_seqcount_invalidate(s)
+
+static inline void write_seqcount_t_invalidate(seqcount_t *s)
{
smp_wmb();
kcsan_nestable_atomic_begin();
@@ -534,7 +781,7 @@ typedef struct {
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
+ unsigned ret = read_seqcount_t_begin(&sl->seqcount);
kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
kcsan_flat_atomic_begin();
@@ -560,7 +807,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
*/
kcsan_flat_atomic_end();
- return read_seqcount_retry(&sl->seqcount, start);
+ return read_seqcount_t_retry(&sl->seqcount, start);
}
/**
@@ -580,7 +827,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -592,7 +839,7 @@ static inline void write_seqlock(seqlock_t *sl)
*/
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
@@ -608,7 +855,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -621,7 +868,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
*/
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
@@ -635,7 +882,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -647,7 +894,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
*/
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -656,7 +903,8 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- __write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_t_begin(&sl->seqcount);
+
return flags;
}
@@ -685,7 +933,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
diff --git a/include/linux/seqlock_types_internal.h b/include/linux/seqlock_types_internal.h
new file mode 100644
index 000000000000..a70bf38e6eb5
--- /dev/null
+++ b/include/linux/seqlock_types_internal.h
@@ -0,0 +1,186 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_SEQLOCK_TYPES_INTERNAL_H
+#define __LINUX_SEQLOCK_TYPES_INTERNAL_H
+
+/*
+ * Sequence counters with associated locks
+ *
+ * Copyright (C) 2020 Linutronix GmbH
+ */
+
+#ifndef __LINUX_SEQLOCK_H
+#error This is an INTERNAL header; it must only be included by seqlock.h
+#endif
+
+#include <linux/mutex.h>
+#include <linux/spinlock.h>
+#include <linux/ww_mutex.h>
+
+/*
+ * @s: pointer to seqcount_t or any of the seqcount_locktype_t variants
+ */
+#define __to_seqcount_t(s) \
+({ \
+ seqcount_t *seq; \
+ \
+ if (__same_type(*(s), seqcount_t)) \
+ seq = (seqcount_t *)(s); \
+ else if (__same_type(*(s), seqcount_spinlock_t)) \
+ seq = &((seqcount_spinlock_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_raw_spinlock_t)) \
+ seq = &((seqcount_raw_spinlock_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_rwlock_t)) \
+ seq = &((seqcount_rwlock_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_mutex_t)) \
+ seq = &((seqcount_mutex_t *)(s))->seqcount; \
+ else if (__same_type(*(s), seqcount_ww_mutex_t)) \
+ seq = &((seqcount_ww_mutex_t *)(s))->seqcount; \
+ else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+ \
+ seq; \
+})
+
+/*
+ * seqcount_LOCKTYPE_t -- write APIs
+ *
+ * For associated lock types which do not implicitly disable preemption,
+ * enforce preemption protection in the write side functions.
+ *
+ * Never use lockdep for the raw write variants.
+ */
+
+#define __associated_lock_is_preemptible(s) \
+({ \
+ bool ret; \
+ \
+ if (__same_type(*(s), seqcount_t) || \
+ __same_type(*(s), seqcount_spinlock_t) || \
+ __same_type(*(s), seqcount_raw_spinlock_t) || \
+ __same_type(*(s), seqcount_rwlock_t)) { \
+ ret = false; \
+ } else if (__same_type(*(s), seqcount_mutex_t) || \
+ __same_type(*(s), seqcount_ww_mutex_t)) { \
+ ret = true; \
+ } else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+ \
+ ret; \
+})
+
+#ifdef CONFIG_LOCKDEP
+
+#define __assert_associated_lock_held(s) \
+do { \
+ if (__same_type(*(s), seqcount_t)) \
+ break; \
+ \
+ if (__same_type(*(s), seqcount_spinlock_t)) \
+ lockdep_assert_held(((seqcount_spinlock_t *)(s))->lock);\
+ else if (__same_type(*(s), seqcount_raw_spinlock_t)) \
+ lockdep_assert_held(((seqcount_raw_spinlock_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_rwlock_t)) \
+ lockdep_assert_held_write(((seqcount_rwlock_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_mutex_t)) \
+ lockdep_assert_held(((seqcount_mutex_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_ww_mutex_t)) \
+ lockdep_assert_held(&((seqcount_ww_mutex_t *)(s))->lock->base); \
+ else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+} while (0)
+
+#else
+
+#define __assert_associated_lock_held(s) \
+do { \
+ (void) __to_seqcount_t(s); \
+} while (0)
+
+#endif /* CONFIG_LOCKDEP */
+
+#define do_raw_write_seqcount_begin(s) \
+do { \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ raw_write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+#define do_raw_write_seqcount_end(s) \
+do { \
+ raw_write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+#define do_write_seqcount_begin_nested(s, subclass) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin_nested(__to_seqcount_t(s), subclass); \
+} while (0)
+
+#define do_write_seqcount_begin(s) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+#define do_write_seqcount_end(s) \
+do { \
+ write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+#define do_write_seqcount_invalidate(s) \
+ write_seqcount_t_invalidate(__to_seqcount_t(s))
+
+#define do_raw_write_seqcount_barrier(s) \
+ raw_write_seqcount_t_barrier(__to_seqcount_t(s))
+
+/*
+ * Latch sequence counters write side critical sections don't need to
+ * run with preemption disabled. Check @raw_write_seqcount_latch().
+ */
+#define do_raw_write_seqcount_latch(s) \
+ raw_write_seqcount_t_latch(__to_seqcount_t(s))
+
+/*
+ * seqcount_LOCKTYPE_t -- read APIs
+ */
+
+#define do___read_seqcount_begin(s) \
+ __read_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_raw_read_seqcount(s) \
+ raw_read_seqcount_t(__to_seqcount_t(s))
+
+#define do_raw_seqcount_begin(s) \
+ raw_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_raw_read_seqcount_begin(s) \
+ raw_read_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_read_seqcount_begin(s) \
+ read_seqcount_t_begin(__to_seqcount_t(s))
+
+#define do_raw_read_seqcount_latch(s) \
+ raw_read_seqcount_t_latch(__to_seqcount_t(s))
+
+#define do___read_seqcount_retry(s, start) \
+ __read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+#define do_read_seqcount_retry(s, start) \
+ read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+#endif /* __LINUX_SEQLOCK_TYPES_INTERNAL_H */
--
2.20.1
Commit 3c3b177a9369 ("reservation: add support for read-only access
using rcu") introduced a sequence counter to manage updates to
reservations. Back then, the reservation object initializer
reservation_object_init() was always inlined.
Having the sequence counter initialization inlined meant that each of
the call sites would have a different lockdep class key, which would've
broken lockdep's deadlock detection. The aforementioned commit thus
introduced, and exported, a custom seqcount lockdep class key and name.
The commit 8735f16803f00 ("dma-buf: cleanup reservation_object_init...")
transformed the reservation object initializer to a normal non-inlined C
function. seqcount_init(), which automatically defines the seqcount
lockdep class key and must be called non-inlined, can now be safely used.
Remove the seqcount custom lockdep class key, name, and export. Use
seqcount_init() inside the dma reservation object initializer.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
---
drivers/dma-buf/dma-resv.c | 9 +--------
include/linux/dma-resv.h | 2 --
2 files changed, 1 insertion(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index b45f8514dc82..15efa0c2dacb 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -51,12 +51,6 @@
DEFINE_WD_CLASS(reservation_ww_class);
EXPORT_SYMBOL(reservation_ww_class);
-struct lock_class_key reservation_seqcount_class;
-EXPORT_SYMBOL(reservation_seqcount_class);
-
-const char reservation_seqcount_string[] = "reservation_seqcount";
-EXPORT_SYMBOL(reservation_seqcount_string);
-
/**
* dma_resv_list_alloc - allocate fence list
* @shared_max: number of fences we need space for
@@ -135,9 +129,8 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
+ seqcount_init(&obj->seq);
- __seqcount_init(&obj->seq, reservation_seqcount_string,
- &reservation_seqcount_class);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
}
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index ee50d10f052b..a6538ae7d93f 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -46,8 +46,6 @@
#include <linux/rcupdate.h>
extern struct ww_class reservation_ww_class;
-extern struct lock_class_key reservation_seqcount_class;
-extern const char reservation_seqcount_string[];
/**
* struct dma_resv_list - a list of shared fences
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
The dma-buf reservation subsystem uses plain sequence counters to manage
updates to reservations. Writer serialization is accomplished through a
wound/wait mutex.
Acquiring a wound/wait mutex does not disable preemption, so this needs
to be done manually before and after the write side critical section.
Use the newly-added seqcount_ww_mutex_t instead:
- It associates the ww_mutex with the sequence count, which enables
lockdep to validate that the write side critical section is properly
serialized.
- It removes the need to explicitly add preempt_disable/enable()
around the write side critical section because the write_begin/end()
functions for this new data type automatically do this.
If lockdep is disabled this ww_mutex lock association is compiled out
and has neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
---
drivers/dma-buf/dma-resv.c | 8 +-------
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 --
include/linux/dma-resv.h | 2 +-
3 files changed, 2 insertions(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 15efa0c2dacb..a7631352a486 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -129,7 +129,7 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
- seqcount_init(&obj->seq);
+ seqcount_ww_mutex_init(&obj->seq, &obj->lock);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
@@ -260,7 +260,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
fobj = dma_resv_get_list(obj);
count = fobj->shared_count;
- preempt_disable();
write_seqcount_begin(&obj->seq);
for (i = 0; i < count; ++i) {
@@ -282,7 +281,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
smp_store_mb(fobj->shared_count, count);
write_seqcount_end(&obj->seq);
- preempt_enable();
dma_fence_put(old);
}
EXPORT_SYMBOL(dma_resv_add_shared_fence);
@@ -309,14 +307,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
if (fence)
dma_fence_get(fence);
- preempt_disable();
write_seqcount_begin(&obj->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(obj->fence_excl, fence);
if (old)
old->shared_count = 0;
write_seqcount_end(&obj->seq);
- preempt_enable();
/* inplace update, no shared fences */
while (i--)
@@ -394,13 +390,11 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
src_list = dma_resv_get_list(dst);
old = dma_resv_get_excl(dst);
- preempt_disable();
write_seqcount_begin(&dst->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(dst->fence_excl, new);
RCU_INIT_POINTER(dst->fence, dst_list);
write_seqcount_end(&dst->seq);
- preempt_enable();
dma_resv_list_free(src_list);
dma_fence_put(old);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
index b91b5171270f..ff4b583cb96a 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
@@ -258,11 +258,9 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
new->shared_count = k;
/* Install the new fence list, seqcount provides the barriers */
- preempt_disable();
write_seqcount_begin(&resv->seq);
RCU_INIT_POINTER(resv->fence, new);
write_seqcount_end(&resv->seq);
- preempt_enable();
/* Drop the references to the removed fences or move them to ef_list */
for (i = j, k = 0; i < old->shared_count; ++i) {
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index a6538ae7d93f..d44a77e8a7e3 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -69,7 +69,7 @@ struct dma_resv_list {
*/
struct dma_resv {
struct ww_mutex lock;
- seqcount_t seq;
+ seqcount_ww_mutex_t seq;
struct dma_fence __rcu *fence_excl;
struct dma_resv_list __rcu *fence;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/sched.h | 2 +-
init/init_task.c | 3 ++-
kernel/fork.c | 2 +-
3 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 03403ca6e44e..1b4e6b8dc523 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1055,7 +1055,7 @@ struct task_struct {
/* Protected by ->alloc_lock: */
nodemask_t mems_allowed;
/* Seqence number to catch updates: */
- seqcount_t mems_allowed_seq;
+ seqcount_spinlock_t mems_allowed_seq;
int cpuset_mem_spread_rotor;
int cpuset_slab_spread_rotor;
#endif
diff --git a/init/init_task.c b/init/init_task.c
index 15089d15010a..94fe3ba1bb60 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -154,7 +154,8 @@ struct task_struct init_task
.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),
#endif
#ifdef CONFIG_CPUSETS
- .mems_allowed_seq = SEQCNT_ZERO(init_task.mems_allowed_seq),
+ .mems_allowed_seq = SEQCNT_SPINLOCK_ZERO(init_task.mems_allowed_seq,
+ &init_task.alloc_lock),
#endif
#ifdef CONFIG_RT_MUTEXES
.pi_waiters = RB_ROOT_CACHED,
diff --git a/kernel/fork.c b/kernel/fork.c
index f44d70307210..eb260c6bdb8b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2032,7 +2032,7 @@ static __latent_entropy struct task_struct *copy_process(
#ifdef CONFIG_CPUSETS
p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
- seqcount_init(&p->mems_allowed_seq);
+ seqcount_spinlock_init(&p->mems_allowed_seq, &p->alloc_lock);
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
p->irq_events = 0;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/net/netfilter/nf_conntrack.h | 2 +-
net/netfilter/nf_conntrack_core.c | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 90690e37a56f..ea4e2010b246 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -286,7 +286,7 @@ int nf_conntrack_hash_resize(unsigned int hashsize);
extern struct hlist_nulls_head *nf_conntrack_hash;
extern unsigned int nf_conntrack_htable_size;
-extern seqcount_t nf_conntrack_generation;
+extern seqcount_spinlock_t nf_conntrack_generation;
extern unsigned int nf_conntrack_max;
/* must be called with rcu read lock held */
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 79cd9dde457b..b8c54d390f93 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -180,7 +180,7 @@ EXPORT_SYMBOL_GPL(nf_conntrack_htable_size);
unsigned int nf_conntrack_max __read_mostly;
EXPORT_SYMBOL_GPL(nf_conntrack_max);
-seqcount_t nf_conntrack_generation __read_mostly;
+seqcount_spinlock_t nf_conntrack_generation __read_mostly;
static unsigned int nf_conntrack_hash_rnd __read_mostly;
static u32 hash_conntrack_raw(const struct nf_conntrack_tuple *tuple,
@@ -2598,7 +2598,8 @@ int nf_conntrack_init_start(void)
/* struct nf_ct_ext uses u8 to store offsets/size */
BUILD_BUG_ON(total_extension_size() > 255u);
- seqcount_init(&nf_conntrack_generation);
+ seqcount_spinlock_init(&nf_conntrack_generation,
+ &nf_conntrack_locks_all_lock);
for (i = 0; i < CONNTRACK_LOCKS; i++)
spin_lock_init(&nf_conntrack_locks[i]);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_rwlock_t data type, which allows to associate a
rwlock with the sequence counter. This enables lockdep to verify that
the rwlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/netfilter/nft_set_rbtree.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/netfilter/nft_set_rbtree.c b/net/netfilter/nft_set_rbtree.c
index 62f416bc0579..9f58261ee4c7 100644
--- a/net/netfilter/nft_set_rbtree.c
+++ b/net/netfilter/nft_set_rbtree.c
@@ -18,7 +18,7 @@
struct nft_rbtree {
struct rb_root root;
rwlock_t lock;
- seqcount_t count;
+ seqcount_rwlock_t count;
struct delayed_work gc_work;
};
@@ -516,7 +516,7 @@ static int nft_rbtree_init(const struct nft_set *set,
struct nft_rbtree *priv = nft_set_priv(set);
rwlock_init(&priv->lock);
- seqcount_init(&priv->count);
+ seqcount_rwlock_init(&priv->count, &priv->lock);
priv->root = RB_ROOT;
INIT_DEFERRABLE_WORK(&priv->gc_work, nft_rbtree_gc);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
A plain seqcount_t does not contain the information of which lock must
be held when entering a write side critical section.
Use the new seqcount_spinlock_t and seqcount_mutex_t data types instead,
which allow to associate a lock with the sequence counter. This enables
lockdep to verify that the lock used for writer serialization is held
when the write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/xfrm/xfrm_policy.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 564aa6492e7c..732a940468b0 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -122,7 +122,7 @@ struct xfrm_pol_inexact_bin {
/* list containing '*:*' policies */
struct hlist_head hhead;
- seqcount_t count;
+ seqcount_spinlock_t count;
/* tree sorted by daddr/prefix */
struct rb_root root_d;
@@ -155,7 +155,7 @@ static struct xfrm_policy_afinfo const __rcu *xfrm_policy_afinfo[AF_INET6 + 1]
__read_mostly;
static struct kmem_cache *xfrm_dst_cache __ro_after_init;
-static __read_mostly seqcount_t xfrm_policy_hash_generation;
+static __read_mostly seqcount_mutex_t xfrm_policy_hash_generation;
static struct rhashtable xfrm_policy_inexact_table;
static const struct rhashtable_params xfrm_pol_inexact_params;
@@ -719,7 +719,7 @@ xfrm_policy_inexact_alloc_bin(const struct xfrm_policy *pol, u8 dir)
INIT_HLIST_HEAD(&bin->hhead);
bin->root_d = RB_ROOT;
bin->root_s = RB_ROOT;
- seqcount_init(&bin->count);
+ seqcount_spinlock_init(&bin->count, &net->xfrm.xfrm_policy_lock);
prev = rhashtable_lookup_get_insert_key(&xfrm_policy_inexact_table,
&bin->k, &bin->head,
@@ -1906,7 +1906,7 @@ static int xfrm_policy_match(const struct xfrm_policy *pol,
static struct xfrm_pol_inexact_node *
xfrm_policy_lookup_inexact_addr(const struct rb_root *r,
- seqcount_t *count,
+ seqcount_spinlock_t *count,
const xfrm_address_t *addr, u16 family)
{
const struct rb_node *parent;
@@ -4153,7 +4153,7 @@ void __init xfrm_init(void)
{
register_pernet_subsys(&xfrm_net_ops);
xfrm_dev_init();
- seqcount_init(&xfrm_policy_hash_generation);
+ seqcount_mutex_init(&xfrm_policy_hash_generation, &hash_resize_mutex);
xfrm_input_init();
#ifdef CONFIG_INET_ESPINTCP
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/nfs/nfs4_fs.h | 2 +-
fs/nfs/nfs4state.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/nfs/nfs4_fs.h b/fs/nfs/nfs4_fs.h
index 2b7f6dcd2eb8..210e590e1f71 100644
--- a/fs/nfs/nfs4_fs.h
+++ b/fs/nfs/nfs4_fs.h
@@ -117,7 +117,7 @@ struct nfs4_state_owner {
unsigned long so_flags;
struct list_head so_states;
struct nfs_seqid_counter so_seqid;
- seqcount_t so_reclaim_seqcount;
+ seqcount_spinlock_t so_reclaim_seqcount;
struct mutex so_delegreturn_mutex;
};
diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
index a8dc25ce48bb..b1dba24918f8 100644
--- a/fs/nfs/nfs4state.c
+++ b/fs/nfs/nfs4state.c
@@ -509,7 +509,7 @@ nfs4_alloc_state_owner(struct nfs_server *server,
nfs4_init_seqid_counter(&sp->so_seqid);
atomic_set(&sp->so_count, 1);
INIT_LIST_HEAD(&sp->so_lru);
- seqcount_init(&sp->so_reclaim_seqcount);
+ seqcount_spinlock_init(&sp->so_reclaim_seqcount, &sp->so_lock);
mutex_init(&sp->so_delegreturn_mutex);
return sp;
}
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/hrtimer.h | 2 +-
kernel/time/hrtimer.c | 13 ++++++++++---
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 15c8ac313678..25993b86ac5c 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -159,7 +159,7 @@ struct hrtimer_clock_base {
struct hrtimer_cpu_base *cpu_base;
unsigned int index;
clockid_t clockid;
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct hrtimer *running;
struct timerqueue_head active;
ktime_t (*get_time)(void);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index d89da1c7e005..c4038511d5c9 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -135,7 +135,11 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
* timer->base->cpu_base
*/
static struct hrtimer_cpu_base migration_cpu_base = {
- .clock_base = { { .cpu_base = &migration_cpu_base, }, },
+ .clock_base = { {
+ .cpu_base = &migration_cpu_base,
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(migration_cpu_base.seq,
+ &migration_cpu_base.lock),
+ }, },
};
#define migration_base migration_cpu_base.clock_base[0]
@@ -1998,8 +2002,11 @@ int hrtimers_prepare_cpu(unsigned int cpu)
int i;
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
- cpu_base->clock_base[i].cpu_base = cpu_base;
- timerqueue_init_head(&cpu_base->clock_base[i].active);
+ struct hrtimer_clock_base *clock_b = &cpu_base->clock_base[i];
+
+ clock_b->cpu_base = cpu_base;
+ seqcount_raw_spinlock_init(&clock_b->seq, &cpu_base->lock);
+ timerqueue_init_head(&clock_b->active);
}
cpu_base->cpu = cpu;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
---
include/linux/kvm_irqfd.h | 2 +-
virt/kvm/eventfd.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dc1da020305b..dac047abdba7 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
wait_queue_entry_t wait;
/* Update side is protected by irqfds.lock */
struct kvm_kernel_irq_routing_entry irq_entry;
- seqcount_t irq_entry_sc;
+ seqcount_spinlock_t irq_entry_sc;
/* Used for level IRQ fast-path */
int gsi;
struct work_struct inject;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index ef7ed916ad4a..d6408bb497dc 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
INIT_LIST_HEAD(&irqfd->list);
INIT_WORK(&irqfd->inject, irqfd_inject);
INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
- seqcount_init(&irqfd->irq_entry_sc);
+ seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
f = fdget(args->fd);
if (!f.file) {
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/userfaultfd.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 52de29000c7e..26e8b23594fb 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -61,7 +61,7 @@ struct userfaultfd_ctx {
/* waitqueue head for events */
wait_queue_head_t event_wqh;
/* a refile sequence protected by fault_pending_wqh lock */
- struct seqcount refile_seq;
+ seqcount_spinlock_t refile_seq;
/* pseudo fd refcounting */
refcount_t refcount;
/* userfaultfd syscall flags */
@@ -1998,7 +1998,7 @@ static void init_once_userfaultfd_ctx(void *mem)
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->event_wqh);
init_waitqueue_head(&ctx->fd_wqh);
- seqcount_init(&ctx->refile_seq);
+ seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
}
SYSCALL_DEFINE1(userfaultfd, int, flags)
--
2.20.1
Asserting that preemption is enabled or disabled is a critical sanity
check. Developers are usually reluctant to add such a check in a
fastpath as reading the preemption count can be costly.
Extend the lockdep API with macros asserting that preemption is disabled
or enabled. If lockdep is disabled, or if the underlying architecture
does not support kernel preemption, this assert has no runtime overhead.
References: f54bb2ec02c8 ("locking/lockdep: Add IRQs disabled/enabled assertion APIs: ...")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/lockdep.h | 18 ++++++++++++++++++
lib/Kconfig.debug | 1 +
2 files changed, 19 insertions(+)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index fd04b9e96091..53eff6b26fac 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -548,6 +548,22 @@ do { \
WARN_ON_ONCE(debug_locks && !this_cpu_read(hardirq_context)); \
} while (0)
+#define lockdep_assert_preemption_enabled() \
+do { \
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
+ debug_locks && \
+ (preempt_count() != 0 || \
+ !this_cpu_read(hardirqs_enabled))); \
+} while (0)
+
+#define lockdep_assert_preemption_disabled() \
+do { \
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
+ debug_locks && \
+ (preempt_count() == 0 && \
+ this_cpu_read(hardirqs_enabled))); \
+} while (0)
+
#else
# define might_lock(lock) do { } while (0)
# define might_lock_read(lock) do { } while (0)
@@ -556,6 +572,8 @@ do { \
# define lockdep_assert_irqs_enabled() do { } while (0)
# define lockdep_assert_irqs_disabled() do { } while (0)
# define lockdep_assert_in_irq() do { } while (0)
+# define lockdep_assert_preemption_enabled() do { } while (0)
+# define lockdep_assert_preemption_disabled() do { } while (0)
#endif
#ifdef CONFIG_PROVE_RAW_LOCK_NESTING
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d74ac0fd6b2d..e5e2e632b749 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1118,6 +1118,7 @@ config PROVE_LOCKING
select DEBUG_RWSEMS
select DEBUG_WW_MUTEX_SLOWPATH
select DEBUG_LOCK_ALLOC
+ select PREEMPT_COUNT if !ARCH_NO_PREEMPT
select TRACE_IRQFLAGS
default n
help
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index ab8067f9ce8c..892aefe88fa7 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6935,7 +6935,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
} else
goto abort;
spin_lock_init(&conf->device_lock);
- seqcount_init(&conf->gen_lock);
+ seqcount_spinlock_init(&conf->gen_lock, &conf->device_lock);
mutex_init(&conf->cache_size_mutex);
init_waitqueue_head(&conf->wait_for_quiescent);
init_waitqueue_head(&conf->wait_for_stripe);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index f90e0704bed9..a2c9e9e9f5ac 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -589,7 +589,7 @@ struct r5conf {
int prev_chunk_sectors;
int prev_algo;
short generation; /* increments with every reshape */
- seqcount_t gen_lock; /* lock against generation changes */
+ seqcount_spinlock_t gen_lock; /* lock against generation changes */
unsigned long reshape_checkpoint; /* Time we last updated
* metadata */
long long min_offset_diff; /* minimum difference between
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
kernel/time/timekeeping.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d20d489841c8..05ecfd8a3314 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -39,18 +39,19 @@ enum timekeeping_adv_mode {
TK_ADV_FREQ
};
+static DEFINE_RAW_SPINLOCK(timekeeper_lock);
+
/*
* The most important data for readout fits into a single 64 byte
* cache line.
*/
static struct {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct timekeeper timekeeper;
} tk_core ____cacheline_aligned = {
- .seq = SEQCNT_ZERO(tk_core.seq),
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_core.seq, &timekeeper_lock),
};
-static DEFINE_RAW_SPINLOCK(timekeeper_lock);
static struct timekeeper shadow_timekeeper;
/**
@@ -63,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct tk_read_base base[2];
};
@@ -80,11 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -157,7 +160,7 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
* tk_clock_read - atomic clocksource read() helper
*
* This helper is necessary to use in the read paths because, while the
- * seqlock ensures we don't return a bad value while structures are updated,
+ * seqcount ensures we don't return a bad value while structures are updated,
* it doesn't protect from potential crashes. There is the possibility that
* the tkr's clocksource may change between the read reference, and the
* clock reference passed to the read function. This can cause crashes if
@@ -222,10 +225,10 @@ static inline u64 timekeeping_get_delta(const struct tk_read_base *tkr)
unsigned int seq;
/*
- * Since we're called holding a seqlock, the data may shift
+ * Since we're called holding a seqcount, the data may shift
* under us while we're doing the calculation. This can cause
* false positives, since we'd note a problem but throw the
- * results away. So nest another seqlock here to atomically
+ * results away. So nest another seqcount here to atomically
* grab the points we are checking with.
*/
do {
@@ -486,7 +489,7 @@ EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
*
* To keep it NMI safe since we're accessing from tracing, we're not using a
* separate timekeeper with updates to monotonic clock and boot offset
- * protected with seqlocks. This has the following minor side effects:
+ * protected with seqcounts. This has the following minor side effects:
*
* (1) Its possible that a timestamp be taken after the boot offset is updated
* but before the timekeeper is updated. If this happens, the new boot offset
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 ++--
include/linux/dcache.h | 2 +-
include/linux/fs_struct.h | 2 +-
4 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index 361ea7ab30ea..ea0485861d93 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1746,7 +1746,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
dentry->d_lockref.count = 1;
dentry->d_flags = 0;
spin_lock_init(&dentry->d_lock);
- seqcount_init(&dentry->d_seq);
+ seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
dentry->d_inode = NULL;
dentry->d_parent = dentry;
dentry->d_sb = sb;
diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index ca639ed967b7..04b3f5b9c629 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -117,7 +117,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
- seqcount_init(&fs->seq);
+ seqcount_spinlock_init(&fs->seq, &fs->lock);
fs->umask = old->umask;
spin_lock(&old->lock);
@@ -163,6 +163,6 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
- .seq = SEQCNT_ZERO(init_fs.seq),
+ .seq = SEQCNT_SPINLOCK_ZERO(init_fs.seq, &init_fs.lock),
.umask = 0022,
};
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index a81f0c3cf352..65d975bf9390 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -89,7 +89,7 @@ extern struct dentry_stat_t dentry_stat;
struct dentry {
/* RCU lookup touched fields */
unsigned int d_flags; /* protected by d_lock */
- seqcount_t d_seq; /* per dentry seqlock */
+ seqcount_spinlock_t d_seq; /* per dentry seqlock */
struct hlist_bl_node d_hash; /* lookup hash list */
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index cf1015abfbf2..783b48dedb72 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -9,7 +9,7 @@
struct fs_struct {
int users;
spinlock_t lock;
- seqcount_t seq;
+ seqcount_spinlock_t seq;
int umask;
int in_exec;
struct path root, pwd;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
block/blk-iocost.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 8ac4aad66ebc..8e940c27c27c 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -406,7 +406,7 @@ struct ioc {
enum ioc_running running;
atomic64_t vtime_rate;
- seqcount_t period_seqcount;
+ seqcount_spinlock_t period_seqcount;
u32 period_at; /* wallclock starttime */
u64 period_at_vtime; /* vtime starttime */
@@ -873,7 +873,6 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
{
- lockdep_assert_held(&ioc->lock);
WARN_ON_ONCE(ioc->running != IOC_RUNNING);
write_seqcount_begin(&ioc->period_seqcount);
@@ -2001,7 +2000,7 @@ static int blk_iocost_init(struct request_queue *q)
ioc->running = IOC_IDLE;
atomic64_set(&ioc->vtime_rate, VTIME_PER_USEC);
- seqcount_init(&ioc->period_seqcount);
+ seqcount_spinlock_init(&ioc->period_seqcount, &ioc->lock);
ioc->period_at = ktime_to_us(ktime_get());
atomic64_set(&ioc->cur_period, 0);
atomic_set(&ioc->hweight_gen, 0);
--
2.20.1
On Tue, Jun 30, 2020 at 07:44:48AM +0200, Ahmed S. Darwish wrote:
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. A plain seqcount_t does not
> contain the information of which lock must be held when entering a write
> side critical section.
>
> Use the new seqcount_spinlock_t data type, which allows to associate a
> spinlock with the sequence counter. This enables lockdep to verify that
> the spinlock used for writer serialization is held when the write side
> critical section is entered.
>
> If lockdep is disabled this lock association is compiled out and has
> neither storage size nor runtime overhead.
>
> Signed-off-by: Ahmed S. Darwish <[email protected]>
Reviewed-by: Daniel Wagner <[email protected]>
On Tue, Jun 30, 2020 at 07:44:36AM +0200, Ahmed S. Darwish wrote:
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index d74ac0fd6b2d..e5e2e632b749 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1118,6 +1118,7 @@ config PROVE_LOCKING
> select DEBUG_RWSEMS
> select DEBUG_WW_MUTEX_SLOWPATH
> select DEBUG_LOCK_ALLOC
> + select PREEMPT_COUNT if !ARCH_NO_PREEMPT
> select TRACE_IRQFLAGS
> default n
> help
I suspect this can be done unconditional, the thing that requires arch
support is CONFIG_PREEMPTION.
On Tue, Jun 30, 2020 at 07:44:33AM +0200, Ahmed S. Darwish wrote:
> +Sequence counters (:c:type:`seqcount_t`)
> +========================================
> +.. code-block:: c
I so hate RST, of course it's C. Also, ISTR Jon saying you can leave
that all out without issue.
On Mon, 6 Jul 2020 23:04:39 +0200
Peter Zijlstra <[email protected]> wrote:
> On Tue, Jun 30, 2020 at 07:44:33AM +0200, Ahmed S. Darwish wrote:
> > +Sequence counters (:c:type:`seqcount_t`)
> > +========================================
>
> > +.. code-block:: c
>
> I so hate RST, of course it's C. Also, ISTR Jon saying you can leave
> that all out without issue.
The "c" enables keyword coloring and such - something I can happily do
without but others seem to care about it. If you take out that line,
though, you'll need a "::" at least to start a literal block.
jon
On Mon, Jul 06, 2020 at 11:04:39PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 07:44:33AM +0200, Ahmed S. Darwish wrote:
> > +Sequence counters (:c:type:`seqcount_t`)
> > +========================================
>
> > +.. code-block:: c
>
> I so hate RST, of course it's C. Also, ISTR Jon saying you can leave
> that all out without issue.
Something like the below, and then there's all that :ref: nonsense in
that's unreadable gibberish.
Index: linux-2.6/Documentation/locking/seqlock.rst
===================================================================
--- linux-2.6.orig/Documentation/locking/seqlock.rst
+++ linux-2.6/Documentation/locking/seqlock.rst
@@ -33,10 +33,8 @@ class, it can spin forever and the kerne
This mechanism cannot be used if the protected data contains pointers,
as the writer can invalidate a pointer that the reader is following.
-.. _seqcount_t:
-
-Sequence counters (:c:type:`seqcount_t`)
-========================================
+Sequence counters (`seqcount_t`)
+================================
This is the the raw counting mechanism, which does not protect against
multiple writers. Write side critical sections must thus be serialized
@@ -56,8 +54,6 @@ requirements, use a :ref:`sequential loc
Initialization:
-.. code-block:: c
-
/* dynamic */
seqcount_t foo_seqcount;
seqcount_init(&foo_seqcount);
@@ -72,9 +68,6 @@ Initialization:
Write path:
-.. _seqcount_write_ops:
-.. code-block:: c
-
/* Serialized context with disabled preemption */
write_seqcount_begin(&foo_seqcount);
@@ -85,9 +78,6 @@ Write path:
Read path:
-.. _seqcount_read_ops:
-.. code-block:: c
-
do {
seq = read_seqcount_begin(&foo_seqcount);
@@ -95,9 +85,7 @@ Read path:
} while (read_seqcount_retry(&foo_seqcount, seq));
-.. _seqcount_locktype_t:
-
-Sequence counters with associated locks (:c:type:`seqcount_LOCKTYPE_t`)
+Sequence counters with associated locks (`seqcount_LOCKTYPE_t`)
-----------------------------------------------------------------------
As :ref:`earlier discussed <seqcount_t>`, seqcount write side critical
@@ -117,11 +105,11 @@ protection is enforced in the write side
The following seqcounts with associated locks are defined:
- - :c:type:`seqcount_spinlock_t`
- - :c:type:`seqcount_raw_spinlock_t`
- - :c:type:`seqcount_rwlock_t`
- - :c:type:`seqcount_mutex_t`
- - :c:type:`seqcount_ww_mutex_t`
+ - `seqcount_spinlock_t`
+ - `seqcount_raw_spinlock_t`
+ - `seqcount_rwlock_t`
+ - `seqcount_mutex_t`
+ - `seqcount_ww_mutex_t`
The plain seqcount read and write APIs branch out to the specific
seqcount_LOCKTYPE_t implementation at compile-time. This avoids kernel
@@ -129,8 +117,6 @@ API explosion per each new seqcount LOCK
Initialization (replace "LOCKTYPE" with one of the supported locks):
-.. code-block:: c
-
/* dynamic */
seqcount_LOCKTYPE_t foo_seqcount;
seqcount_LOCKTYPE_init(&foo_seqcount, &lock);
@@ -149,9 +135,7 @@ while running from a context with the as
Read path: same as in :ref:`plain seqcount_t <seqcount_read_ops>`.
-.. _seqlock_t:
-
-Sequential locks (:c:type:`seqlock_t`)
+Sequential locks (`seqlock_t`)
======================================
This contains the :ref:`sequence counting mechanism <seqcount_t>`
@@ -164,8 +148,6 @@ halves respectively.
Initialization:
-.. code-block:: c
-
/* dynamic */
seqlock_t foo_seqlock;
seqlock_init(&foo_seqlock);
@@ -180,8 +162,6 @@ Initialization:
Write path:
-.. code-block:: c
-
write_seqlock(&foo_seqlock);
/* ... [[write-side critical section]] ... */
@@ -194,8 +174,6 @@ Read path, three categories:
retry if a writer is in progress by detecting change in the sequence
number. Writers do not wait for a sequence reader.
- .. code-block:: c
-
do {
seq = read_seqbegin(&foo_seqlock);
@@ -208,8 +186,6 @@ Read path, three categories:
from entering its critical section. This read lock is
exclusive. Unlike rwlock_t, only one locking reader can acquire it.
- .. code-block:: c
-
read_seqlock_excl(&foo_seqlock);
/* ... [[read-side critical section]] ... */
@@ -224,8 +200,6 @@ Read path, three categories:
the next iteration marker), the lockless read is transformed to a
full locking read and no retry loop is necessary.
- .. code-block:: c
-
/* marker; even initialization */
int seq = 0;
do {
On Tue, Jun 30, 2020 at 07:44:38AM +0200, Ahmed S. Darwish wrote:
> +#include <linux/seqlock_types_internal.h>
Why? why not include those lines directly here? Having it in a separate
file actually makes it harder to read.
On 2020-07-06 22:50:04 [+0200], Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 07:44:36AM +0200, Ahmed S. Darwish wrote:
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index d74ac0fd6b2d..e5e2e632b749 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -1118,6 +1118,7 @@ config PROVE_LOCKING
> > select DEBUG_RWSEMS
> > select DEBUG_WW_MUTEX_SLOWPATH
> > select DEBUG_LOCK_ALLOC
> > + select PREEMPT_COUNT if !ARCH_NO_PREEMPT
> > select TRACE_IRQFLAGS
> > default n
> > help
>
> I suspect this can be done unconditional, the thing that requires arch
> support is CONFIG_PREEMPTION.
|$ git grep -C 2 PREEMPT_COUNT lib/Kconfig.debug
|lib/Kconfig.debug-config DEBUG_ATOMIC_SLEEP
|lib/Kconfig.debug- bool "Sleep inside atomic section checking"
|lib/Kconfig.debug: select PREEMPT_COUNT
|lib/Kconfig.debug- depends on DEBUG_KERNEL
|lib/Kconfig.debug- depends on !ARCH_NO_PREEMPT
There will be a build fault if you force preempt_count.
Sebastian
On Mon, Jul 06, 2020 at 11:21:48PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 07:44:38AM +0200, Ahmed S. Darwish wrote:
> > +#include <linux/seqlock_types_internal.h>
>
> Why? why not include those lines directly here? Having it in a separate
> file actually makes it harder to read.
>
The seqlock_types_internal.h file contains mostly a heavy sequence of
typeof() branching macros, which is not related to the core logic of
sequence counters or sequential locks:
#define __to_seqcount_t(s)
({
seqcount_t *seq;
if (__same_type(*(s), seqcount_t))
seq = (seqcount_t *)(s);
else if (__same_type(*(s), seqcount_spinlock_t))
seq = &((seqcount_spinlock_t *)(s))->seqcount;
else if (__same_type(*(s), seqcount_raw_spinlock_t))
seq = &((seqcount_raw_spinlock_t *)(s))->seqcount;
else if (__same_type(*(s), seqcount_rwlock_t))
seq = &((seqcount_rwlock_t *)(s))->seqcount;
else if (__same_type(*(s), seqcount_mutex_t))
seq = &((seqcount_mutex_t *)(s))->seqcount;
else if (__same_type(*(s), seqcount_ww_mutex_t))
seq = &((seqcount_ww_mutex_t *)(s))->seqcount;
else
BUILD_BUG_ON_MSG(1, "Unknown seqcount type");
seq;
})
#define __associated_lock_exists_and_is_preemptible(s)
({
bool ret;
/* No associated lock for these */
if (__same_type(*(s), seqcount_t) ||
__same_type(*(s), seqcount_irq_t)) {
ret = false;
} else if (__same_type(*(s), seqcount_spinlock_t) ||
__same_type(*(s), seqcount_raw_spinlock_t) ||
__same_type(*(s), seqcount_rwlock_t)) {
ret = false;
} else if (__same_type(*(s), seqcount_mutex_t) ||
__same_type(*(s), seqcount_ww_mutex_t)) {
ret = true;
} else
BUILD_BUG_ON_MSG(1, "Unknown seqcount type");
ret;
})
#define __assert_seqcount_write_section_is_protected(s)
do {
/* Can't assert anything with plain seqcount_t */
if (__same_type(*(s), seqcount_t))
break;
if (__same_type(*(s), seqcount_spinlock_t))
lockdep_assert_held(((seqcount_spinlock_t *)(s))->lock);
else if (__same_type(*(s), seqcount_raw_spinlock_t))
lockdep_assert_held(((seqcount_raw_spinlock_t *)(s))->lock);
else if (__same_type(*(s), seqcount_rwlock_t))
lockdep_assert_held_write(((seqcount_rwlock_t *)(s))->lock);
else if (__same_type(*(s), seqcount_mutex_t))
lockdep_assert_held(((seqcount_mutex_t *)(s))->lock);
else if (__same_type(*(s), seqcount_ww_mutex_t))
lockdep_assert_held(&((seqcount_ww_mutex_t *)(s))->lock->base);
else if (__same_type(*(s), seqcount_irq_t))
lockdep_assert_irqs_disabled();
else
BUILD_BUG_ON_MSG(1, "Unknown seqcount type");
} while (0)
and so on and so forth. With the final patch that enables PREEMPT_RT
support (not yet submitted upstream), this file gets even more fugly:
#define __rt_lock_unlock_associated_sleeping_lock(s)
do {
if (__same_type(*(s), seqcount_t) ||
__same_type(*(s), seqcount_raw_spinlock_t)) {
break;
}
if (__same_type(*(s), seqcount_spinlock_t)) {
spin_lock(((seqcount_spinlock_t *) s)->lock);
spin_unlock(((seqcount_spinlock_t *) s)->lock);
} else if (__same_type(*(s), seqcount_rwlock_t)) {
read_lock(((seqcount_rwlock_t *) s)->lock);
read_unlock(((seqcount_rwlock_t *) s)->lock);
} else if (__same_type(*(s), seqcount_mutex_t)) {
mutex_lock(((seqcount_mutex_t *) s)->lock);
mutex_unlock(((seqcount_mutex_t *) s)->lock);
} else if (__same_type(*(s), seqcount_ww_mutex_t)) {
ww_mutex_lock(((seqcount_ww_mutex_t *) s)->lock, NULL);
ww_mutex_unlock(((seqcount_ww_mutex_t *) s)->lock);
} else
BUILD_BUG_ON_MSG(1, "Unknown seqcount type");
} while (0)
and even more macros not included here for brevity.
IMHO it makes sense to isolate these "not core logic" parts. Adding all
of this to plain seqlock.h makes it almost unreadable.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Mon, Jul 06, 2020 at 11:04:39PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 07:44:33AM +0200, Ahmed S. Darwish wrote:
> > +Sequence counters (:c:type:`seqcount_t`)
> > +========================================
>
> > +.. code-block:: c
>
> I so hate RST, of course it's C. Also, ISTR Jon saying you can leave
> that all out without issue.
There you go again.
linux$ git grep 'code-block:: c' | wc -l
==> 470
linux$ git grep ':ref:' | wc -l
==> 3171
linux$ git grep ':c:type:' | wc -l
==> 1533
In writing this, Documentation/doc-guide/ was followed, and literally
the thousands of examples above.
But honestly, I don't want to block merging documentation because of
this, and the point of making the docs as readable as possible from text
editors also has merit.
So... I'll just make that file as "RST-lite" as possible.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Tue, Jul 07, 2020 at 12:12:01PM +0200, Ahmed S. Darwish wrote:
> On Mon, Jul 06, 2020 at 11:04:39PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 30, 2020 at 07:44:33AM +0200, Ahmed S. Darwish wrote:
> > > +Sequence counters (:c:type:`seqcount_t`)
> > > +========================================
> >
> > > +.. code-block:: c
> >
> > I so hate RST, of course it's C. Also, ISTR Jon saying you can leave
> > that all out without issue.
>
> There you go again.
>
> linux$ git grep 'code-block:: c' | wc -l
> ==> 470
>
> linux$ git grep ':ref:' | wc -l
> ==> 3171
>
> linux$ git grep ':c:type:' | wc -l
> ==> 1533
>
> In writing this, Documentation/doc-guide/ was followed, and literally
> the thousands of examples above.
Then this is a very bad guide. Jon, why is it recommending such horrible
crap? Should we add another field to MAINTAINERS to indicate RST/TXT
preference? Because I'm getting fed up trying to fix up all this
unreadable gunk.
I'm >< close to writing an script that will unconditionally and silently
convert everything to txt before committing.
> But honestly, I don't want to block merging documentation because of
> this, and the point of making the docs as readable as possible from text
> editors also has merit.
>
> So... I'll just make that file as "RST-lite" as possible.
Thanks!
On Tue, Jul 07, 2020 at 10:40:24AM +0200, Ahmed S. Darwish wrote:
> On Mon, Jul 06, 2020 at 11:21:48PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 30, 2020 at 07:44:38AM +0200, Ahmed S. Darwish wrote:
> > > +#include <linux/seqlock_types_internal.h>
> >
> > Why? why not include those lines directly here? Having it in a separate
> > file actually makes it harder to read.
> >
>
> The seqlock_types_internal.h file contains mostly a heavy sequence of
> typeof() branching macros, which is not related to the core logic of
> sequence counters or sequential locks:
> IMHO it makes sense to isolate these "not core logic" parts. Adding all
> of this to plain seqlock.h makes it almost unreadable.
So I applied it all yesterday and tried to make sense of the resulting
indirections and couldn't find stuff -- because it was hidding in
another file.
Specifically I disliked that *seqcount_t* naming and didn't see how it
all connected.
And that other file is less than 200 lines, which resulted in me
wondering why it needed to be hidden away like that.
Anyway, let me muck around with that a bit.
On Tue, Jul 07, 2020 at 03:04:10PM +0200, Peter Zijlstra wrote:
> Anyway, let me muck around with that a bit.
How's this? it removes a level of indirection and a bunch of repetition.
It doesn't provide SEQCNT_LOCKTYPE_ZERO() for each LOCKTYPE, but you can
use this one macro for any LOCKTYPE.
It's also more than 200 lines shorter.
---
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8b97204f35a7..2b599cf03db4 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
- *
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
- *
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
*
+ * See Documentation/locking/seqlock.rst for full description.
*
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/
#include <linux/spinlock.h>
@@ -41,8 +20,8 @@
#include <asm/processor.h>
/*
- * The seqlock interface does not prescribe a precise sequence of read
- * begin/retry/end. For readers, typically there is a call to
+ * The seqlock seqcount_t interface does not prescribe a precise sequence of
+ * read begin/retry/end. For readers, typically there is a call to
* read_seqcount_begin() and read_seqcount_retry(), however, there are more
* esoteric cases which do not follow this pattern.
*
@@ -56,10 +35,28 @@
#define KCSAN_SEQLOCK_REGION_MAX 1000
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * This is the raw counting mechanism, without any writer protection.
+ *
+ * Write side critical sections must be serialized and non-preemptible.
+ *
+ * If readers can be invoked from hardirq or softirq contexts,
+ * interrupts or bottom halves must also be respectively disabled before
+ * entering the write section.
+ *
+ * This mechanism can't be used if the protected data contains pointers,
+ * as the writer can invalidate a pointer that a reader is following.
+ *
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_t) instead.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
typedef struct seqcount {
unsigned sequence;
@@ -82,6 +79,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
+/**
+ * seqcount_init() - runtime initializer for seqcount_t
+ * @s: Pointer to the &typedef seqcount_t instance
+ */
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
@@ -105,12 +106,122 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
# define seqcount_lockdep_reader_access(x)
#endif
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
+/**
+ * SEQCNT_ZERO() - static initializer for seqcount_t
+ * @name: Name of the &typedef seqcount_t instance
+ */
+#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
+
+/*
+ * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ *
+ * A sequence counter which associates the lock used for writer
+ * serialization at initialization time. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ *
+ * For associated locks which do not implicitly disable preemption,
+ * preemption protection is enforced in the write side function.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
+#ifdef CONFIG_LOCKDEP
+#define __SEQCOUNT_LOCKDEP(expr) expr
+#else
+#define __SEQCOUNT_LOCKDEP(expr)
+#endif
+
+#define SEQCOUNT_LOCKTYPE(name, locktype) \
+typedef struct seqcount_##name { \
+ seqcount_t seqcount; \
+ __SEQCOUNT_LOCKDEP(locktype *lock); \
+} seqcount_##name##_t; \
+ \
+static __always_inline void \
+seqcount_##name##_init(seqcount_##name##_t *s, locktype *l) \
+{ \
+ seqcount_init(&s->seqcount); \
+ __SEQCOUNT_LOCKDEP(s->lock = l); \
+}
+
+#define SEQCNT_LOCKTYPE_ZERO(_name, _lock) { \
+ .seqcount = SEQCNT_ZERO(_name.seqcount), \
+ __SEQCOUNT_LOCKDEP(.lock = (_lock)) \
+}
+
+#include <linux/spinlock.h>
+#include <linux/ww_mutex.h>
+
+SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t)
+SEQCOUNT_LOCKTYPE(spinlock, spinlock_t)
+SEQCOUNT_LOCKTYPE(rwlock, rwlock_t)
+SEQCOUNT_LOCKTYPE(mutex, struct mutex)
+SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex)
+
+#define __to_seqcount_t(s) (seqcount_t *)(s)
+
+/*
+ * seqcount_LOCKTYPE_t -- write APIs
+ *
+ * For associated lock types which do not implicitly disable preemption,
+ * enforce preemption protection in the write side functions.
+ *
+ * Never use lockdep for the raw write variants.
+ */
+
+#define __associated_lock_is_preemptible(s) \
+({ \
+ bool ret; \
+ \
+ if (__same_type(*(s), seqcount_t) || \
+ __same_type(*(s), seqcount_raw_spinlock_t)) { \
+ ret = false; \
+ } else if (__same_type(*(s), seqcount_spinlock_t) || \
+ __same_type(*(s), seqcount_rwlock_t)) { \
+ ret = IS_BUILTIN(CONFIG_PREEMPT_RT); \
+ } else if (__same_type(*(s), seqcount_mutex_t) || \
+ __same_type(*(s), seqcount_ww_mutex_t)) { \
+ ret = true; \
+ } else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+ \
+ ret; \
+})
+
+#ifdef CONFIG_LOCKDEP
+
+#define __assert_associated_lock_held(s) \
+do { \
+ if (__same_type(*(s), seqcount_t)) \
+ break; \
+ \
+ if (__same_type(*(s), seqcount_spinlock_t)) \
+ lockdep_assert_held(((seqcount_spinlock_t *)(s))->lock);\
+ else if (__same_type(*(s), seqcount_raw_spinlock_t)) \
+ lockdep_assert_held(((seqcount_raw_spinlock_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_rwlock_t)) \
+ lockdep_assert_held_write(((seqcount_rwlock_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_mutex_t)) \
+ lockdep_assert_held(((seqcount_mutex_t *)(s))->lock); \
+ else if (__same_type(*(s), seqcount_ww_mutex_t)) \
+ lockdep_assert_held(&((seqcount_ww_mutex_t *)(s))->lock->base); \
+ else \
+ BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
+} while (0)
+
+#else
+
+#define __assert_associated_lock_held(s) \
+do { \
+ (void) __to_seqcount_t(s); \
+} while (0)
+
+#endif /* CONFIG_LOCKDEP */
/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
+ * __read_seqcount_begin() - begin a seqcount read section (without barrier)
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
@@ -121,7 +232,9 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+#define __read_seqcount_begin(s) do___read_seqcount_begin(__to_seqcount_t(s))
+
+static inline unsigned do___read_seqcount_begin(const seqcount_t *s)
{
unsigned ret;
@@ -136,15 +249,18 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
+ * raw_read_seqcount() - Read the seqcount raw counter value
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
+ * seqcount_t, without any lockdep checks and without checking or
+ * masking the sequence counter LSB. Calling code is responsible for
+ * handling that.
*/
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
+#define raw_read_seqcount(s) do_raw_read_seqcount(__to_seqcount_t(s))
+
+static inline unsigned do_raw_read_seqcount(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -153,42 +269,46 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
+ * raw_read_seqcount_begin() - start a seqcount read section w/o lockdep
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount_begin opens a read critical section of the given
- * seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
+ * seqcount_t, but without any lockdep checking. Validity of the read
+ * section must be checked with read_seqcount_retry().
*/
-static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+#define raw_read_seqcount_begin(s) do_raw_read_seqcount_begin(__to_seqcount_t(s))
+
+static inline unsigned do_raw_read_seqcount_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
+ unsigned ret = do___read_seqcount_begin(s);
smp_rmb();
return ret;
}
/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
+ * read_seqcount_begin() - start a seqcount read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * read_seqcount_begin opens a read critical section of the given
+ * seqcount_t. Validity of the read section must be checked with
+ * read_seqcount_retry().
*/
-static inline unsigned read_seqcount_begin(const seqcount_t *s)
+#define read_seqcount_begin(s) do_read_seqcount_begin(__to_seqcount_t(s))
+
+static inline unsigned do_read_seqcount_begin(const seqcount_t *s)
{
seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
+ return do_raw_read_seqcount_begin(s);
}
/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
+ * raw_seqcount_begin() - begin a seq-read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
- * raw_seqcount_begin opens a read critical section of the given seqcount.
+ * raw_seqcount_begin opens a read critical section of the given seqcount_t.
* Validity of the critical section is tested by checking read_seqcount_retry
* function.
*
@@ -197,7 +317,9 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
* read_seqcount_retry() instead of stabilizing at the beginning of the
* critical section.
*/
-static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+#define raw_seqcount_begin(s) do_raw_seqcount_begin(__to_seqcount_t(s))
+
+static inline unsigned do_raw_seqcount_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -206,8 +328,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
}
/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
+ * __read_seqcount_retry() - end a seq-read critical section (without barrier)
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -219,38 +341,56 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define __read_seqcount_retry(s, start) do___read_seqcount_retry(__to_seqcount_t(s), start)
+
+static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
{
kcsan_atomic_next(0);
return unlikely(READ_ONCE(s->sequence) != start);
}
/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
+ * read_seqcount_retry() - end a seq-read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
- * read_seqcount_retry closes a read critical section of the given seqcount.
+ * read_seqcount_retry closes a read critical section of given seqcount_t.
* If the critical section was invalid, it must be ignored (and typically
* retried).
*/
-static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define read_seqcount_retry(s, start) do_read_seqcount_retry(__to_seqcount_t(s), start)
+
+static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return do___read_seqcount_retry(s, start);
}
+#define raw_write_seqcount_begin(s) \
+do { \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ do_raw_write_seqcount_begin(__to_seqcount_t(s)); \
+} while (0)
-
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+static inline void do_raw_write_seqcount_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
}
-static inline void raw_write_seqcount_end(seqcount_t *s)
+#define raw_write_seqcount_end(s) \
+do { \
+ do_raw_write_seqcount_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void do_raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
@@ -258,12 +398,12 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
}
/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_barrier() - do a seq write barrier
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
- * collapse the two back-to-back wmb()s.
+ * collapse the two back-to-back wmb()s::
*
* Note that writes surrounding the barrier should be declared atomic (e.g.
* via WRITE_ONCE): a) to ensure the writes become visible to other threads
@@ -298,7 +438,9 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* WRITE_ONCE(X, false);
* }
*/
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
+#define raw_write_seqcount_barrier(s) do_raw_write_seqcount_barrier(__to_seqcount_t(s))
+
+static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -307,7 +449,24 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
kcsan_nestable_atomic_end();
}
-static inline int raw_read_seqcount_latch(seqcount_t *s)
+/**
+ * raw_read_seqcount_latch() - pick even or odd seqcount latch data copy
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ *
+ * Use seqcount latching to switch between two storage places with
+ * sequence protection to allow interruptible, preemptible, writer
+ * sections.
+ *
+ * Check raw_write_seqcount_latch() for more details and a full reader
+ * and writer usage example.
+ *
+ * Return: sequence counter. Use the lowest bit as index for picking
+ * which data copy to read. Full counter must then be checked with
+ * read_seqcount_retry().
+ */
+#define raw_read_seqcount_latch(s) do_raw_read_seqcount_latch(__to_seqcount_t(s))
+
+static inline int do_raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
@@ -315,8 +474,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
}
/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_latch() - redirect readers to even/odd copy
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -332,101 +491,164 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* Very simply put: we first modify one copy and then the other. This ensures
* there is always one copy in a stable state, ready to give us an answer.
*
- * The basic form is a data structure like:
+ * The basic form is a data structure like::
*
- * struct latch_struct {
- * seqcount_t seq;
- * struct data_struct data[2];
- * };
+ * struct latch_struct {
+ * seqcount_t seq;
+ * struct data_struct data[2];
+ * };
*
* Where a modification, which is assumed to be externally serialized, does the
- * following:
+ * following::
*
- * void latch_modify(struct latch_struct *latch, ...)
- * {
- * smp_wmb(); <- Ensure that the last data[1] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * void latch_modify(struct latch_struct *latch, ...)
+ * {
+ * smp_wmb(); // Ensure that the last data[1] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[0], ...);
+ * modify(latch->data[0], ...);
*
- * smp_wmb(); <- Ensure that the data[0] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * smp_wmb(); // Ensure that the data[0] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[1], ...);
- * }
+ * modify(latch->data[1], ...);
+ * }
*
- * The query will have a form like:
+ * The query will have a form like::
*
- * struct entry *latch_query(struct latch_struct *latch, ...)
- * {
- * struct entry *entry;
- * unsigned seq, idx;
+ * struct entry *latch_query(struct latch_struct *latch, ...)
+ * {
+ * struct entry *entry;
+ * unsigned seq, idx;
*
- * do {
- * seq = raw_read_seqcount_latch(&latch->seq);
+ * do {
+ * seq = raw_read_seqcount_latch(&latch->seq);
*
- * idx = seq & 0x01;
- * entry = data_query(latch->data[idx], ...);
+ * idx = seq & 0x01;
+ * entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes necessary smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq);
*
- * return entry;
- * }
+ * return entry;
+ * }
*
* So during the modification, queries are first redirected to data[1]. Then we
* modify data[0]. When that is complete, we redirect queries back to data[0]
* and we can modify data[1].
*
- * NOTE: The non-requirement for atomic modifications does _NOT_ include
- * the publishing of new entries in the case where data is a dynamic
- * data structure.
+ * NOTE:
+ *
+ * The non-requirement for atomic modifications does _NOT_ include
+ * the publishing of new entries in the case where data is a dynamic
+ * data structure.
*
- * An iteration might start in data[0] and get suspended long enough
- * to miss an entire modification sequence, once it resumes it might
- * observe the new entry.
+ * An iteration might start in data[0] and get suspended long enough
+ * to miss an entire modification sequence, once it resumes it might
+ * observe the new entry.
*
- * NOTE: When data is a dynamic data structure; one should use regular RCU
- * patterns to manage the lifetimes of the objects within.
+ * NOTE:
+ *
+ * When data is a dynamic data structure; one should use regular RCU
+ * patterns to manage the lifetimes of the objects within.
*/
-static inline void raw_write_seqcount_latch(seqcount_t *s)
+#define raw_write_seqcount_latch(s) do_raw_write_seqcount_latch(__to_seqcount_t(s))
+
+static inline void do_raw_write_seqcount_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
smp_wmb(); /* increment "sequence" before following stores */
}
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ do_raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
+#define write_seqcount_begin_nested(s, subclass) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ do_write_seqcount_begin_nested(__to_seqcount_t(s), subclass); \
+} while (0)
+
+static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+// lockdep_assert_preemption_disabled();
+ __write_seqcount_begin_nested(s, subclass);
+}
+
/*
- * Sequence counter only version assumes that callers are using their
- * own mutexing.
+ * write_seqcount_t_begin() without lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption is
+ * already disabled. For example, seqlock_t write side functions.
*/
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+static inline void __write_seqcount_begin(seqcount_t *s)
{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+ __write_seqcount_begin_nested(s, 0);
}
-static inline void write_seqcount_begin(seqcount_t *s)
+/**
+ * write_seqcount_begin() - start a seqcount write-side critical section
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ *
+ * write_seqcount_begin opens a write-side critical section of the given
+ * seqcount. Seqcount write-side critical sections must be externally
+ * serialized and non-preemptible.
+ */
+#define write_seqcount_begin(s) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ do_write_seqcount_begin(__to_seqcount_t(s)); \
+} while (0)
+
+static inline void do_write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
-static inline void write_seqcount_end(seqcount_t *s)
+/**
+ * write_seqcount_end() - end a seqcount_t write-side critical section
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * The write section must've been opened with write_seqcount_begin().
+ */
+#define write_seqcount_end(s) \
+do { \
+ do_write_seqcount_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void do_write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
raw_write_seqcount_end(s);
}
/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
+ * write_seqcount_invalidate() - invalidate in-progress read-side seq operations
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
*/
-static inline void write_seqcount_invalidate(seqcount_t *s)
+#define write_seqcount_invalidate(s) do_write_seqcount_invalidate(__to_seqcount_t(s))
+
+static inline void do_write_seqcount_invalidate(seqcount_t *s)
{
smp_wmb();
kcsan_nestable_atomic_begin();
@@ -434,42 +656,74 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * For more info, see:
+ * - Comments on top of seqcount_t
+ * - Documentation/locking/seqlock.rst
+ */
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+/**
+ * seqlock_init() - dynamic initializer for seqlock_t
+ * @sl: Pointer to the &typedef seqlock_t instance
+ */
+#define seqlock_init(sl) \
do { \
- seqcount_init(&(x)->seqcount); \
- spin_lock_init(&(x)->lock); \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+/**
+ * DEFINE_SEQLOCK() - Define a statically-allocated seqlock_t
+ * @sl: Name of the &typedef seqlock_t instance
+ */
+#define DEFINE_SEQLOCK(sl) \
+ seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
-/*
- * Read side functions for starting and finalizing a read side section.
+/**
+ * read_seqbegin() - start a seqlock_t read-side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqbegin opens a read side critical section of the given
+ * seqlock_t. Validity of the critical section is tested by checking
+ * read_seqretry().
+ *
+ * Return: count to be passed to read_seqretry()
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
+ unsigned ret = do_read_seqcount_begin(&sl->seqcount);
kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
kcsan_flat_atomic_begin();
return ret;
}
+/**
+ * read_seqretry() - end and validate a seqlock_t read side section
+ * @sl: Pointer to &typedef seqlock_t
+ * @start: count, from read_seqbegin()
+ *
+ * read_seqretry closes the given seqlock_t read side critical section,
+ * and checks its validity. If the read section was invalid, it must be
+ * ignored and retried.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
/*
@@ -478,47 +732,94 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
*/
kcsan_flat_atomic_end();
- return read_seqcount_retry(&sl->seqcount, start);
+ return do_read_seqcount_retry(&sl->seqcount, start);
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * write_seqlock() - start a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_seqlock opens a write side critical section of the given
+ * seqlock_t. It also acquires the spinlock_t embedded inside the
+ * sequential lock. All the seqlock_t write side critical sections are
+ * thus automatically serialized and non-preemptible.
+ *
+ * Use the ``_irqsave`` and ``_bh`` variants instead if the read side
+ * can be invoked from a hardirq or softirq context.
+ *
+ * The opened write side section must be closed with write_sequnlock().
*/
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock() - end a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock closes the (serialized and non-preemptible) write
+ * side critical section of given seqlock_t.
+ */
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
+/**
+ * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of write_seqlock(). Use only if the read side section
+ * can be invoked from a softirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_bh().
+ */
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock_bh closes the serialized, non-preemptible,
+ * softirqs-disabled, seqlock_t write side critical section opened with
+ * write_seqlock_bh().
+ */
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
+/**
+ * write_seqlock_irq() - start a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * This is the ``_irq`` variant of write_seqlock(). Use only if the read
+ * section of given seqlock_t can be invoked from a hardirq context.
+ */
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_irq() - end a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of write_sequnlock(). The write side section of
+ * given seqlock_t must've been opened with write_seqlock_irq().
+ */
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -527,44 +828,98 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
+
return flags;
}
+/**
+ * write_seqlock_irqsave() - start a non-interruptible seqlock_t write section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to write_sequnlock_irqrestore().
+ *
+ * ``_irqsave`` variant of write_seqlock(). Use if the read section of
+ * given seqlock_t can be invoked from a hardirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_irqrestore().
+ */
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
+/**
+ * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
+ *
+ * ``_irqrestore`` variant of write_sequnlock(). The write section of
+ * given seqlock_t must've been opened with write_seqlock_irqsave().
+ */
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * read_seqlock_excl() - begin a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqlock_excl opens a locking reader critical section for the
+ * given seqlock_t. A locking reader exclusively locks out other writers
+ * and other *locking* readers, but doesn't update the sequence number.
+ *
+ * Locking readers act like a normal spin_lock()/spin_unlock().
+ *
+ * The opened read side section must be closed with read_sequnlock_excl().
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
+/**
+ * read_sequnlock_excl() - end a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_sequnlock_excl closes the locking reader critical section opened
+ * with read_seqlock_excl().
+ */
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
+ * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq : Marker and return parameter. If the passed value is even, the
+ * reader will become a *lockless* seqlock_t sequence counter reader as
+ * in read_seqbegin(). If the passed value is odd, the reader will
+ * become a fully locking reader, as in read_seqlock_excl(). In the
+ * first call to read_seqbegin_or_lock(), the caller **must** initialize
+ * and pass an even value to @seq so a lockless read is optimistically
+ * tried first.
+ *
+ * read_seqbegin_or_lock is an API designed to optimistically try a
+ * normal lockless seqlock_t read section first, as in read_seqbegin().
+ * If an odd counter is found, the normal lockless read trial has
+ * failed, and the next reader iteration transforms to a full seqlock_t
+ * locking reader as in read_seqlock_excl().
*
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
+ * This is typically used to avoid lockless seqlock_t readers starvation
+ * (too much retry loops) in the case of a sharp spike in write
+ * activity.
+ *
+ * The opened read section must be closed with done_seqretry(). Check
+ * Documentation/locking/seqlock.rst for template example code.
+ *
+ * Return: The encountered sequence counter value, returned through the
+ * @seq parameter, which is overloaded as a return parameter. The
+ * returned value must be checked with need_seqretry(). If the read
+ * section must be retried, the returned value must also be passed to
+ * the @seq parameter of the next read_seqbegin_or_lock() iteration.
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
@@ -574,32 +929,90 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
read_seqlock_excl(lock);
}
+/**
+ * need_seqretry() - validate seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * need_seqretry checks if the seqlock_t read-side critical section
+ * started with read_seqbegin_or_lock() is valid. If it was not, the
+ * caller must retry the read-side section.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
+/**
+ * done_seqretry() - end seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * done_seqretry finishes the seqlock_t read side critical section
+ * started by read_seqbegin_or_lock(). The read section must've been
+ * already validated with need_seqretry().
+ */
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
+/**
+ * read_seqlock_excl_bh() - start a locking reader seqlock_t section
+ * with softirqs disabled
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of read_seqlock_excl(). Use this variant if the
+ * seqlock_t write side section, *or other read sections*, can be
+ * invoked from a softirq context
+ *
+ * The opened section must be closed with read_sequnlock_excl_bh().
+ */
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
+/**
+ * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of read_sequnlock_excl(). The closed section must've
+ * been opened with read_seqlock_excl_bh().
+ */
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
+/**
+ * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of read_seqlock_excl(). Use this only if the
+ * seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from a hardirq context.
+ *
+ * The opened read section must be closed with read_sequnlock_excl_irq().
+ */
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
+/**
+ * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of read_sequnlock_excl(). The closed section must've
+ * been opened with read_seqlock_excl_irq().
+ */
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
@@ -613,15 +1026,59 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
+ * locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to read_sequnlock_excl_irqrestore().
+ *
+ * ``_irqsave`` variant of read_seqlock_excl(). Use this only if the
+ * seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from a hardirq context.
+ *
+ * Opened section must be closed with read_sequnlock_excl_irqrestore().
+ */
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+/**
+ * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from
+ * read_seqlock_excl_irqsave()
+ *
+ * ``_irqrestore`` variant of read_sequnlock_excl(). The closed section
+ * must've been opened with read_seqlock_excl_irqsave().
+ */
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
spin_unlock_irqrestore(&sl->lock, flags);
}
+/**
+ * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
+ * a non-interruptible locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
+ *
+ * This is the ``_irqsave`` variant of read_seqbegin_or_lock(). Use if
+ * the seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from hardirq context.
+ *
+ * The validity of the read section must be checked with need_seqretry().
+ * The opened section must be closed with done_seqretry_irqrestore().
+ *
+ * Return:
+ *
+ * 1. The saved local interrupts state in case of a locking reader, to be
+ * passed to done_seqretry_irqrestore().
+ *
+ * 2. The encountered sequence counter value, returned through @seq which
+ * is overloaded as a return parameter. Check read_seqbegin_or_lock().
+ */
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
@@ -635,6 +1092,18 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
return flags;
}
+/**
+ * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
+ * non-interruptible locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Count, from read_seqbegin_or_lock_irqsave()
+ * @flags: Caller's saved local interrupt state in case of a locking
+ * reader, also from read_seqbegin_or_lock_irqsave()
+ *
+ * This is the ``_irqrestore`` variant of done_seqretry(). The read
+ * section must've been opened with read_seqbegin_or_lock_irqsave(), and
+ * validated with need_seqretry().
+ */
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
On Tue, Jul 07, 2020 at 04:37:26PM +0200, Peter Zijlstra wrote:
> + * Use the ``_irqsave`` and ``_bh`` variants instead if the read side
> + * ``_bh`` variant of write_seqlock(). Use only if the read side section
> + * ``_irq`` variant of write_sequnlock(). The write side section of
> + * ``_irqsave`` variant of write_seqlock(). Use if the read section of
> + * ``_irqrestore`` variant of write_sequnlock(). The write section of
> + * ``_bh`` variant of read_seqlock_excl(). Use this variant if the
> + * ``_bh`` variant of read_sequnlock_excl(). The closed section must've
> + * ``_irq`` variant of read_seqlock_excl(). Use this only if the
> + * ``_irq`` variant of read_sequnlock_excl(). The closed section must've
> + * ``_irqsave`` variant of read_seqlock_excl(). Use this only if the
> + * ``_irqrestore`` variant of read_sequnlock_excl(). The closed section
> + * This is the ``_irqsave`` variant of read_seqbegin_or_lock(). Use if
> + * This is the ``_irqrestore`` variant of done_seqretry(). The read
Can we also get rid of that '' nonsense, the gods of ASCII gifted us "
for this.
On Tue, Jul 07, 2020 at 04:37:26PM +0200, Peter Zijlstra wrote:
>
> How's this? it removes a level of indirection and a bunch of repetition.
ACK, for the extra level of indirection removed.
> It's also more than 200 lines shorter.
...
> +#define __to_seqcount_t(s) (seqcount_t *)(s)
...
> +#define read_seqcount_begin(s) do_read_seqcount_begin(__to_seqcount_t(s))
> +
> +static inline unsigned do_read_seqcount_begin(const seqcount_t *s)
> +{
...
Hmm, the __to_seqcount_t(s) force cast is not good. It will break the
arguments type-safety of seqcount macros that do not have either:
__associated_lock_is_preemptible() or
__assert_associated_lock_held()
in their path. This basically includes all the read path macros, and
even some others (e.g. write_seqcount_invalidate()).
With the suggested force cast above, I can literally *pass anything* to
read_seqcount_begin() and friends, and the compiler won't say a thing.
So, I'll restore __to_seqcount_t(s) that to its original implementation:
/*
* @s: pointer to seqcount_t or any of the seqcount_locktype_t variants
*/
#define __to_seqcount_t(s) \
({ \
seqcount_t *seq; \
\
if (__same_type(*(s), seqcount_t)) \
seq = (seqcount_t *)(s); \
else if (__same_type(*(s), seqcount_spinlock_t)) \
seq = &((seqcount_spinlock_t *)(s))->seqcount; \
else if (__same_type(*(s), seqcount_raw_spinlock_t)) \
seq = &((seqcount_raw_spinlock_t *)(s))->seqcount; \
else if (__same_type(*(s), seqcount_rwlock_t)) \
seq = &((seqcount_rwlock_t *)(s))->seqcount; \
else if (__same_type(*(s), seqcount_mutex_t)) \
seq = &((seqcount_mutex_t *)(s))->seqcount; \
else if (__same_type(*(s), seqcount_ww_mutex_t)) \
seq = &((seqcount_ww_mutex_t *)(s))->seqcount; \
else \
BUILD_BUG_ON_MSG(1, "Unknown seqcount type"); \
\
seq; \
})
Yes, I know, it's not the prettiest thing in the world, but I'd take
ugly over breaking the compiler type checks any day.
>
> It doesn't provide SEQCNT_LOCKTYPE_ZERO() for each LOCKTYPE, but you can
> use this one macro for any LOCKTYPE.
>
From call-sites perspectives, SEQCNT_SPINLOCK_ZERO() is much more
readable than SEQCNT_LOCKTYPE_ZERO() and so on. It only costs us 5 lines
*for all* the seqcount lock types. IMHO it's worth the trade-off.
>
> So I applied it all yesterday and tried to make sense of the resulting
> indirections and couldn't find stuff -- because it was hidding in
> another file.
>
> Specifically I disliked that *seqcount_t* naming and didn't see how it
> all connected.
>
Hmm, the idea was that write_seqcount_t_begin() and friends applies only
to plain old "seqcount_t". But, yes, I agree, it's confusing as hell.
The way you've organized the macros is much more readable, so thanks a
lot, I'll incorporate that in v4.
Kind regards,
--
Ahmed S. Darwish
Linutronix GmbH
On Wed, Jul 08, 2020 at 11:12:01AM +0200, Peter Zijlstra wrote:
> On Tue, Jul 07, 2020 at 04:37:26PM +0200, Peter Zijlstra wrote:
> > + * Use the ``_irqsave`` and ``_bh`` variants instead if the read side
> > + * ``_bh`` variant of write_seqlock(). Use only if the read side section
> > + * ``_irq`` variant of write_sequnlock(). The write side section of
> > + * ``_irqsave`` variant of write_seqlock(). Use if the read section of
> > + * ``_irqrestore`` variant of write_sequnlock(). The write section of
> > + * ``_bh`` variant of read_seqlock_excl(). Use this variant if the
> > + * ``_bh`` variant of read_sequnlock_excl(). The closed section must've
> > + * ``_irq`` variant of read_seqlock_excl(). Use this only if the
> > + * ``_irq`` variant of read_sequnlock_excl(). The closed section must've
> > + * ``_irqsave`` variant of read_seqlock_excl(). Use this only if the
> > + * ``_irqrestore`` variant of read_sequnlock_excl(). The closed section
> > + * This is the ``_irqsave`` variant of read_seqbegin_or_lock(). Use if
> > + * This is the ``_irqrestore`` variant of done_seqretry(). The read
>
> Can we also get rid of that '' nonsense, the gods of ASCII gifted us "
> for this.
Hehe, why not. I welcome back our ASCII overlords.
On Wed, Jul 08, 2020 at 12:33:14PM +0200, Ahmed S. Darwish wrote:
> > +#define read_seqcount_begin(s) do_read_seqcount_begin(__to_seqcount_t(s))
> > +
> > +static inline unsigned do_read_seqcount_begin(const seqcount_t *s)
> > +{
> ...
>
> Hmm, the __to_seqcount_t(s) force cast is not good. It will break the
> arguments type-safety of seqcount macros that do not have either:
>
> __associated_lock_is_preemptible() or
> __assert_associated_lock_held()
>
> in their path. This basically includes all the read path macros, and
> even some others (e.g. write_seqcount_invalidate()).
>
> With the suggested force cast above, I can literally *pass anything* to
> read_seqcount_begin() and friends, and the compiler won't say a thing.
>
> So, I'll restore __to_seqcount_t(s) that to its original implementation:
Right, I figured that the write side would be enough to catch glaring
abuse. But sure.
It's a bummer we didn't upgrade the minimum compiler version to 4.9,
that would've given us _Generic(), which allows one to write this
slightly less verbose I think.
How about something disguisting like this then?
#ifdef CONFIG_LOCKDEP
#define __SEQCOUNT_LOCKDEP(expr) expr
#else
#define __SEQCOUNT_LOCKDEP(expr)
#endif
typedef seqcount_t * __seqprop_ptr_t;
typedef bool __seqprop_preempt_t;
typedef void __seqprop_assert_t;
static __always_inline __seqprop_ptr_t
__seqprop_seqcount_ptr(seqcount_t *s)
{
return s;
}
static __always_inline __seqprop_preempt_t
__seqprop_seqcount_preempt(seqcount_t *s)
{
return false;
}
static __always_inline __seqprop_assert_t
__seqprop_seqcount_assert(seqcount_t *s)
{
}
#define SEQCOUNT_LOCKTYPE(name, locktype, preempt, lockmember) \
typedef struct seqcount_##name { \
seqcount_t seqcount; \
__SEQCOUNT_LOCKDEP(locktype *lock); \
} seqcount_##name##_t; \
\
static __always_inline void \
seqcount_##name##_init(seqcount_##name##_t *s, locktype *l) \
{ \
seqcount_init(&s->seqcount); \
__SEQCOUNT_LOCKDEP(s->lock = l); \
} \
\
static __always_inline __seqprop_ptr_t \
__seqprop_##name##_ptr(seqcount_##name##_t *s) \
{ \
return &s->seqcount; \
} \
\
static __always_inline __seqprop_preempt_t \
__seqprop_##name##_preempt(seqcount_##name##_t *s) \
{ \
return preempt; \
} \
\
static __always_inline __seqprop_assert_t \
__seqprop_##name##_assert(seqcount_##name##_t *s) \
{ \
__SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember)); \
}
#define SEQCNT_LOCKTYPE_ZERO(_name, _lock) { \
.seqcount = SEQCNT_ZERO(_name.seqcount), \
__SEQCOUNT_LOCKDEP(.lock = (_lock)) \
}
#include <linux/spinlock.h>
#include <linux/ww_mutex.h>
#define __SEQ_RT IS_BUILTIN(CONFIG_PREEMPT_RT)
SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, lock)
SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, lock)
SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, lock)
SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, lock)
SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex,true, lock->base)
#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900) || defined(__CHECKER__)
#define __seqprop_pick(const_expr, s, locktype, prop, otherwise) \
__builtin_choose_expr(const_expr, \
__seqprop_##locktype##_##prop((void *)(s)), \
otherwise)
extern void __seqprop_invalid(void);
#define __seqprop(s, prop) \
__seqprop_pick(__same_type(*(s), seqcount_t), (s), seqcount, prop, \
__seqprop_pick(__same_type(*(s), seqcount_raw_spinlock_t), (s), raw_spinlock, prop, \
__seqprop_pick(__same_type(*(s), seqcount_spinlock_t), (s), spinlock, prop, \
__seqprop_pick(__same_type(*(s), seqcount_rwlock_t), (s), rwlock, prop, \
__seqprop_pick(__same_type(*(s), seqcount_mutex_t), (s), mutex, prop, \
__seqprop_pick(__same_type(*(s), seqcount_ww_mutex_t), (s), ww_mutex, prop, \
__seqprop_invalid()))))))
#else
#define __seqprop_case(s, locktype, prop) \
seqcount_##locktype##_t: __seqprop_##locktype##_##prop((void *)s)
#define __seqprop(s, prop) \
_Generic(*(s), \
seqcount_t: __seqprop_seqcount_##prop((void*)s),\
__seqprop_case((s), raw_spinlock, prop), \
__seqprop_case((s), spinlock, prop), \
__seqprop_case((s), rwlock, prop), \
__seqprop_case((s), mutex, prop), \
__seqprop_case((s), ww_mutex, prop))
#endif
#define __to_seqcount_t(s) __seqprop(s, ptr)
#define __associated_lock_is_preemptible(s) __seqprop(s, preempt)
#define __assert_associated_lock_held(s) __seqprop(s, assert)
On Wed, Jul 08, 2020 at 02:29:38PM +0200, Peter Zijlstra wrote:
> #define SEQCOUNT_LOCKTYPE(name, locktype, preempt, lockmember) \
> typedef struct seqcount_##name { \
> seqcount_t seqcount; \
> __SEQCOUNT_LOCKDEP(locktype *lock); \
> } seqcount_##name##_t; \
> \
> static __always_inline void \
> seqcount_##name##_init(seqcount_##name##_t *s, locktype *l) \
> { \
> seqcount_init(&s->seqcount); \
> __SEQCOUNT_LOCKDEP(s->lock = l); \
> } \
> \
> static __always_inline __seqprop_ptr_t \
> __seqprop_##name##_ptr(seqcount_##name##_t *s) \
> { \
> return &s->seqcount; \
> } \
> \
> static __always_inline __seqprop_preempt_t \
> __seqprop_##name##_preempt(seqcount_##name##_t *s) \
> { \
> return preempt; \
> } \
> \
> static __always_inline __seqprop_assert_t \
> __seqprop_##name##_assert(seqcount_##name##_t *s) \
> { \
> __SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember)); \
> }
For PREEMPT_RT's magic thing, you can add:
static __always_inline void \
__seqprop_##name##_lock(seqcount_##name##_t *s) \
{ \
if (!__SEQ_RT || !preempt) \
return; \
\
lockbase##_lock(&s->lock); \
lockbase##_unlock(&s->lock); \
}
and:
#define __rt_lock_unlock_associated_sleeping_lock(s) __seqprop(s, lock)
On Wed, Jul 08, 2020 at 04:13:32PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 08, 2020 at 02:29:38PM +0200, Peter Zijlstra wrote:
>
> > #define SEQCOUNT_LOCKTYPE(name, locktype, preempt, lockmember) \
> > typedef struct seqcount_##name { \
> > seqcount_t seqcount; \
> > __SEQCOUNT_LOCKDEP(locktype *lock); \
> > } seqcount_##name##_t; \
> > \
> > static __always_inline void \
> > seqcount_##name##_init(seqcount_##name##_t *s, locktype *l) \
> > { \
> > seqcount_init(&s->seqcount); \
> > __SEQCOUNT_LOCKDEP(s->lock = l); \
> > } \
> > \
> > static __always_inline __seqprop_ptr_t \
> > __seqprop_##name##_ptr(seqcount_##name##_t *s) \
> > { \
> > return &s->seqcount; \
> > } \
> > \
> > static __always_inline __seqprop_preempt_t \
> > __seqprop_##name##_preempt(seqcount_##name##_t *s) \
> > { \
> > return preempt; \
> > } \
> > \
> > static __always_inline __seqprop_assert_t \
> > __seqprop_##name##_assert(seqcount_##name##_t *s) \
> > { \
> > __SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember)); \
> > }
>
> For PREEMPT_RT's magic thing, you can add:
>
> static __always_inline void \
> __seqprop_##name##_lock(seqcount_##name##_t *s) \
> { \
> if (!__SEQ_RT || !preempt) \
> return; \
> \
> lockbase##_lock(&s->lock); \
> lockbase##_unlock(&s->lock); \
> }
>
> and:
>
> #define __rt_lock_unlock_associated_sleeping_lock(s) __seqprop(s, lock)
Or possible have it like:
if (!__SEQ_RT || !preempt)
return smp_cond_load_relaxed(&s->seqcount->sequence, !(VAL & 1));
lockbase##_lock(&s->lock);
lockbase##_unlock(&s->lock);
return READ_ONCE(s->seqcount->sequence);
and then replace most of __read_seqcount_begin() with it.
On Wed, Jul 08, 2020 at 02:29:38PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 08, 2020 at 12:33:14PM +0200, Ahmed S. Darwish wrote:
>
> > > +#define read_seqcount_begin(s) do_read_seqcount_begin(__to_seqcount_t(s))
> > > +
> > > +static inline unsigned do_read_seqcount_begin(const seqcount_t *s)
> > > +{
> > ...
> >
> > Hmm, the __to_seqcount_t(s) force cast is not good. It will break the
> > arguments type-safety of seqcount macros that do not have either:
> >
> > __associated_lock_is_preemptible() or
> > __assert_associated_lock_held()
> >
> > in their path. This basically includes all the read path macros, and
> > even some others (e.g. write_seqcount_invalidate()).
> >
> > With the suggested force cast above, I can literally *pass anything* to
> > read_seqcount_begin() and friends, and the compiler won't say a thing.
> >
> > So, I'll restore __to_seqcount_t(s) that to its original implementation:
>
> Right, I figured that the write side would be enough to catch glaring
> abuse. But sure.
>
> It's a bummer we didn't upgrade the minimum compiler version to 4.9,
> that would've given us _Generic(), which allows one to write this
> slightly less verbose I think.
>
Looking at 5429ef62bcf3 ("compiler/gcc: Raise minimum GCC version for
kernel builds to 4.8"), it seems that the decision of picking gcc 4.8
vs. 4.9 was kinda arbitrary.
Anyway, please continue below.
> How about something disguisting like this then?
>
...
> #define __SEQ_RT IS_BUILTIN(CONFIG_PREEMPT_RT)
>
> SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, lock)
> SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, lock)
> SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, lock)
> SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, lock)
> SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, lock->base)
>
> #if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900) || defined(__CHECKER__)
>
> #define __seqprop_pick(const_expr, s, locktype, prop, otherwise) \
> __builtin_choose_expr(const_expr, \
> __seqprop_##locktype##_##prop((void *)(s)), \
> otherwise)
>
> extern void __seqprop_invalid(void);
>
> #define __seqprop(s, prop) \
> __seqprop_pick(__same_type(*(s), seqcount_t), (s), seqcount, prop, \
> __seqprop_pick(__same_type(*(s), seqcount_raw_spinlock_t), (s), raw_spinlock, prop, \
> __seqprop_pick(__same_type(*(s), seqcount_spinlock_t), (s), spinlock, prop, \
> __seqprop_pick(__same_type(*(s), seqcount_rwlock_t), (s), rwlock, prop, \
> __seqprop_pick(__same_type(*(s), seqcount_mutex_t), (s), mutex, prop, \
> __seqprop_pick(__same_type(*(s), seqcount_ww_mutex_t), (s), ww_mutex, prop, \
> __seqprop_invalid()))))))
>
> #else
>
> #define __seqprop_case(s, locktype, prop) \
> seqcount_##locktype##_t: __seqprop_##locktype##_##prop((void *)s)
>
> #define __seqprop(s, prop) \
> _Generic(*(s), \
> seqcount_t: __seqprop_seqcount_##prop((void*)s),\
> __seqprop_case((s), raw_spinlock, prop), \
> __seqprop_case((s), spinlock, prop), \
> __seqprop_case((s), rwlock, prop), \
> __seqprop_case((s), mutex, prop), \
> __seqprop_case((s), ww_mutex, prop))
>
> #endif
>
> #define __to_seqcount_t(s) __seqprop(s, ptr)
> #define __associated_lock_is_preemptible(s) __seqprop(s, preempt)
> #define __assert_associated_lock_held(s) __seqprop(s, assert)
Hmm, I'll prototype the whole thing (along with PREEMPT_RT associated
lock()/unlock() as you've mentioned in the other e-mail), and come back.
Honestly, I have a first impression that this is heading into too much
complexity and compaction, but let's finish the whole thing first.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Wed, Jul 08, 2020 at 05:09:30PM +0200, Ahmed S. Darwish wrote:
> On Wed, Jul 08, 2020 at 02:29:38PM +0200, Peter Zijlstra wrote:
> > How about something disguisting like this then?
> >
> ...
> > #define __SEQ_RT IS_BUILTIN(CONFIG_PREEMPT_RT)
> >
> > SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, lock)
> > SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, lock)
> > SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, lock)
> > SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, lock)
> > SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, lock->base)
> >
> > #if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900) || defined(__CHECKER__)
> >
> > #define __seqprop_pick(const_expr, s, locktype, prop, otherwise) \
> > __builtin_choose_expr(const_expr, \
> > __seqprop_##locktype##_##prop((void *)(s)), \
> > otherwise)
> >
> > extern void __seqprop_invalid(void);
> >
> > #define __seqprop(s, prop) \
> > __seqprop_pick(__same_type(*(s), seqcount_t), (s), seqcount, prop, \
> > __seqprop_pick(__same_type(*(s), seqcount_raw_spinlock_t), (s), raw_spinlock, prop, \
> > __seqprop_pick(__same_type(*(s), seqcount_spinlock_t), (s), spinlock, prop, \
> > __seqprop_pick(__same_type(*(s), seqcount_rwlock_t), (s), rwlock, prop, \
> > __seqprop_pick(__same_type(*(s), seqcount_mutex_t), (s), mutex, prop, \
> > __seqprop_pick(__same_type(*(s), seqcount_ww_mutex_t), (s), ww_mutex, prop, \
> > __seqprop_invalid()))))))
> >
> > #else
> >
> > #define __seqprop_case(s, locktype, prop) \
> > seqcount_##locktype##_t: __seqprop_##locktype##_##prop((void *)s)
> >
> > #define __seqprop(s, prop) \
> > _Generic(*(s), \
> > seqcount_t: __seqprop_seqcount_##prop((void*)s),\
> > __seqprop_case((s), raw_spinlock, prop), \
> > __seqprop_case((s), spinlock, prop), \
> > __seqprop_case((s), rwlock, prop), \
> > __seqprop_case((s), mutex, prop), \
> > __seqprop_case((s), ww_mutex, prop))
> >
> > #endif
> >
> > #define __to_seqcount_t(s) __seqprop(s, ptr)
> > #define __associated_lock_is_preemptible(s) __seqprop(s, preempt)
> > #define __assert_associated_lock_held(s) __seqprop(s, assert)
>
> Hmm, I'll prototype the whole thing (along with PREEMPT_RT associated
> lock()/unlock() as you've mentioned in the other e-mail), and come back.
>
> Honestly, I have a first impression that this is heading into too much
> complexity and compaction, but let's finish the whole thing first.
So the thing I pasted compiles kernel/sched/cputime.o, but that only
uses the old seqcount_t thing, not any of the fancy new stuff, still the
compiler groks it all.
And while the gcc-4.8 code is horrendous crap, the rest should be pretty
straight forward and concentrates on the pieces where there are
differences.
I even considered:
#define __SEQPROP(name, prop, expr) \
static __always_inline __seqprop_##prop##_t \
__seqprop##name##_##prop(seqcount##name##_t *s) \
{ \
expr; \
}
Such that we could write:
__SEQPROP(, ptr, return s)
__SEQPROP(, preempt, return false)
__SEQPROP(, assert, )
__SEQPROP(_##locktype, ptr, return &s->seqcount) \
__SEQPROP(_##locktype, preempt, return preempt) \
__SEQPROP(_##locktype, assert, __SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember))) \
But I figured _that_ might've been one step too far ;-)
On Wed, Jul 08, 2020 at 05:35:22PM +0200, Peter Zijlstra wrote:
...
>
> And while the gcc-4.8 code is horrendous crap, the rest should be pretty
> straight forward and concentrates on the pieces where there are
> differences.
>
Is there any possibility of upgrading the minimum gcc version to 4.9? Is
there any supported architecture that is still stuck on 4.8?
...
> I even considered:
>
> #define __SEQPROP(name, prop, expr) \
> static __always_inline __seqprop_##prop##_t \
> __seqprop##name##_##prop(seqcount##name##_t *s) \
> { \
> expr; \
> }
>
> Such that we could write:
>
> __SEQPROP(, ptr, return s)
> __SEQPROP(, preempt, return false)
> __SEQPROP(, assert, )
>
> __SEQPROP(_##locktype, ptr, return &s->seqcount) \
> __SEQPROP(_##locktype, preempt, return preempt) \
> __SEQPROP(_##locktype, assert, __SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember))) \
>
> But I figured _that_ might've been one step too far ;-)
Initially I implemented something like this during internal,
pre-upstream, versions of this patch series. We've decided afterwards
that the macro compression level is so high that the whole thing is not
so easily understandable.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Wed, Jul 08, 2020 at 05:35:22PM +0200, Peter Zijlstra wrote:
> But I figured _that_ might've been one step too far ;-)
Damn, now you made me do it... and it's not too horrible. Included the
-rt part.
---
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8b97204f35a7..cc15a6aaab00 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
- *
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
- *
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
*
+ * See Documentation/locking/seqlock.rst for full description.
*
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/
#include <linux/spinlock.h>
@@ -41,8 +20,8 @@
#include <asm/processor.h>
/*
- * The seqlock interface does not prescribe a precise sequence of read
- * begin/retry/end. For readers, typically there is a call to
+ * The seqlock seqcount_t interface does not prescribe a precise sequence of
+ * read begin/retry/end. For readers, typically there is a call to
* read_seqcount_begin() and read_seqcount_retry(), however, there are more
* esoteric cases which do not follow this pattern.
*
@@ -56,10 +35,28 @@
#define KCSAN_SEQLOCK_REGION_MAX 1000
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * This is the raw counting mechanism, without any writer protection.
+ *
+ * Write side critical sections must be serialized and non-preemptible.
+ *
+ * If readers can be invoked from hardirq or softirq contexts,
+ * interrupts or bottom halves must also be respectively disabled before
+ * entering the write section.
+ *
+ * This mechanism can't be used if the protected data contains pointers,
+ * as the writer can invalidate a pointer that a reader is following.
+ *
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_t) instead.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
typedef struct seqcount {
unsigned sequence;
@@ -82,6 +79,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
+/**
+ * seqcount_init() - runtime initializer for seqcount_t
+ * @s: Pointer to the &typedef seqcount_t instance
+ */
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
@@ -105,12 +106,139 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
# define seqcount_lockdep_reader_access(x)
#endif
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
+/**
+ * SEQCNT_ZERO() - static initializer for seqcount_t
+ * @name: Name of the &typedef seqcount_t instance
+ */
+#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
+
+/*
+ * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ *
+ * A sequence counter which associates the lock used for writer
+ * serialization at initialization time. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ *
+ * For associated locks which do not implicitly disable preemption,
+ * preemption protection is enforced in the write side function.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
+#define __SEQ_LOCK(expr) expr
+#else
+#define __SEQ_LOCK(expr)
+#endif
+
+typedef seqcount_t * __seqprop_ptr_t;
+typedef bool __seqprop_preempt_t;
+typedef void __seqprop_assert_t;
+typedef unsigned __seqprop_begin_t;
+
+#define __SEQPROP(name, prop) \
+static __always_inline __seqprop_##prop##_t \
+__seqprop##name##_##prop(seqcount##name##_t *s)
+
+__SEQPROP(, ptr) { return s; }
+__SEQPROP(, preempt) { return false; }
+__SEQPROP(, assert) { }
+__SEQPROP(, begin) { return smp_cond_load_relaxed(&s->sequence, !(VAL & 1)); }
+
+#define SEQCOUNT_LOCKTYPE(name, locktype, lockbase, blocking, lockmember) \
+typedef struct seqcount_##name { \
+ seqcount_t seqcount; \
+ __SEQ_LOCK(locktype *lock); \
+} seqcount_##name##_t; \
+ \
+static __always_inline void \
+seqcount_##name##_init(seqcount_##name##_t *s, locktype *l) \
+{ \
+ seqcount_init(&s->seqcount); \
+ __SEQ_LOCK(s->lock = l); \
+} \
+ \
+__SEQPROP(_##name, ptr) { return &s->seqcount; } \
+__SEQPROP(_##name, preempt) { return blocking; } \
+__SEQPROP(_##name, assert) { \
+ __SEQ_LOCK(lockdep_assert_held(s->lockmember)); \
+} \
+__SEQPROP(_##name, begin) { \
+ if (!__SEQ_RT || !blocking) \
+ return __seqprop_begin(&s->seqcount); \
+ \
+ __SEQ_LOCK(lockbase##_lock(&s->lock); \
+ lockbase##_unlock(&s->lock)); \
+ \
+ return READ_ONCE(s->seqcount.sequence); \
+}
+
+#define SEQCNT_LOCKTYPE_ZERO(_name, _lock) { \
+ .seqcount = SEQCNT_ZERO(_name.seqcount), \
+ __SEQ_LOCK(.lock = (_lock)) \
+}
+
+#include <linux/spinlock.h>
+#include <linux/ww_mutex.h>
+
+#define __SEQ_RT IS_BUILTIN(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, raw_spin, false, lock)
+SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, spin, __SEQ_RT, lock)
+SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, read, __SEQ_RT, lock)
+SEQCOUNT_LOCKTYPE(mutex, struct mutex, mutex, true, lock)
+SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex,ww_mutex, true, lock->base)
+/*
+ * seqcount_LOCKTYPE_t -- write APIs
+ *
+ * For associated lock types which do not implicitly disable preemption,
+ * enforce preemption protection in the write side functions.
+ *
+ * Never use lockdep for the raw write variants.
+ */
+
+#define __seqprop_pick(s, name, prop, otherwise) \
+ __builtin_choose_expr(__same_type(*(s), seqcount##name##_t, \
+ __seqprop##name##_##prop((void *)(s)), \
+ otherwise)
+
+extern void __seqprop_invalid(void);
+
+#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900) || defined(__CHECKER__)
+
+#define __seqprop(s, prop) \
+ __seqprop_pick((s), , prop, \
+ __seqprop_pick((s), _raw_spinlock, prop, \
+ __seqprop_pick((s), _spinlock, prop, \
+ __seqprop_pick((s), _rwlock, prop, \
+ __seqprop_pick((s), _mutex, prop, \
+ __seqprop_pick((s), _ww_mutex, prop, \
+ __seqprop_invalid()))))))
+
+#else
+
+#define __seqprop_case(s, name, prop) \
+ seqcount##name##_t: __seqprop##name##_##prop((void *)s)
+
+#define __seqprop(s, prop) \
+ _Generic(*(s), \
+ __seqprop_case((s), , prop), \
+ __seqprop_case((s), _raw_spinlock, prop), \
+ __seqprop_case((s), _spinlock, prop), \
+ __seqprop_case((s), _rwlock, prop), \
+ __seqprop_case((s), _mutex, prop), \
+ __seqprop_case((s), _ww_mutex, prop))
+
+#endif
+
+#define __to_seqcount_t(s) __seqprop(s, ptr)
+#define __associated_lock_is_preemptible(s) __seqprop(s, preempt)
+#define __assert_associated_lock_held(s) __seqprop(s, assert)
/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
+ * __read_seqcount_begin() - begin a seqcount read section (without barrier)
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
@@ -121,7 +249,15 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+#define __read_seqcount_begin(s) \
+({ \
+ unsigned ret = __seqprop(s, begin); \
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
+ ret; \
+})
+
+
+static inline unsigned do___read_seqcount_begin(const seqcount_t *s)
{
unsigned ret;
@@ -136,15 +272,18 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
+ * raw_read_seqcount() - Read the seqcount raw counter value
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
+ * seqcount_t, without any lockdep checks and without checking or
+ * masking the sequence counter LSB. Calling code is responsible for
+ * handling that.
*/
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
+#define raw_read_seqcount(s) do_raw_read_seqcount(__to_seqcount_t(s))
+
+static inline unsigned do_raw_read_seqcount(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -153,42 +292,44 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
+ * raw_read_seqcount_begin() - start a seqcount read section w/o lockdep
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount_begin opens a read critical section of the given
- * seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
+ * seqcount_t, but without any lockdep checking. Validity of the read
+ * section must be checked with read_seqcount_retry().
*/
-static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+#define raw_read_seqcount_begin(s) do_raw_read_seqcount_begin(__to_seqcount_t(s))
+
+static inline unsigned do_raw_read_seqcount_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
+ unsigned ret = do___read_seqcount_begin(s);
smp_rmb();
return ret;
}
/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
+ * read_seqcount_begin() - start a seqcount read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * read_seqcount_begin opens a read critical section of the given
+ * seqcount_t. Validity of the read section must be checked with
+ * read_seqcount_retry().
*/
-static inline unsigned read_seqcount_begin(const seqcount_t *s)
-{
- seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
-}
+#define read_seqcount_begin(s) \
+({ \
+ seqcount_lockdep_reader_access(s); \
+ __read_seqcount_begin(s); \
+})
/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
+ * raw_seqcount_begin() - begin a seq-read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* Returns: count to be passed to read_seqcount_retry
*
- * raw_seqcount_begin opens a read critical section of the given seqcount.
+ * raw_seqcount_begin opens a read critical section of the given seqcount_t.
* Validity of the critical section is tested by checking read_seqcount_retry
* function.
*
@@ -197,7 +338,9 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
* read_seqcount_retry() instead of stabilizing at the beginning of the
* critical section.
*/
-static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+#define raw_seqcount_begin(s) do_raw_seqcount_begin(__to_seqcount_t(s))
+
+static inline unsigned do_raw_seqcount_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -206,8 +349,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
}
/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
+ * __read_seqcount_retry() - end a seq-read critical section (without barrier)
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
@@ -219,38 +362,56 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
* Use carefully, only in critical code, and comment how the barrier is
* provided.
*/
-static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define __read_seqcount_retry(s, start) do___read_seqcount_retry(__to_seqcount_t(s), start)
+
+static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
{
kcsan_atomic_next(0);
return unlikely(READ_ONCE(s->sequence) != start);
}
/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
+ * read_seqcount_retry() - end a seq-read critical section
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin
* Returns: 1 if retry is required, else 0
*
- * read_seqcount_retry closes a read critical section of the given seqcount.
+ * read_seqcount_retry closes a read critical section of given seqcount_t.
* If the critical section was invalid, it must be ignored (and typically
* retried).
*/
-static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define read_seqcount_retry(s, start) do_read_seqcount_retry(__to_seqcount_t(s), start)
+
+static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return do___read_seqcount_retry(s, start);
}
+#define raw_write_seqcount_begin(s) \
+do { \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ do_raw_write_seqcount_begin(__to_seqcount_t(s)); \
+} while (0)
-
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+static inline void do_raw_write_seqcount_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
}
-static inline void raw_write_seqcount_end(seqcount_t *s)
+#define raw_write_seqcount_end(s) \
+do { \
+ do_raw_write_seqcount_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void do_raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
@@ -258,12 +419,12 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
}
/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_barrier() - do a seq write barrier
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
- * collapse the two back-to-back wmb()s.
+ * collapse the two back-to-back wmb()s::
*
* Note that writes surrounding the barrier should be declared atomic (e.g.
* via WRITE_ONCE): a) to ensure the writes become visible to other threads
@@ -298,7 +459,9 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* WRITE_ONCE(X, false);
* }
*/
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
+#define raw_write_seqcount_barrier(s) do_raw_write_seqcount_barrier(__to_seqcount_t(s))
+
+static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -307,7 +470,24 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
kcsan_nestable_atomic_end();
}
-static inline int raw_read_seqcount_latch(seqcount_t *s)
+/**
+ * raw_read_seqcount_latch() - pick even or odd seqcount latch data copy
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ *
+ * Use seqcount latching to switch between two storage places with
+ * sequence protection to allow interruptible, preemptible, writer
+ * sections.
+ *
+ * Check raw_write_seqcount_latch() for more details and a full reader
+ * and writer usage example.
+ *
+ * Return: sequence counter. Use the lowest bit as index for picking
+ * which data copy to read. Full counter must then be checked with
+ * read_seqcount_retry().
+ */
+#define raw_read_seqcount_latch(s) do_raw_read_seqcount_latch(__to_seqcount_t(s))
+
+static inline int do_raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
@@ -315,8 +495,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
}
/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_latch() - redirect readers to even/odd copy
+ * @s: pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -332,101 +512,164 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* Very simply put: we first modify one copy and then the other. This ensures
* there is always one copy in a stable state, ready to give us an answer.
*
- * The basic form is a data structure like:
+ * The basic form is a data structure like::
*
- * struct latch_struct {
- * seqcount_t seq;
- * struct data_struct data[2];
- * };
+ * struct latch_struct {
+ * seqcount_t seq;
+ * struct data_struct data[2];
+ * };
*
* Where a modification, which is assumed to be externally serialized, does the
- * following:
+ * following::
*
- * void latch_modify(struct latch_struct *latch, ...)
- * {
- * smp_wmb(); <- Ensure that the last data[1] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * void latch_modify(struct latch_struct *latch, ...)
+ * {
+ * smp_wmb(); // Ensure that the last data[1] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[0], ...);
+ * modify(latch->data[0], ...);
*
- * smp_wmb(); <- Ensure that the data[0] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * smp_wmb(); // Ensure that the data[0] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[1], ...);
- * }
+ * modify(latch->data[1], ...);
+ * }
*
- * The query will have a form like:
+ * The query will have a form like::
*
- * struct entry *latch_query(struct latch_struct *latch, ...)
- * {
- * struct entry *entry;
- * unsigned seq, idx;
+ * struct entry *latch_query(struct latch_struct *latch, ...)
+ * {
+ * struct entry *entry;
+ * unsigned seq, idx;
*
- * do {
- * seq = raw_read_seqcount_latch(&latch->seq);
+ * do {
+ * seq = raw_read_seqcount_latch(&latch->seq);
*
- * idx = seq & 0x01;
- * entry = data_query(latch->data[idx], ...);
+ * idx = seq & 0x01;
+ * entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes necessary smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq);
*
- * return entry;
- * }
+ * return entry;
+ * }
*
* So during the modification, queries are first redirected to data[1]. Then we
* modify data[0]. When that is complete, we redirect queries back to data[0]
* and we can modify data[1].
*
- * NOTE: The non-requirement for atomic modifications does _NOT_ include
- * the publishing of new entries in the case where data is a dynamic
- * data structure.
+ * NOTE:
+ *
+ * The non-requirement for atomic modifications does _NOT_ include
+ * the publishing of new entries in the case where data is a dynamic
+ * data structure.
+ *
+ * An iteration might start in data[0] and get suspended long enough
+ * to miss an entire modification sequence, once it resumes it might
+ * observe the new entry.
*
- * An iteration might start in data[0] and get suspended long enough
- * to miss an entire modification sequence, once it resumes it might
- * observe the new entry.
+ * NOTE:
*
- * NOTE: When data is a dynamic data structure; one should use regular RCU
- * patterns to manage the lifetimes of the objects within.
+ * When data is a dynamic data structure; one should use regular RCU
+ * patterns to manage the lifetimes of the objects within.
*/
-static inline void raw_write_seqcount_latch(seqcount_t *s)
+#define raw_write_seqcount_latch(s) do_raw_write_seqcount_latch(__to_seqcount_t(s))
+
+static inline void do_raw_write_seqcount_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
smp_wmb(); /* increment "sequence" before following stores */
}
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ do_raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
+#define write_seqcount_begin_nested(s, subclass) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ do_write_seqcount_begin_nested(__to_seqcount_t(s), subclass); \
+} while (0)
+
+static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+// lockdep_assert_preemption_disabled();
+ __write_seqcount_begin_nested(s, subclass);
+}
+
/*
- * Sequence counter only version assumes that callers are using their
- * own mutexing.
+ * write_seqcount_t_begin() without lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption is
+ * already disabled. For example, seqlock_t write side functions.
*/
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+static inline void __write_seqcount_begin(seqcount_t *s)
{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+ __write_seqcount_begin_nested(s, 0);
}
-static inline void write_seqcount_begin(seqcount_t *s)
+/**
+ * write_seqcount_begin() - start a seqcount write-side critical section
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
+ *
+ * write_seqcount_begin opens a write-side critical section of the given
+ * seqcount. Seqcount write-side critical sections must be externally
+ * serialized and non-preemptible.
+ */
+#define write_seqcount_begin(s) \
+do { \
+ __assert_associated_lock_held(s); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ do_write_seqcount_begin(__to_seqcount_t(s)); \
+} while (0)
+
+static inline void do_write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
-static inline void write_seqcount_end(seqcount_t *s)
+/**
+ * write_seqcount_end() - end a seqcount_t write-side critical section
+ * @s: Pointer to &typedef seqcount_t
+ *
+ * The write section must've been opened with write_seqcount_begin().
+ */
+#define write_seqcount_end(s) \
+do { \
+ do_write_seqcount_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void do_write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
raw_write_seqcount_end(s);
}
/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
+ * write_seqcount_invalidate() - invalidate in-progress read-side seq operations
+ * @s: Pointer to &typedef seqcount_t or any of the seqcount_locktype_t variants
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
*/
-static inline void write_seqcount_invalidate(seqcount_t *s)
+#define write_seqcount_invalidate(s) do_write_seqcount_invalidate(__to_seqcount_t(s))
+
+static inline void do_write_seqcount_invalidate(seqcount_t *s)
{
smp_wmb();
kcsan_nestable_atomic_begin();
@@ -434,32 +677,53 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * For more info, see:
+ * - Comments on top of seqcount_t
+ * - Documentation/locking/seqlock.rst
+ */
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+/**
+ * seqlock_init() - dynamic initializer for seqlock_t
+ * @sl: Pointer to the &typedef seqlock_t instance
+ */
+#define seqlock_init(sl) \
do { \
- seqcount_init(&(x)->seqcount); \
- spin_lock_init(&(x)->lock); \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+/**
+ * DEFINE_SEQLOCK() - Define a statically-allocated seqlock_t
+ * @sl: Name of the &typedef seqlock_t instance
+ */
+#define DEFINE_SEQLOCK(sl) \
+ seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
-/*
- * Read side functions for starting and finalizing a read side section.
+/**
+ * read_seqbegin() - start a seqlock_t read-side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqbegin opens a read side critical section of the given
+ * seqlock_t. Validity of the critical section is tested by checking
+ * read_seqretry().
+ *
+ * Return: count to be passed to read_seqretry()
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
@@ -470,6 +734,17 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
return ret;
}
+/**
+ * read_seqretry() - end and validate a seqlock_t read side section
+ * @sl: Pointer to &typedef seqlock_t
+ * @start: count, from read_seqbegin()
+ *
+ * read_seqretry closes the given seqlock_t read side critical section,
+ * and checks its validity. If the read section was invalid, it must be
+ * ignored and retried.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
/*
@@ -478,47 +753,94 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
*/
kcsan_flat_atomic_end();
- return read_seqcount_retry(&sl->seqcount, start);
+ return do_read_seqcount_retry(&sl->seqcount, start);
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * write_seqlock() - start a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_seqlock opens a write side critical section of the given
+ * seqlock_t. It also acquires the spinlock_t embedded inside the
+ * sequential lock. All the seqlock_t write side critical sections are
+ * thus automatically serialized and non-preemptible.
+ *
+ * Use the ``_irqsave`` and ``_bh`` variants instead if the read side
+ * can be invoked from a hardirq or softirq context.
+ *
+ * The opened write side section must be closed with write_sequnlock().
*/
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock() - end a seqlock_t write side critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock closes the (serialized and non-preemptible) write
+ * side critical section of given seqlock_t.
+ */
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
+/**
+ * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of write_seqlock(). Use only if the read side section
+ * can be invoked from a softirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_bh().
+ */
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * write_sequnlock_bh closes the serialized, non-preemptible,
+ * softirqs-disabled, seqlock_t write side critical section opened with
+ * write_seqlock_bh().
+ */
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
+/**
+ * write_seqlock_irq() - start a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * This is the ``_irq`` variant of write_seqlock(). Use only if the read
+ * section of given seqlock_t can be invoked from a hardirq context.
+ */
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_irq() - end a non-interruptible seqlock_t write side section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of write_sequnlock(). The write side section of
+ * given seqlock_t must've been opened with write_seqlock_irq().
+ */
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -527,44 +849,98 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
+
return flags;
}
+/**
+ * write_seqlock_irqsave() - start a non-interruptible seqlock_t write section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to write_sequnlock_irqrestore().
+ *
+ * ``_irqsave`` variant of write_seqlock(). Use if the read section of
+ * given seqlock_t can be invoked from a hardirq context.
+ *
+ * The opened write section must be closed with write_sequnlock_irqrestore().
+ */
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
+/**
+ * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
+ *
+ * ``_irqrestore`` variant of write_sequnlock(). The write section of
+ * given seqlock_t must've been opened with write_seqlock_irqsave().
+ */
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_end(&sl->seqcount);
+ do_write_seqcount_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * read_seqlock_excl() - begin a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_seqlock_excl opens a locking reader critical section for the
+ * given seqlock_t. A locking reader exclusively locks out other writers
+ * and other *locking* readers, but doesn't update the sequence number.
+ *
+ * Locking readers act like a normal spin_lock()/spin_unlock().
+ *
+ * The opened read side section must be closed with read_sequnlock_excl().
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
+/**
+ * read_sequnlock_excl() - end a seqlock_t locking reader critical section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * read_sequnlock_excl closes the locking reader critical section opened
+ * with read_seqlock_excl().
+ */
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
+ * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq : Marker and return parameter. If the passed value is even, the
+ * reader will become a *lockless* seqlock_t sequence counter reader as
+ * in read_seqbegin(). If the passed value is odd, the reader will
+ * become a fully locking reader, as in read_seqlock_excl(). In the
+ * first call to read_seqbegin_or_lock(), the caller **must** initialize
+ * and pass an even value to @seq so a lockless read is optimistically
+ * tried first.
+ *
+ * read_seqbegin_or_lock is an API designed to optimistically try a
+ * normal lockless seqlock_t read section first, as in read_seqbegin().
+ * If an odd counter is found, the normal lockless read trial has
+ * failed, and the next reader iteration transforms to a full seqlock_t
+ * locking reader as in read_seqlock_excl().
+ *
+ * This is typically used to avoid lockless seqlock_t readers starvation
+ * (too much retry loops) in the case of a sharp spike in write
+ * activity.
*
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
+ * The opened read section must be closed with done_seqretry(). Check
+ * Documentation/locking/seqlock.rst for template example code.
+ *
+ * Return: The encountered sequence counter value, returned through the
+ * @seq parameter, which is overloaded as a return parameter. The
+ * returned value must be checked with need_seqretry(). If the read
+ * section must be retried, the returned value must also be passed to
+ * the @seq parameter of the next read_seqbegin_or_lock() iteration.
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
@@ -574,32 +950,90 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
read_seqlock_excl(lock);
}
+/**
+ * need_seqretry() - validate seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * need_seqretry checks if the seqlock_t read-side critical section
+ * started with read_seqbegin_or_lock() is valid. If it was not, the
+ * caller must retry the read-side section.
+ *
+ * Return: 1 if a retry is required, 0 otherwise
+ */
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
+/**
+ * done_seqretry() - end seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * done_seqretry finishes the seqlock_t read side critical section
+ * started by read_seqbegin_or_lock(). The read section must've been
+ * already validated with need_seqretry().
+ */
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
+/**
+ * read_seqlock_excl_bh() - start a locking reader seqlock_t section
+ * with softirqs disabled
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of read_seqlock_excl(). Use this variant if the
+ * seqlock_t write side section, *or other read sections*, can be
+ * invoked from a softirq context
+ *
+ * The opened section must be closed with read_sequnlock_excl_bh().
+ */
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
+/**
+ * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_bh`` variant of read_sequnlock_excl(). The closed section must've
+ * been opened with read_seqlock_excl_bh().
+ */
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
+/**
+ * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
+ * reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of read_seqlock_excl(). Use this only if the
+ * seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from a hardirq context.
+ *
+ * The opened read section must be closed with read_sequnlock_excl_irq().
+ */
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
+/**
+ * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ *
+ * ``_irq`` variant of read_sequnlock_excl(). The closed section must've
+ * been opened with read_seqlock_excl_irq().
+ */
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
@@ -613,15 +1047,59 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
+ * locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to read_sequnlock_excl_irqrestore().
+ *
+ * ``_irqsave`` variant of read_seqlock_excl(). Use this only if the
+ * seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from a hardirq context.
+ *
+ * Opened section must be closed with read_sequnlock_excl_irqrestore().
+ */
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+/**
+ * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
+ * locking reader section
+ * @sl: Pointer to &typedef seqlock_t
+ * @flags: Caller's saved interrupt state, from
+ * read_seqlock_excl_irqsave()
+ *
+ * ``_irqrestore`` variant of read_sequnlock_excl(). The closed section
+ * must've been opened with read_seqlock_excl_irqsave().
+ */
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
spin_unlock_irqrestore(&sl->lock, flags);
}
+/**
+ * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
+ * a non-interruptible locking reader
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
+ *
+ * This is the ``_irqsave`` variant of read_seqbegin_or_lock(). Use if
+ * the seqlock_t write side critical section, *or other read side sections*,
+ * can be invoked from hardirq context.
+ *
+ * The validity of the read section must be checked with need_seqretry().
+ * The opened section must be closed with done_seqretry_irqrestore().
+ *
+ * Return:
+ *
+ * 1. The saved local interrupts state in case of a locking reader, to be
+ * passed to done_seqretry_irqrestore().
+ *
+ * 2. The encountered sequence counter value, returned through @seq which
+ * is overloaded as a return parameter. Check read_seqbegin_or_lock().
+ */
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
@@ -635,6 +1113,18 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
return flags;
}
+/**
+ * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
+ * non-interruptible locking reader section
+ * @lock: Pointer to &typedef seqlock_t
+ * @seq: Count, from read_seqbegin_or_lock_irqsave()
+ * @flags: Caller's saved local interrupt state in case of a locking
+ * reader, also from read_seqbegin_or_lock_irqsave()
+ *
+ * This is the ``_irqrestore`` variant of done_seqretry(). The read
+ * section must've been opened with read_seqbegin_or_lock_irqsave(), and
+ * validated with need_seqretry().
+ */
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
On Wed, Jul 08, 2020 at 05:58:13PM +0200, Ahmed S. Darwish wrote:
> On Wed, Jul 08, 2020 at 05:35:22PM +0200, Peter Zijlstra wrote:
> ...
> >
> > And while the gcc-4.8 code is horrendous crap, the rest should be pretty
> > straight forward and concentrates on the pieces where there are
> > differences.
> >
>
> Is there any possibility of upgrading the minimum gcc version to 4.9? Is
> there any supported architecture that is still stuck on 4.8?
Upgrading also got mention here:
https://lkml.kernel.org/r/CAHk-=wgD+q+oDdtukYC74_cDX5i0Ynf0GLhuNe2Faaokejj6fQ@mail.gmail.com
But it didn't get much response.
On Wed, Jul 08, 2020 at 05:58:13PM +0200, Ahmed S. Darwish wrote:
> > I even considered:
> >
> > #define __SEQPROP(name, prop, expr) \
> > static __always_inline __seqprop_##prop##_t \
> > __seqprop##name##_##prop(seqcount##name##_t *s) \
> > { \
> > expr; \
> > }
> >
> > Such that we could write:
> >
> > __SEQPROP(, ptr, return s)
> > __SEQPROP(, preempt, return false)
> > __SEQPROP(, assert, )
> >
> > __SEQPROP(_##locktype, ptr, return &s->seqcount) \
> > __SEQPROP(_##locktype, preempt, return preempt) \
> > __SEQPROP(_##locktype, assert, __SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember))) \
> >
> > But I figured _that_ might've been one step too far ;-)
>
> Initially I implemented something like this during internal,
> pre-upstream, versions of this patch series. We've decided afterwards
> that the macro compression level is so high that the whole thing is not
> so easily understandable.
I've been reading too much tracing code lately... :-) This is only 2
levels of expansion and fits on a single screen.
Hi,
This is v4 of the seqlock patch series:
[PATCH v1 00/25]
https://lore.kernel.org/lkml/[email protected]
[PATCH v2 00/06] (bugfixes-only, merged)
https://lore.kernel.org/lkml/[email protected]
[PATCH v2 00/18]
https://lore.kernel.org/lkml/[email protected]
[PATCH v3 00/20]
https://lore.kernel.org/lkml/[email protected]
It is based over:
git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git :: locking/core
Changelog
=========
- Unconditionally use C11 _Generic() expressions for seqcount_locktype_t
switching. Thanks to Peter, pushing for 6ec4476ac825 ("Raise gcc
version requirement to 4.9").
- Compress the new seqcount_locktype_t code code by using generative
macros, as suggested by Peter here:
https://lkml.kernel.org/r/[email protected]
Keep *all* functions that are to be invoked by call-sites out of such
generative macros though. This simplifies the generative macros code,
and (more importantly) make the newly exported seqlock.h API explicit.
- Make all documentation "RST-lite", for better readability from text
editors.
- Add additional clean-ups at the start of the series for better
overall readability of seqlock.h code, and for future extensibility.
Thanks,
8<--------------
Ahmed S. Darwish (24):
Documentation: locking: Describe seqlock design and usage
seqlock: Properly format kernel-doc code samples
seqlock: seqcount_t latch: End read sections with
read_seqcount_retry()
seqlock: Reorder seqcount_t and seqlock_t API definitions
seqlock: Add kernel-doc for seqcount_t and seqlock_t APIs
seqlock: Implement raw_seqcount_begin() in terms of
raw_read_seqcount()
lockdep: Add preemption enabled/disabled assertion APIs
seqlock: lockdep assert non-preemptibility on seqcount_t write
seqlock: Extend seqcount API with associated locks
seqlock: Align multi-line macros newline escapes at 72 columns
dma-buf: Remove custom seqcount lockdep class key
dma-buf: Use sequence counter with associated wound/wait mutex
sched: tasks: Use sequence counter with associated spinlock
netfilter: conntrack: Use sequence counter with associated spinlock
netfilter: nft_set_rbtree: Use sequence counter with associated rwlock
xfrm: policy: Use sequence counters with associated lock
timekeeping: Use sequence counter with associated raw spinlock
vfs: Use sequence counter with associated spinlock
raid5: Use sequence counter with associated spinlock
iocost: Use sequence counter with associated spinlock
NFSv4: Use sequence counter with associated spinlock
userfaultfd: Use sequence counter with associated spinlock
kvm/eventfd: Use sequence counter with associated spinlock
hrtimer: Use sequence counter with associated raw spinlock
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 222 ++++
block/blk-iocost.c | 5 +-
drivers/dma-buf/dma-resv.c | 15 +-
.../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 -
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 +-
fs/nfs/nfs4_fs.h | 2 +-
fs/nfs/nfs4state.c | 2 +-
fs/userfaultfd.c | 4 +-
include/linux/dcache.h | 2 +-
include/linux/dma-resv.h | 4 +-
include/linux/fs_struct.h | 2 +-
include/linux/hrtimer.h | 2 +-
include/linux/kvm_irqfd.h | 2 +-
include/linux/lockdep.h | 19 +
include/linux/sched.h | 2 +-
include/linux/seqlock.h | 1139 +++++++++++++----
include/net/netfilter/nf_conntrack.h | 2 +-
init/init_task.c | 3 +-
kernel/fork.c | 2 +-
kernel/time/hrtimer.c | 13 +-
kernel/time/timekeeping.c | 19 +-
lib/Kconfig.debug | 1 +
net/netfilter/nf_conntrack_core.c | 5 +-
net/netfilter/nft_set_rbtree.c | 4 +-
net/xfrm/xfrm_policy.c | 10 +-
virt/kvm/eventfd.c | 2 +-
30 files changed, 1173 insertions(+), 323 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
base-commit: a9232dc5607dbada801f2fe83ea307cda762969a
--
2.20.1
The seqcount_t latch reader example at the raw_write_seqcount_latch()
kernel-doc comment ends the latch read section with a manual smp memory
barrier and sequence counter comparison.
This is technically correct, but it is suboptimal: read_seqcount_retry()
already contains the same logic of an smp memory barrier and sequence
counter comparison.
End the latch read critical section example with read_seqcount_retry().
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 6c4f68ef1393..d724b5e5408d 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -363,8 +363,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes needed smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq));
*
* return entry;
* }
--
2.20.1
Asserting that preemption is enabled or disabled is a critical sanity
check. Developers are usually reluctant to add such a check in a
fastpath as reading the preemption count can be costly.
Extend the lockdep API with macros asserting that preemption is disabled
or enabled. If lockdep is disabled, or if the underlying architecture
does not support kernel preemption, this assert has no runtime overhead.
References: f54bb2ec02c8 ("locking/lockdep: Add IRQs disabled/enabled assertion APIs: ...")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/lockdep.h | 19 +++++++++++++++++++
lib/Kconfig.debug | 1 +
2 files changed, 20 insertions(+)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 7aafba0ddcf9..39a35699d0d6 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -549,6 +549,22 @@ do { \
WARN_ON_ONCE(debug_locks && !this_cpu_read(hardirq_context)); \
} while (0)
+#define lockdep_assert_preemption_enabled() \
+do { \
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
+ debug_locks && \
+ (preempt_count() != 0 || \
+ !this_cpu_read(hardirqs_enabled))); \
+} while (0)
+
+#define lockdep_assert_preemption_disabled() \
+do { \
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
+ debug_locks && \
+ (preempt_count() == 0 && \
+ this_cpu_read(hardirqs_enabled))); \
+} while (0)
+
#else
# define might_lock(lock) do { } while (0)
# define might_lock_read(lock) do { } while (0)
@@ -557,6 +573,9 @@ do { \
# define lockdep_assert_irqs_enabled() do { } while (0)
# define lockdep_assert_irqs_disabled() do { } while (0)
# define lockdep_assert_in_irq() do { } while (0)
+
+# define lockdep_assert_preemption_enabled() do { } while (0)
+# define lockdep_assert_preemption_disabled() do { } while (0)
#endif
#ifdef CONFIG_PROVE_RAW_LOCK_NESTING
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9ad9210d70a1..5379931ba3b5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1117,6 +1117,7 @@ config PROVE_LOCKING
select DEBUG_RWSEMS
select DEBUG_WW_MUTEX_SLOWPATH
select DEBUG_LOCK_ALLOC
+ select PREEMPT_COUNT if !ARCH_NO_PREEMPT
select TRACE_IRQFLAGS
default n
help
--
2.20.1
Preemption must be disabled before entering a sequence count write side
critical section. Failing to do so, the seqcount read side can preempt
the write side section and spin for the entire scheduler tick. If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.
Assert through lockdep that preemption is disabled for seqcount writers.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 29 +++++++++++++++++++++++------
1 file changed, 23 insertions(+), 6 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index e885702d8b82..54bc20496392 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -266,6 +266,12 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
/**
* write_seqcount_begin_nested() - start a seqcount_t write section with
* custom lockdep nesting level
@@ -276,8 +282,19 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
*/
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+ lockdep_assert_preemption_disabled();
+ __write_seqcount_begin_nested(s, subclass);
+}
+
+/*
+ * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption is
+ * already disabled. For example, seqlock_t write side functions.
+ */
+static inline void __write_seqcount_begin(seqcount_t *s)
+{
+ __write_seqcount_begin_nested(s, 0);
}
/**
@@ -575,7 +592,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -601,7 +618,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -628,7 +645,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -649,7 +666,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
return flags;
}
--
2.20.1
The seqlock.h seqcount_t and seqlock_t API definitions are presented in
the chronological order of their development rather than the order that
makes most sense to readers. This makes it hard to follow and understand
the header file code.
Group and reorder all of the exported seqlock.h functions according to
their function.
First, group together the seqcount_t standard read path functions:
- __read_seqcount_begin()
- raw_read_seqcount_begin()
- read_seqcount_begin()
since each function is implemented exactly in terms of the one above
it. Then, group the special-case seqcount_t readers on their own as:
- raw_read_seqcount()
- raw_seqcount_begin()
since the only difference between the two functions is that the second
one masks the sequence counter LSB while the first one does not. Note
that raw_seqcount_begin() can actually be implemented in terms of
raw_read_seqcount(), which will be done in a follow-up commit.
Then, group the seqcount_t write path functions, instead of injecting
unrelated seqcount_t latch functions between them, and order them as:
- raw_write_seqcount_begin()
- raw_write_seqcount_end()
- write_seqcount_begin_nested()
- write_seqcount_begin()
- write_seqcount_end()
- raw_write_seqcount_barrier()
- write_seqcount_invalidate()
which is the expected natural order. This also isolates the seqcount_t
latch functions into their own area, at the end of the sequence counters
section, and before jumping to the next one: sequential locks
(seqlock_t).
Do a similar grouping and reordering for seqlock_t "locking" readers vs.
the "conditionally locking or lockless" ones.
No implementation code was changed in any of the reordering above.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 174 ++++++++++++++++++++--------------------
1 file changed, 86 insertions(+), 88 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d724b5e5408d..4c1456008d89 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -128,23 +128,6 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
return ret;
}
-/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
- */
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
-{
- unsigned ret = READ_ONCE(s->sequence);
- smp_rmb();
- kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
- return ret;
-}
-
/**
* raw_read_seqcount_begin - start seq-read critical section w/o lockdep
* @s: pointer to seqcount_t
@@ -176,6 +159,23 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
return raw_read_seqcount_begin(s);
}
+/**
+ * raw_read_seqcount - Read the raw seqcount
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * raw_read_seqcount opens a read critical section of the given
+ * seqcount without any lockdep checking and without checking or
+ * masking the LSB. Calling code is responsible for handling that.
+ */
+static inline unsigned raw_read_seqcount(const seqcount_t *s)
+{
+ unsigned ret = READ_ONCE(s->sequence);
+ smp_rmb();
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
+ return ret;
+}
+
/**
* raw_seqcount_begin - begin a seq-read critical section
* @s: pointer to seqcount_t
@@ -234,8 +234,6 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_retry(s, start);
}
-
-
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
@@ -250,6 +248,23 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
+static inline void write_seqcount_begin(seqcount_t *s)
+{
+ write_seqcount_begin_nested(s, 0);
+}
+
+static inline void write_seqcount_end(seqcount_t *s)
+{
+ seqcount_release(&s->dep_map, _RET_IP_);
+ raw_write_seqcount_end(s);
+}
+
/**
* raw_write_seqcount_barrier - do a seq write barrier
* @s: pointer to seqcount_t
@@ -300,6 +315,21 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * write_seqcount_invalidate - invalidate in-progress read-side seq operations
+ * @s: pointer to seqcount_t
+ *
+ * After write_seqcount_invalidate, no read-side seq operations will complete
+ * successfully and see data older than this.
+ */
+static inline void write_seqcount_invalidate(seqcount_t *s)
+{
+ smp_wmb();
+ kcsan_nestable_atomic_begin();
+ s->sequence+=2;
+ kcsan_nestable_atomic_end();
+}
+
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
@@ -395,38 +425,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
-}
-
-static inline void write_seqcount_begin(seqcount_t *s)
-{
- write_seqcount_begin_nested(s, 0);
-}
-
-static inline void write_seqcount_end(seqcount_t *s)
-{
- seqcount_release(&s->dep_map, _RET_IP_);
- raw_write_seqcount_end(s);
-}
-
-/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
- *
- * After write_seqcount_invalidate, no read-side seq operations will complete
- * successfully and see data older than this.
- */
-static inline void write_seqcount_invalidate(seqcount_t *s)
-{
- smp_wmb();
- kcsan_nestable_atomic_begin();
- s->sequence+=2;
- kcsan_nestable_atomic_end();
-}
-
/*
* Sequential locks (seqlock_t)
*
@@ -555,6 +553,43 @@ static inline void read_sequnlock_excl(seqlock_t *sl)
spin_unlock(&sl->lock);
}
+static inline void read_seqlock_excl_bh(seqlock_t *sl)
+{
+ spin_lock_bh(&sl->lock);
+}
+
+static inline void read_sequnlock_excl_bh(seqlock_t *sl)
+{
+ spin_unlock_bh(&sl->lock);
+}
+
+static inline void read_seqlock_excl_irq(seqlock_t *sl)
+{
+ spin_lock_irq(&sl->lock);
+}
+
+static inline void read_sequnlock_excl_irq(seqlock_t *sl)
+{
+ spin_unlock_irq(&sl->lock);
+}
+
+static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&sl->lock, flags);
+ return flags;
+}
+
+#define read_seqlock_excl_irqsave(lock, flags) \
+ do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+
+static inline void
+read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
+{
+ spin_unlock_irqrestore(&sl->lock, flags);
+}
+
/**
* read_seqbegin_or_lock - begin a sequence number check or locking block
* @lock: sequence lock
@@ -584,43 +619,6 @@ static inline void done_seqretry(seqlock_t *lock, int seq)
read_sequnlock_excl(lock);
}
-static inline void read_seqlock_excl_bh(seqlock_t *sl)
-{
- spin_lock_bh(&sl->lock);
-}
-
-static inline void read_sequnlock_excl_bh(seqlock_t *sl)
-{
- spin_unlock_bh(&sl->lock);
-}
-
-static inline void read_seqlock_excl_irq(seqlock_t *sl)
-{
- spin_lock_irq(&sl->lock);
-}
-
-static inline void read_sequnlock_excl_irq(seqlock_t *sl)
-{
- spin_unlock_irq(&sl->lock);
-}
-
-static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
-{
- unsigned long flags;
-
- spin_lock_irqsave(&sl->lock, flags);
- return flags;
-}
-
-#define read_seqlock_excl_irqsave(lock, flags) \
- do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
-
-static inline void
-read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
-{
- spin_unlock_irqrestore(&sl->lock, flags);
-}
-
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
--
2.20.1
seqlock.h is now included by kernel's RST documentation, but a small
number of the the exported seqlock.h functions are kernel-doc annotated.
Add kernel-doc for all seqlock.h exported APIs.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 425 ++++++++++++++++++++++++++++++++--------
1 file changed, 348 insertions(+), 77 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 4c1456008d89..85fb3ac93ffb 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -75,6 +75,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
+/**
+ * seqcount_init() - runtime initializer for seqcount_t
+ * @s: Pointer to the seqcount_t instance
+ */
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
@@ -98,13 +102,15 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
# define seqcount_lockdep_reader_access(x)
#endif
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
-
+/**
+ * SEQCNT_ZERO() - static initializer for seqcount_t
+ * @name: Name of the seqcount_t instance
+ */
+#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
+ * @s: Pointer to seqcount_t
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -113,6 +119,8 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* Use carefully, only in critical code, and comment how the barrier is
* provided.
+ *
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
@@ -129,13 +137,10 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
+ * @s: Pointer to seqcount_t
*
- * raw_read_seqcount_begin opens a read critical section of the given
- * seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
@@ -145,13 +150,10 @@ static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
}
/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * read_seqcount_begin() - begin a seqcount_t read critical section
+ * @s: Pointer to seqcount_t
*
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
@@ -160,13 +162,15 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount() - read the raw seqcount_t counter value
+ * @s: Pointer to seqcount_t
*
* raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
+ * seqcount_t, without any lockdep checking, and without checking or
+ * masking the sequence counter LSB. Calling code is responsible for
+ * handling that.
+ *
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned raw_read_seqcount(const seqcount_t *s)
{
@@ -177,18 +181,21 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
+ * lockdep and w/o counter stabilization
+ * @s: Pointer to seqcount_t
*
- * raw_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * raw_seqcount_begin opens a read critical section of the given
+ * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
+ * for the count to stabilize. If a writer is active when it begins, it
+ * will fail the read_seqcount_retry() at the end of the read critical
+ * section instead of stabilizing at the beginning of it.
*
- * Unlike read_seqcount_begin(), this function will not wait for the count
- * to stabilize. If a writer is active when we begin, we will fail the
- * read_seqcount_retry() instead of stabilizing at the beginning of the
- * critical section.
+ * Use this only in special kernel hot paths where the read section is
+ * small and has a high probability of success through other external
+ * means. It will save a single branching instruction.
+ *
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
@@ -199,10 +206,9 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
}
/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * @start: count, from read_seqcount_begin
- * Returns: 1 if retry is required, else 0
+ * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
+ * @s: Pointer to seqcount_t
+ * @start: count, from read_seqcount_begin()
*
* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -211,6 +217,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
*
* Use carefully, only in critical code, and comment how the barrier is
* provided.
+ *
+ * Return: true if a read section retry is required, else false
*/
static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
@@ -219,14 +227,15 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
}
/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
- * @start: count, from read_seqcount_begin
- * Returns: 1 if retry is required, else 0
+ * read_seqcount_retry() - end a seqcount_t read critical section
+ * @s: Pointer to seqcount_t
+ * @start: count, from read_seqcount_begin()
*
- * read_seqcount_retry closes a read critical section of the given seqcount.
- * If the critical section was invalid, it must be ignored (and typically
- * retried).
+ * read_seqcount_retry closes the read critical section of given
+ * seqcount_t. If the critical section was invalid, it must be ignored
+ * (and typically retried).
+ *
+ * Return: true if a read section retry is required, else false
*/
static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
{
@@ -234,6 +243,10 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_retry(s, start);
}
+/**
+ * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
+ * @s: Pointer to seqcount_t
+ */
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
@@ -241,6 +254,10 @@ static inline void raw_write_seqcount_begin(seqcount_t *s)
smp_wmb();
}
+/**
+ * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
+ * @s: Pointer to seqcount_t
+ */
static inline void raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
@@ -248,17 +265,42 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * write_seqcount_begin_nested() - start a seqcount_t write section with
+ * custom lockdep nesting level
+ * @s: Pointer to seqcount_t
+ * @subclass: lockdep nesting level
+ *
+ * See Documentation/locking/lockdep-design.rst
+ */
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
+/**
+ * write_seqcount_begin() - start a seqcount_t write side critical section
+ * @s: Pointer to seqcount_t
+ *
+ * write_seqcount_begin opens a write side critical section of the given
+ * seqcount_t.
+ *
+ * Context: seqcount_t write side critical sections must be serialized and
+ * non-preemptible. If readers can be invoked from hardirq or softirq
+ * context, interrupts or bottom halves must be respectively disabled.
+ */
static inline void write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
+/**
+ * write_seqcount_end() - end a seqcount_t write side critical section
+ * @s: Pointer to seqcount_t
+ *
+ * The write section must've been opened with write_seqcount_begin().
+ */
static inline void write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
@@ -266,12 +308,12 @@ static inline void write_seqcount_end(seqcount_t *s)
}
/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_barrier() - do a seqcount_t write barrier
+ * @s: Pointer to seqcount_t
*
- * This can be used to provide an ordering guarantee instead of the
- * usual consistency guarantee. It is one wmb cheaper, because we can
- * collapse the two back-to-back wmb()s.
+ * This can be used to provide an ordering guarantee instead of the usual
+ * consistency guarantee. It is one wmb cheaper, because it can collapse
+ * the two back-to-back wmb()s.
*
* Note that writes surrounding the barrier should be declared atomic (e.g.
* via WRITE_ONCE): a) to ensure the writes become visible to other threads
@@ -316,11 +358,12 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
}
/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
+ * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
+ * side operations
+ * @s: Pointer to seqcount_t
*
- * After write_seqcount_invalidate, no read-side seq operations will complete
- * successfully and see data older than this.
+ * After write_seqcount_invalidate, no seqcount_t read side operations
+ * will complete successfully and see data older than this.
*/
static inline void write_seqcount_invalidate(seqcount_t *s)
{
@@ -330,6 +373,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
+ * @s: Pointer to seqcount_t
+ *
+ * Use seqcount_t latching to switch between two storage places protected
+ * by a sequence counter. Doing so allows having interruptible, preemptible,
+ * seqcount_t write side critical sections.
+ *
+ * Check raw_write_seqcount_latch() for more details and a full reader and
+ * writer usage example.
+ *
+ * Return: sequence counter raw value. Use the lowest bit as an index for
+ * picking which data copy to read. The full counter value must then be
+ * checked with read_seqcount_retry().
+ */
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
@@ -338,8 +396,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
}
/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_latch() - redirect readers to even/odd copy
+ * @s: Pointer to seqcount_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -446,17 +504,28 @@ typedef struct {
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+/**
+ * seqlock_init() - dynamic initializer for seqlock_t
+ * @sl: Pointer to the seqlock_t instance
+ */
+#define seqlock_init(sl) \
do { \
- seqcount_init(&(x)->seqcount); \
- spin_lock_init(&(x)->lock); \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+/**
+ * DEFINE_SEQLOCK() - Define a statically allocated seqlock_t
+ * @sl: Name of the seqlock_t instance
+ */
+#define DEFINE_SEQLOCK(sl) \
+ seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
-/*
- * Read side functions for starting and finalizing a read side section.
+/**
+ * read_seqbegin() - start a seqlock_t read side critical section
+ * @sl: Pointer to seqlock_t
+ *
+ * Return: count, to be passed to read_seqretry()
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
@@ -467,6 +536,17 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
return ret;
}
+/**
+ * read_seqretry() - end a seqlock_t read side section
+ * @sl: Pointer to seqlock_t
+ * @start: count, from read_seqbegin()
+ *
+ * read_seqretry closes the read side critical section of given seqlock_t.
+ * If the critical section was invalid, it must be ignored (and typically
+ * retried).
+ *
+ * Return: true if a read section retry is required, else false
+ */
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
/*
@@ -478,10 +558,18 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
return read_seqcount_retry(&sl->seqcount, start);
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * write_seqlock() - start a seqlock_t write side critical section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_seqlock opens a write side critical section for the given
+ * seqlock_t. It also implicitly acquires the spinlock_t embedded inside
+ * that sequential lock. All seqlock_t write side sections are thus
+ * automatically serialized and non-preemptible.
+ *
+ * Context: if the seqlock_t read section, or other write side critical
+ * sections, can be invoked from hardirq or softirq contexts, use the
+ * _irqsave or _bh variants of this function instead.
*/
static inline void write_seqlock(seqlock_t *sl)
{
@@ -489,30 +577,66 @@ static inline void write_seqlock(seqlock_t *sl)
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock() - end a seqlock_t write side critical section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_sequnlock closes the (serialized and non-preemptible) write side
+ * critical section of given seqlock_t.
+ */
static inline void write_sequnlock(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
+/**
+ * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * _bh variant of write_seqlock(). Use only if the read side section, or
+ * other write side sections, can be invoked from softirq contexts.
+ */
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_sequnlock_bh closes the serialized, non-preemptible, and
+ * softirqs-disabled, seqlock_t write side critical section opened with
+ * write_seqlock_bh().
+ */
static inline void write_sequnlock_bh(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
+/**
+ * write_seqlock_irq() - start a non-interruptible seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * _irq variant of write_seqlock(). Use only if the read side section, or
+ * other write sections, can be invoked from hardirq contexts.
+ */
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_sequnlock_irq closes the serialized and non-interruptible
+ * seqlock_t write side section opened with write_seqlock_irq().
+ */
static inline void write_sequnlock_irq(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
@@ -528,9 +652,28 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
+ * section
+ * @lock: Pointer to seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to write_sequnlock_irqrestore().
+ *
+ * _irqsave variant of write_seqlock(). Use it only if the read side
+ * section, or other write sections, can be invoked from hardirq context.
+ */
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
+/**
+ * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
+ * section
+ * @sl: Pointer to seqlock_t
+ * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
+ *
+ * write_sequnlock_irqrestore closes the serialized and non-interruptible
+ * seqlock_t write section previously opened with write_seqlock_irqsave().
+ */
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
@@ -538,36 +681,79 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * read_seqlock_excl() - begin a seqlock_t locking reader section
+ * @sl: Pointer to seqlock_t
+ *
+ * read_seqlock_excl opens a seqlock_t locking reader critical section. A
+ * locking reader exclusively locks out *both* other writers *and* other
+ * locking readers, but it does not update the embedded sequence number.
+ *
+ * Locking readers act like a normal spin_lock()/spin_unlock().
+ *
+ * Context: if the seqlock_t write section, *or other read sections*, can
+ * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
+ * variant of this function instead.
+ *
+ * The opened read section must be closed with read_sequnlock_excl().
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
+/**
+ * read_sequnlock_excl() - end a seqlock_t locking reader critical section
+ * @sl: Pointer to seqlock_t
+ */
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
+/**
+ * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
+ * softirqs disabled
+ * @sl: Pointer to seqlock_t
+ *
+ * _bh variant of read_seqlock_excl(). Use this variant only if the
+ * seqlock_t write side section, *or other read sections*, can be invoked
+ * from softirq contexts.
+ */
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
+/**
+ * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
+ * reader section
+ * @sl: Pointer to seqlock_t
+ */
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
+/**
+ * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
+ * reader section
+ * @sl: Pointer to seqlock_t
+ *
+ * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
+ * write side section, *or other read sections*, can be invoked from a
+ * hardirq context.
+ */
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
+/**
+ * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
+ * locking reader section
+ * @sl: Pointer to seqlock_t
+ */
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
@@ -581,9 +767,26 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
+ * locking reader section
+ * @lock: Pointer to seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to read_sequnlock_excl_irqrestore().
+ *
+ * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
+ * write side section, *or other read sections*, can be invoked from a
+ * hardirq context.
+ */
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+/**
+ * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
+ * locking reader section
+ * @sl: Pointer to seqlock_t
+ * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
+ */
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
@@ -591,14 +794,35 @@ read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
}
/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
- *
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
+ * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
+ * @lock: Pointer to seqlock_t
+ * @seq : Marker and return parameter. If the passed value is even, the
+ * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
+ * If the passed value is odd, the reader will become a *locking* reader
+ * as in read_seqlock_excl(). In the first call to this function, the
+ * caller *must* initialize and pass an even value to @seq; this way, a
+ * lockless read can be optimistically tried first.
+ *
+ * read_seqbegin_or_lock is an API designed to optimistically try a normal
+ * lockless seqlock_t read section first. If an odd counter is found, the
+ * lockless read trial has failed, and the next read iteration transforms
+ * itself into a full seqlock_t locking reader.
+ *
+ * This is typically used to avoid seqlock_t lockless readers starvation
+ * (too much retry loops) in the case of a sharp spike in write side
+ * activity.
+ *
+ * Context: if the seqlock_t write section, *or other read sections*, can
+ * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
+ * variant of this function instead.
+ *
+ * Check Documentation/locking/seqlock.rst for template example code.
+ *
+ * Return: the encountered sequence counter value, through the @seq
+ * parameter, which is overloaded as a return parameter. This returned
+ * value must be checked with need_seqretry(). If the read section need to
+ * be retried, this returned value must also be passed as the @seq
+ * parameter of the next read_seqbegin_or_lock() iteration.
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
@@ -608,17 +832,52 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
read_seqlock_excl(lock);
}
+/**
+ * need_seqretry() - validate seqlock_t "locking or lockless" read section
+ * @lock: Pointer to seqlock_t
+ * @seq: sequence count, from read_seqbegin_or_lock()
+ *
+ * Return: true if a read section retry is required, false otherwise
+ */
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
+/**
+ * done_seqretry() - end seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * done_seqretry finishes the seqlock_t read side critical section started
+ * with read_seqbegin_or_lock() and validated by need_seqretry().
+ */
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
+/**
+ * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
+ * a non-interruptible locking reader
+ * @lock: Pointer to seqlock_t
+ * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
+ *
+ * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
+ * the seqlock_t write section, *or other read sections*, can be invoked
+ * from hardirq context.
+ *
+ * Note: Interrupts will be disabled only for "locking reader" mode.
+ *
+ * Return:
+ *
+ * 1. The saved local interrupts state in case of a locking reader, to
+ * be passed to done_seqretry_irqrestore().
+ *
+ * 2. The encountered sequence counter value, returned through @seq
+ * overloaded as a return parameter. Check read_seqbegin_or_lock().
+ */
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
@@ -632,6 +891,18 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
return flags;
}
+/**
+ * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
+ * non-interruptible locking reader section
+ * @lock: Pointer to seqlock_t
+ * @seq: Count, from read_seqbegin_or_lock_irqsave()
+ * @flags: Caller's saved local interrupt state in case of a locking
+ * reader, also from read_seqbegin_or_lock_irqsave()
+ *
+ * This is the _irqrestore variant of done_seqretry(). The read section
+ * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
+ * by need_seqretry().
+ */
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 ++--
include/linux/dcache.h | 2 +-
include/linux/fs_struct.h | 2 +-
4 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index 361ea7ab30ea..ea0485861d93 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1746,7 +1746,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
dentry->d_lockref.count = 1;
dentry->d_flags = 0;
spin_lock_init(&dentry->d_lock);
- seqcount_init(&dentry->d_seq);
+ seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
dentry->d_inode = NULL;
dentry->d_parent = dentry;
dentry->d_sb = sb;
diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index ca639ed967b7..04b3f5b9c629 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -117,7 +117,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
- seqcount_init(&fs->seq);
+ seqcount_spinlock_init(&fs->seq, &fs->lock);
fs->umask = old->umask;
spin_lock(&old->lock);
@@ -163,6 +163,6 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
- .seq = SEQCNT_ZERO(init_fs.seq),
+ .seq = SEQCNT_SPINLOCK_ZERO(init_fs.seq, &init_fs.lock),
.umask = 0022,
};
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index a81f0c3cf352..65d975bf9390 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -89,7 +89,7 @@ extern struct dentry_stat_t dentry_stat;
struct dentry {
/* RCU lookup touched fields */
unsigned int d_flags; /* protected by d_lock */
- seqcount_t d_seq; /* per dentry seqlock */
+ seqcount_spinlock_t d_seq; /* per dentry seqlock */
struct hlist_bl_node d_hash; /* lookup hash list */
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index cf1015abfbf2..783b48dedb72 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -9,7 +9,7 @@
struct fs_struct {
int users;
spinlock_t lock;
- seqcount_t seq;
+ seqcount_spinlock_t seq;
int umask;
int in_exec;
struct path root, pwd;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index ab8067f9ce8c..892aefe88fa7 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6935,7 +6935,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
} else
goto abort;
spin_lock_init(&conf->device_lock);
- seqcount_init(&conf->gen_lock);
+ seqcount_spinlock_init(&conf->gen_lock, &conf->device_lock);
mutex_init(&conf->cache_size_mutex);
init_waitqueue_head(&conf->wait_for_quiescent);
init_waitqueue_head(&conf->wait_for_stripe);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index f90e0704bed9..a2c9e9e9f5ac 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -589,7 +589,7 @@ struct r5conf {
int prev_chunk_sectors;
int prev_algo;
short generation; /* increments with every reshape */
- seqcount_t gen_lock; /* lock against generation changes */
+ seqcount_spinlock_t gen_lock; /* lock against generation changes */
unsigned long reshape_checkpoint; /* Time we last updated
* metadata */
long long min_offset_diff; /* minimum difference between
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/hrtimer.h | 2 +-
kernel/time/hrtimer.c | 13 ++++++++++---
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 15c8ac313678..25993b86ac5c 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -159,7 +159,7 @@ struct hrtimer_clock_base {
struct hrtimer_cpu_base *cpu_base;
unsigned int index;
clockid_t clockid;
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct hrtimer *running;
struct timerqueue_head active;
ktime_t (*get_time)(void);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index d89da1c7e005..c4038511d5c9 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -135,7 +135,11 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
* timer->base->cpu_base
*/
static struct hrtimer_cpu_base migration_cpu_base = {
- .clock_base = { { .cpu_base = &migration_cpu_base, }, },
+ .clock_base = { {
+ .cpu_base = &migration_cpu_base,
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(migration_cpu_base.seq,
+ &migration_cpu_base.lock),
+ }, },
};
#define migration_base migration_cpu_base.clock_base[0]
@@ -1998,8 +2002,11 @@ int hrtimers_prepare_cpu(unsigned int cpu)
int i;
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
- cpu_base->clock_base[i].cpu_base = cpu_base;
- timerqueue_init_head(&cpu_base->clock_base[i].active);
+ struct hrtimer_clock_base *clock_b = &cpu_base->clock_base[i];
+
+ clock_b->cpu_base = cpu_base;
+ seqcount_raw_spinlock_init(&clock_b->seq, &cpu_base->lock);
+ timerqueue_init_head(&clock_b->active);
}
cpu_base->cpu = cpu;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
---
include/linux/kvm_irqfd.h | 2 +-
virt/kvm/eventfd.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dc1da020305b..dac047abdba7 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
wait_queue_entry_t wait;
/* Update side is protected by irqfds.lock */
struct kvm_kernel_irq_routing_entry irq_entry;
- seqcount_t irq_entry_sc;
+ seqcount_spinlock_t irq_entry_sc;
/* Used for level IRQ fast-path */
int gsi;
struct work_struct inject;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index ef7ed916ad4a..d6408bb497dc 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
INIT_LIST_HEAD(&irqfd->list);
INIT_WORK(&irqfd->inject, irqfd_inject);
INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
- seqcount_init(&irqfd->irq_entry_sc);
+ seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
f = fdget(args->fd);
if (!f.file) {
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
fs/userfaultfd.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 52de29000c7e..26e8b23594fb 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -61,7 +61,7 @@ struct userfaultfd_ctx {
/* waitqueue head for events */
wait_queue_head_t event_wqh;
/* a refile sequence protected by fault_pending_wqh lock */
- struct seqcount refile_seq;
+ seqcount_spinlock_t refile_seq;
/* pseudo fd refcounting */
refcount_t refcount;
/* userfaultfd syscall flags */
@@ -1998,7 +1998,7 @@ static void init_once_userfaultfd_ctx(void *mem)
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->event_wqh);
init_waitqueue_head(&ctx->fd_wqh);
- seqcount_init(&ctx->refile_seq);
+ seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
}
SYSCALL_DEFINE1(userfaultfd, int, flags)
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
kernel/time/timekeeping.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d20d489841c8..05ecfd8a3314 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -39,18 +39,19 @@ enum timekeeping_adv_mode {
TK_ADV_FREQ
};
+static DEFINE_RAW_SPINLOCK(timekeeper_lock);
+
/*
* The most important data for readout fits into a single 64 byte
* cache line.
*/
static struct {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct timekeeper timekeeper;
} tk_core ____cacheline_aligned = {
- .seq = SEQCNT_ZERO(tk_core.seq),
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_core.seq, &timekeeper_lock),
};
-static DEFINE_RAW_SPINLOCK(timekeeper_lock);
static struct timekeeper shadow_timekeeper;
/**
@@ -63,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct tk_read_base base[2];
};
@@ -80,11 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -157,7 +160,7 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
* tk_clock_read - atomic clocksource read() helper
*
* This helper is necessary to use in the read paths because, while the
- * seqlock ensures we don't return a bad value while structures are updated,
+ * seqcount ensures we don't return a bad value while structures are updated,
* it doesn't protect from potential crashes. There is the possibility that
* the tkr's clocksource may change between the read reference, and the
* clock reference passed to the read function. This can cause crashes if
@@ -222,10 +225,10 @@ static inline u64 timekeeping_get_delta(const struct tk_read_base *tkr)
unsigned int seq;
/*
- * Since we're called holding a seqlock, the data may shift
+ * Since we're called holding a seqcount, the data may shift
* under us while we're doing the calculation. This can cause
* false positives, since we'd note a problem but throw the
- * results away. So nest another seqlock here to atomically
+ * results away. So nest another seqcount here to atomically
* grab the points we are checking with.
*/
do {
@@ -486,7 +489,7 @@ EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
*
* To keep it NMI safe since we're accessing from tracing, we're not using a
* separate timekeeper with updates to monotonic clock and boot offset
- * protected with seqlocks. This has the following minor side effects:
+ * protected with seqcounts. This has the following minor side effects:
*
* (1) Its possible that a timestamp be taken after the boot offset is updated
* but before the timekeeper is updated. If this happens, the new boot offset
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_rwlock_t data type, which allows to associate a
rwlock with the sequence counter. This enables lockdep to verify that
the rwlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/netfilter/nft_set_rbtree.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/netfilter/nft_set_rbtree.c b/net/netfilter/nft_set_rbtree.c
index b6aad3fc46c3..4b2834fd17b2 100644
--- a/net/netfilter/nft_set_rbtree.c
+++ b/net/netfilter/nft_set_rbtree.c
@@ -18,7 +18,7 @@
struct nft_rbtree {
struct rb_root root;
rwlock_t lock;
- seqcount_t count;
+ seqcount_rwlock_t count;
struct delayed_work gc_work;
};
@@ -523,7 +523,7 @@ static int nft_rbtree_init(const struct nft_set *set,
struct nft_rbtree *priv = nft_set_priv(set);
rwlock_init(&priv->lock);
- seqcount_init(&priv->count);
+ seqcount_rwlock_init(&priv->count, &priv->lock);
priv->root = RB_ROOT;
INIT_DEFERRABLE_WORK(&priv->gc_work, nft_rbtree_gc);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/net/netfilter/nf_conntrack.h | 2 +-
net/netfilter/nf_conntrack_core.c | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 90690e37a56f..ea4e2010b246 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -286,7 +286,7 @@ int nf_conntrack_hash_resize(unsigned int hashsize);
extern struct hlist_nulls_head *nf_conntrack_hash;
extern unsigned int nf_conntrack_htable_size;
-extern seqcount_t nf_conntrack_generation;
+extern seqcount_spinlock_t nf_conntrack_generation;
extern unsigned int nf_conntrack_max;
/* must be called with rcu read lock held */
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 79cd9dde457b..b8c54d390f93 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -180,7 +180,7 @@ EXPORT_SYMBOL_GPL(nf_conntrack_htable_size);
unsigned int nf_conntrack_max __read_mostly;
EXPORT_SYMBOL_GPL(nf_conntrack_max);
-seqcount_t nf_conntrack_generation __read_mostly;
+seqcount_spinlock_t nf_conntrack_generation __read_mostly;
static unsigned int nf_conntrack_hash_rnd __read_mostly;
static u32 hash_conntrack_raw(const struct nf_conntrack_tuple *tuple,
@@ -2598,7 +2598,8 @@ int nf_conntrack_init_start(void)
/* struct nf_ct_ext uses u8 to store offsets/size */
BUILD_BUG_ON(total_extension_size() > 255u);
- seqcount_init(&nf_conntrack_generation);
+ seqcount_spinlock_init(&nf_conntrack_generation,
+ &nf_conntrack_locks_all_lock);
for (i = 0; i < CONNTRACK_LOCKS; i++)
spin_lock_init(&nf_conntrack_locks[i]);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
The dma-buf reservation subsystem uses plain sequence counters to manage
updates to reservations. Writer serialization is accomplished through a
wound/wait mutex.
Acquiring a wound/wait mutex does not disable preemption, so this needs
to be done manually before and after the write side critical section.
Use the newly-added seqcount_ww_mutex_t instead:
- It associates the ww_mutex with the sequence count, which enables
lockdep to validate that the write side critical section is properly
serialized.
- It removes the need to explicitly add preempt_disable/enable()
around the write side critical section because the write_begin/end()
functions for this new data type automatically do this.
If lockdep is disabled this ww_mutex lock association is compiled out
and has neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
---
drivers/dma-buf/dma-resv.c | 8 +-------
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 --
include/linux/dma-resv.h | 2 +-
3 files changed, 2 insertions(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 15efa0c2dacb..a7631352a486 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -129,7 +129,7 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
- seqcount_init(&obj->seq);
+ seqcount_ww_mutex_init(&obj->seq, &obj->lock);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
@@ -260,7 +260,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
fobj = dma_resv_get_list(obj);
count = fobj->shared_count;
- preempt_disable();
write_seqcount_begin(&obj->seq);
for (i = 0; i < count; ++i) {
@@ -282,7 +281,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
smp_store_mb(fobj->shared_count, count);
write_seqcount_end(&obj->seq);
- preempt_enable();
dma_fence_put(old);
}
EXPORT_SYMBOL(dma_resv_add_shared_fence);
@@ -309,14 +307,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
if (fence)
dma_fence_get(fence);
- preempt_disable();
write_seqcount_begin(&obj->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(obj->fence_excl, fence);
if (old)
old->shared_count = 0;
write_seqcount_end(&obj->seq);
- preempt_enable();
/* inplace update, no shared fences */
while (i--)
@@ -394,13 +390,11 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
src_list = dma_resv_get_list(dst);
old = dma_resv_get_excl(dst);
- preempt_disable();
write_seqcount_begin(&dst->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(dst->fence_excl, new);
RCU_INIT_POINTER(dst->fence, dst_list);
write_seqcount_end(&dst->seq);
- preempt_enable();
dma_resv_list_free(src_list);
dma_fence_put(old);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
index b91b5171270f..ff4b583cb96a 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
@@ -258,11 +258,9 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
new->shared_count = k;
/* Install the new fence list, seqcount provides the barriers */
- preempt_disable();
write_seqcount_begin(&resv->seq);
RCU_INIT_POINTER(resv->fence, new);
write_seqcount_end(&resv->seq);
- preempt_enable();
/* Drop the references to the removed fences or move them to ef_list */
for (i = j, k = 0; i < old->shared_count; ++i) {
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index a6538ae7d93f..d44a77e8a7e3 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -69,7 +69,7 @@ struct dma_resv_list {
*/
struct dma_resv {
struct ww_mutex lock;
- seqcount_t seq;
+ seqcount_ww_mutex_t seq;
struct dma_fence __rcu *fence_excl;
struct dma_resv_list __rcu *fence;
--
2.20.1
Commit 3c3b177a9369 ("reservation: add support for read-only access
using rcu") introduced a sequence counter to manage updates to
reservations. Back then, the reservation object initializer
reservation_object_init() was always inlined.
Having the sequence counter initialization inlined meant that each of
the call sites would have a different lockdep class key, which would've
broken lockdep's deadlock detection. The aforementioned commit thus
introduced, and exported, a custom seqcount lockdep class key and name.
The commit 8735f16803f00 ("dma-buf: cleanup reservation_object_init...")
transformed the reservation object initializer to a normal non-inlined C
function. seqcount_init(), which automatically defines the seqcount
lockdep class key and must be called non-inlined, can now be safely used.
Remove the seqcount custom lockdep class key, name, and export. Use
seqcount_init() inside the dma reservation object initializer.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
---
drivers/dma-buf/dma-resv.c | 9 +--------
include/linux/dma-resv.h | 2 --
2 files changed, 1 insertion(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index b45f8514dc82..15efa0c2dacb 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -51,12 +51,6 @@
DEFINE_WD_CLASS(reservation_ww_class);
EXPORT_SYMBOL(reservation_ww_class);
-struct lock_class_key reservation_seqcount_class;
-EXPORT_SYMBOL(reservation_seqcount_class);
-
-const char reservation_seqcount_string[] = "reservation_seqcount";
-EXPORT_SYMBOL(reservation_seqcount_string);
-
/**
* dma_resv_list_alloc - allocate fence list
* @shared_max: number of fences we need space for
@@ -135,9 +129,8 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
+ seqcount_init(&obj->seq);
- __seqcount_init(&obj->seq, reservation_seqcount_string,
- &reservation_seqcount_class);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
}
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index ee50d10f052b..a6538ae7d93f 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -46,8 +46,6 @@
#include <linux/rcupdate.h>
extern struct ww_class reservation_ww_class;
-extern struct lock_class_key reservation_seqcount_class;
-extern const char reservation_seqcount_string[];
/**
* struct dma_resv_list - a list of shared fences
--
2.20.1
raw_seqcount_begin() has the same code as raw_read_seqcount(), with the
exception of masking the sequence counter's LSB before returning it to
the caller.
Note, raw_seqcount_begin() masks the counter's LSB before returning it
to the caller so that read_seqcount_retry() can fail if the counter is
odd -- without the overhead of an extra branching instruction.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 85fb3ac93ffb..e885702d8b82 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -199,10 +199,11 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
*/
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
- unsigned ret = READ_ONCE(s->sequence);
- smp_rmb();
- kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
- return ret & ~1;
+ /*
+ * If the counter is odd, let read_seqcount_retry() fail
+ * by decrementing the counter.
+ */
+ return raw_read_seqcount(s) & ~1;
}
/**
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Reviewed-by: Daniel Wagner <[email protected]>
---
block/blk-iocost.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 8ac4aad66ebc..8e940c27c27c 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -406,7 +406,7 @@ struct ioc {
enum ioc_running running;
atomic64_t vtime_rate;
- seqcount_t period_seqcount;
+ seqcount_spinlock_t period_seqcount;
u32 period_at; /* wallclock starttime */
u64 period_at_vtime; /* vtime starttime */
@@ -873,7 +873,6 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
{
- lockdep_assert_held(&ioc->lock);
WARN_ON_ONCE(ioc->running != IOC_RUNNING);
write_seqcount_begin(&ioc->period_seqcount);
@@ -2001,7 +2000,7 @@ static int blk_iocost_init(struct request_queue *q)
ioc->running = IOC_IDLE;
atomic64_set(&ioc->vtime_rate, VTIME_PER_USEC);
- seqcount_init(&ioc->period_seqcount);
+ seqcount_spinlock_init(&ioc->period_seqcount, &ioc->lock);
ioc->period_at = ktime_to_us(ktime_get());
atomic64_set(&ioc->cur_period, 0);
atomic_set(&ioc->hweight_gen, 0);
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
A plain seqcount_t does not contain the information of which lock must
be held when entering a write side critical section.
Use the new seqcount_spinlock_t and seqcount_mutex_t data types instead,
which allow to associate a lock with the sequence counter. This enables
lockdep to verify that the lock used for writer serialization is held
when the write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
net/xfrm/xfrm_policy.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 564aa6492e7c..732a940468b0 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -122,7 +122,7 @@ struct xfrm_pol_inexact_bin {
/* list containing '*:*' policies */
struct hlist_head hhead;
- seqcount_t count;
+ seqcount_spinlock_t count;
/* tree sorted by daddr/prefix */
struct rb_root root_d;
@@ -155,7 +155,7 @@ static struct xfrm_policy_afinfo const __rcu *xfrm_policy_afinfo[AF_INET6 + 1]
__read_mostly;
static struct kmem_cache *xfrm_dst_cache __ro_after_init;
-static __read_mostly seqcount_t xfrm_policy_hash_generation;
+static __read_mostly seqcount_mutex_t xfrm_policy_hash_generation;
static struct rhashtable xfrm_policy_inexact_table;
static const struct rhashtable_params xfrm_pol_inexact_params;
@@ -719,7 +719,7 @@ xfrm_policy_inexact_alloc_bin(const struct xfrm_policy *pol, u8 dir)
INIT_HLIST_HEAD(&bin->hhead);
bin->root_d = RB_ROOT;
bin->root_s = RB_ROOT;
- seqcount_init(&bin->count);
+ seqcount_spinlock_init(&bin->count, &net->xfrm.xfrm_policy_lock);
prev = rhashtable_lookup_get_insert_key(&xfrm_policy_inexact_table,
&bin->k, &bin->head,
@@ -1906,7 +1906,7 @@ static int xfrm_policy_match(const struct xfrm_policy *pol,
static struct xfrm_pol_inexact_node *
xfrm_policy_lookup_inexact_addr(const struct rb_root *r,
- seqcount_t *count,
+ seqcount_spinlock_t *count,
const xfrm_address_t *addr, u16 family)
{
const struct rb_node *parent;
@@ -4153,7 +4153,7 @@ void __init xfrm_init(void)
{
register_pernet_subsys(&xfrm_net_ops);
xfrm_dev_init();
- seqcount_init(&xfrm_policy_hash_generation);
+ seqcount_mutex_init(&xfrm_policy_hash_generation, &hash_resize_mutex);
xfrm_input_init();
#ifdef CONFIG_INET_ESPINTCP
--
2.20.1
Align the code samples and note sections inside kernel-doc comments with
tabs. This way they can be properly parsed and rendered by Sphinx. It
also makes the code samples easier to read from text editors.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 108 +++++++++++++++++++++-------------------
1 file changed, 56 insertions(+), 52 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 299d68f10325..6c4f68ef1393 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -263,32 +263,32 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* atomically, avoiding compiler optimizations; b) to document which writes are
* meant to propagate to the reader critical section. This is necessary because
* neither writes before and after the barrier are enclosed in a seq-writer
- * critical section that would ensure readers are aware of ongoing writes.
+ * critical section that would ensure readers are aware of ongoing writes::
*
- * seqcount_t seq;
- * bool X = true, Y = false;
+ * seqcount_t seq;
+ * bool X = true, Y = false;
*
- * void read(void)
- * {
- * bool x, y;
+ * void read(void)
+ * {
+ * bool x, y;
*
- * do {
- * int s = read_seqcount_begin(&seq);
+ * do {
+ * int s = read_seqcount_begin(&seq);
*
- * x = X; y = Y;
+ * x = X; y = Y;
*
- * } while (read_seqcount_retry(&seq, s));
+ * } while (read_seqcount_retry(&seq, s));
*
- * BUG_ON(!x && !y);
+ * BUG_ON(!x && !y);
* }
*
* void write(void)
* {
- * WRITE_ONCE(Y, true);
+ * WRITE_ONCE(Y, true);
*
- * raw_write_seqcount_barrier(seq);
+ * raw_write_seqcount_barrier(seq);
*
- * WRITE_ONCE(X, false);
+ * WRITE_ONCE(X, false);
* }
*/
static inline void raw_write_seqcount_barrier(seqcount_t *s)
@@ -325,64 +325,68 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* Very simply put: we first modify one copy and then the other. This ensures
* there is always one copy in a stable state, ready to give us an answer.
*
- * The basic form is a data structure like:
+ * The basic form is a data structure like::
*
- * struct latch_struct {
- * seqcount_t seq;
- * struct data_struct data[2];
- * };
+ * struct latch_struct {
+ * seqcount_t seq;
+ * struct data_struct data[2];
+ * };
*
* Where a modification, which is assumed to be externally serialized, does the
- * following:
+ * following::
*
- * void latch_modify(struct latch_struct *latch, ...)
- * {
- * smp_wmb(); <- Ensure that the last data[1] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * void latch_modify(struct latch_struct *latch, ...)
+ * {
+ * smp_wmb(); // Ensure that the last data[1] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[0], ...);
+ * modify(latch->data[0], ...);
*
- * smp_wmb(); <- Ensure that the data[0] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * smp_wmb(); // Ensure that the data[0] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[1], ...);
- * }
+ * modify(latch->data[1], ...);
+ * }
*
- * The query will have a form like:
+ * The query will have a form like::
*
- * struct entry *latch_query(struct latch_struct *latch, ...)
- * {
- * struct entry *entry;
- * unsigned seq, idx;
+ * struct entry *latch_query(struct latch_struct *latch, ...)
+ * {
+ * struct entry *entry;
+ * unsigned seq, idx;
*
- * do {
- * seq = raw_read_seqcount_latch(&latch->seq);
+ * do {
+ * seq = raw_read_seqcount_latch(&latch->seq);
*
- * idx = seq & 0x01;
- * entry = data_query(latch->data[idx], ...);
+ * idx = seq & 0x01;
+ * entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * smp_rmb();
+ * } while (seq != latch->seq);
*
- * return entry;
- * }
+ * return entry;
+ * }
*
* So during the modification, queries are first redirected to data[1]. Then we
* modify data[0]. When that is complete, we redirect queries back to data[0]
* and we can modify data[1].
*
- * NOTE: The non-requirement for atomic modifications does _NOT_ include
- * the publishing of new entries in the case where data is a dynamic
- * data structure.
+ * NOTE:
*
- * An iteration might start in data[0] and get suspended long enough
- * to miss an entire modification sequence, once it resumes it might
- * observe the new entry.
+ * The non-requirement for atomic modifications does _NOT_ include
+ * the publishing of new entries in the case where data is a dynamic
+ * data structure.
*
- * NOTE: When data is a dynamic data structure; one should use regular RCU
- * patterns to manage the lifetimes of the objects within.
+ * An iteration might start in data[0] and get suspended long enough
+ * to miss an entire modification sequence, once it resumes it might
+ * observe the new entry.
+ *
+ * NOTE:
+ *
+ * When data is a dynamic data structure; one should use regular RCU
+ * patterns to manage the lifetimes of the objects within.
*/
static inline void raw_write_seqcount_latch(seqcount_t *s)
{
--
2.20.1
Parent commit, "seqlock: Extend seqcount API with associated locks",
introduced a big number of multi-line macros that are newline-escaped
at 72 columns.
For overall cohesion, align the earlier-existing macros similarly.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 29 +++++++++++++++--------------
1 file changed, 15 insertions(+), 14 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8c16a494c968..b48729988325 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -80,17 +80,18 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
}
#ifdef CONFIG_DEBUG_LOCK_ALLOC
-# define SEQCOUNT_DEP_MAP_INIT(lockname) \
- .dep_map = { .name = #lockname } \
+
+# define SEQCOUNT_DEP_MAP_INIT(lockname) \
+ .dep_map = { .name = #lockname }
/**
* seqcount_init() - runtime initializer for seqcount_t
* @s: Pointer to the seqcount_t instance
*/
-# define seqcount_init(s) \
- do { \
- static struct lock_class_key __key; \
- __seqcount_init((s), #s, &__key); \
+# define seqcount_init(s) \
+ do { \
+ static struct lock_class_key __key; \
+ __seqcount_init((s), #s, &__key); \
} while (0)
static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
@@ -842,20 +843,20 @@ typedef struct {
spinlock_t lock;
} seqlock_t;
-#define __SEQLOCK_UNLOCKED(lockname) \
- { \
- .seqcount = SEQCNT_ZERO(lockname), \
- .lock = __SPIN_LOCK_UNLOCKED(lockname) \
+#define __SEQLOCK_UNLOCKED(lockname) \
+ { \
+ .seqcount = SEQCNT_ZERO(lockname), \
+ .lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
/**
* seqlock_init() - dynamic initializer for seqlock_t
* @sl: Pointer to the seqlock_t instance
*/
-#define seqlock_init(sl) \
- do { \
- seqcount_init(&(sl)->seqcount); \
- spin_lock_init(&(sl)->lock); \
+#define seqlock_init(sl) \
+ do { \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
/**
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/sched.h | 2 +-
init/init_task.c | 3 ++-
kernel/fork.c | 2 +-
3 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3903a9500926..02b7fbd17bf6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1054,7 +1054,7 @@ struct task_struct {
/* Protected by ->alloc_lock: */
nodemask_t mems_allowed;
/* Seqence number to catch updates: */
- seqcount_t mems_allowed_seq;
+ seqcount_spinlock_t mems_allowed_seq;
int cpuset_mem_spread_rotor;
int cpuset_slab_spread_rotor;
#endif
diff --git a/init/init_task.c b/init/init_task.c
index 15089d15010a..94fe3ba1bb60 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -154,7 +154,8 @@ struct task_struct init_task
.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),
#endif
#ifdef CONFIG_CPUSETS
- .mems_allowed_seq = SEQCNT_ZERO(init_task.mems_allowed_seq),
+ .mems_allowed_seq = SEQCNT_SPINLOCK_ZERO(init_task.mems_allowed_seq,
+ &init_task.alloc_lock),
#endif
#ifdef CONFIG_RT_MUTEXES
.pi_waiters = RB_ROOT_CACHED,
diff --git a/kernel/fork.c b/kernel/fork.c
index 70d9d0a4de2a..fc72f09a61b2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2032,7 +2032,7 @@ static __latent_entropy struct task_struct *copy_process(
#ifdef CONFIG_CPUSETS
p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
- seqcount_init(&p->mems_allowed_seq);
+ seqcount_spinlock_init(&p->mems_allowed_seq, &p->alloc_lock);
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
p->irq_events = 0;
--
2.20.1
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the write side critical section.
There is no built-in debugging mechanism to verify that the lock used
for writer serialization is held and preemption is disabled. Some usage
sites like dma-buf have explicit lockdep checks for the writer-side
lock, but this covers only a small portion of the sequence counter usage
in the kernel.
Add new sequence counter types which allows to associate a lock to the
sequence counter at initialization time. The seqcount API functions are
extended to provide appropriate lockdep assertions depending on the
seqcount/lock type.
For sequence counters with associated locks that do not implicitly
disable preemption, preemption protection is enforced in the sequence
counter write side functions. This removes the need to explicitly add
preempt_disable/enable() around the write side critical sections: the
write_begin/end() functions for these new sequence counter types
automatically do this.
Introduce the following seqcount types with associated locks:
seqcount_spinlock_t
seqcount_raw_spinlock_t
seqcount_rwlock_t
seqcount_mutex_t
seqcount_ww_mutex_t
Extend the seqcount read and write functions to branch out to the
specific seqcount_LOCKTYPE_t implementation at compile-time. This avoids
kernel API explosion per each new seqcount_LOCKTYPE_t added. Add such
compile-time type detection logic into a new, internal, seqlock header.
Document the proper seqcount_LOCKTYPE_t usage, and rationale, at
Documentation/locking/seqlock.rst.
If lockdep is disabled, this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/seqlock.rst | 52 ++++
include/linux/seqlock.h | 462 +++++++++++++++++++++++++-----
2 files changed, 446 insertions(+), 68 deletions(-)
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index 366dd368d90a..62c5ad98c11c 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -87,6 +87,58 @@ Read path::
} while (read_seqcount_retry(&foo_seqcount, seq));
+.. _seqcount_locktype_t:
+
+Sequence counters with associated locks (``seqcount_LOCKTYPE_t``)
+-----------------------------------------------------------------
+
+As discussed at :ref:`seqcount_t`, sequence count write side critical
+sections must be serialized and non-preemptible. This variant of
+sequence counters associate the lock used for writer serialization at
+initialization time, which enables lockdep to validate that the write
+side critical sections are properly serialized.
+
+This lock association is a NOOP if lockdep is disabled and has neither
+storage nor runtime overhead. If lockdep is enabled, the lock pointer is
+stored in struct seqcount and lockdep's "lock is held" assertions are
+injected at the beginning of the write side critical section to validate
+that it is properly protected.
+
+For lock types which do not implicitly disable preemption, preemption
+protection is enforced in the write side function.
+
+The following sequence counters with associated locks are defined:
+
+ - ``seqcount_spinlock_t``
+ - ``seqcount_raw_spinlock_t``
+ - ``seqcount_rwlock_t``
+ - ``seqcount_mutex_t``
+ - ``seqcount_ww_mutex_t``
+
+The plain seqcount read and write APIs branch out to the specific
+seqcount_LOCKTYPE_t implementation at compile-time. This avoids kernel
+API explosion per each new seqcount LOCKTYPE.
+
+Initialization (replace "LOCKTYPE" with one of the supported locks)::
+
+ /* dynamic */
+ seqcount_LOCKTYPE_t foo_seqcount;
+ seqcount_LOCKTYPE_init(&foo_seqcount, &lock);
+
+ /* static */
+ static seqcount_LOCKTYPE_t foo_seqcount =
+ SEQCNT_LOCKTYPE_ZERO(foo_seqcount, &lock);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_LOCKTYPE_ZERO(foo.seq, &lock),
+ } foo;
+
+Write path: same as in :ref:`seqcount_t`, while running from a context
+with the associated LOCKTYPE lock acquired.
+
+Read path: same as in :ref:`seqcount_t`.
+
.. _seqlock_t:
Sequential locks (``seqlock_t``)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 54bc20496392..8c16a494c968 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -10,13 +10,17 @@
*
* Copyrights:
* - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
+ * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
*/
-#include <linux/spinlock.h>
-#include <linux/preempt.h>
-#include <linux/lockdep.h>
#include <linux/compiler.h>
#include <linux/kcsan-checks.h>
+#include <linux/lockdep.h>
+#include <linux/mutex.h>
+#include <linux/preempt.h>
+#include <linux/spinlock.h>
+#include <linux/ww_mutex.h>
+
#include <asm/processor.h>
/*
@@ -48,6 +52,10 @@
* This mechanism can't be used if the protected data contains pointers,
* as the writer can invalidate a pointer that a reader is following.
*
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_t) instead.
+ *
* If it's desired to automatically handle the sequence counter writer
* serialization and non-preemptibility requirements, use a sequential
* lock (seqlock_t) instead.
@@ -108,9 +116,267 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*/
#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
+/*
+ * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ *
+ * A sequence counter which associates the lock used for writer
+ * serialization at initialization time. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ *
+ * For associated locks which do not implicitly disable preemption,
+ * preemption protection is enforced in the write side function.
+ *
+ * Lockdep is never used in any for the raw write variants.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
+#ifdef CONFIG_LOCKDEP
+#define __SEQ_LOCKDEP(expr) expr
+#else
+#define __SEQ_LOCKDEP(expr)
+#endif
+
+#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+ __SEQ_LOCKDEP(.lock = (assoc_lock)) \
+}
+
+#define seqcount_locktype_init(s, assoc_lock) \
+do { \
+ seqcount_init(&(s)->seqcount); \
+ __SEQ_LOCKDEP((s)->lock = (assoc_lock)); \
+} while (0)
+
+/**
+ * typedef seqcount_spinlock_t - sequence counter with spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * spinlock. The spinlock is associated to the sequence count in the
+ * static initializer or init function. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_spinlock {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(spinlock_t *lock);
+} seqcount_spinlock_t;
+
+/**
+ * SEQCNT_SPINLOCK_ZERO - static initializer for seqcount_spinlock_t
+ * @name: Name of the seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define SEQCNT_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_spinlock_init - runtime initializer for seqcount_spinlock_t
+ * @s: Pointer to the seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define seqcount_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_raw_spinlock_t - sequence count with raw spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated raw spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * raw spinlock. The raw spinlock is associated to the sequence count in
+ * the static initializer or init function. This enables lockdep to
+ * validate that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_raw_spinlock {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(raw_spinlock_t *lock);
+} seqcount_raw_spinlock_t;
+
+/**
+ * SEQCNT_RAW_SPINLOCK_ZERO - static initializer for seqcount_raw_spinlock_t
+ * @name: Name of the seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_raw_spinlock_init - runtime initializer for seqcount_raw_spinlock_t
+ * @s: Pointer to the seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define seqcount_raw_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_rwlock_t - sequence count with rwlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated rwlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * rwlock. The rwlock is associated to the sequence count in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ */
+typedef struct seqcount_rwlock {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(rwlock_t *lock);
+} seqcount_rwlock_t;
+
+/**
+ * SEQCNT_RWLOCK_ZERO - static initializer for seqcount_rwlock_t
+ * @name: Name of the seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define SEQCNT_RWLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_rwlock_init - runtime initializer for seqcount_rwlock_t
+ * @s: Pointer to the seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define seqcount_rwlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_mutex_t - sequence count with mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * mutex. The mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_mutex_t.
+ */
+typedef struct seqcount_mutex {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(struct mutex *lock);
+} seqcount_mutex_t;
+
+/**
+ * SEQCNT_MUTEX_ZERO - static initializer for seqcount_mutex_t
+ * @name: Name of the seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define SEQCNT_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_mutex_init - runtime initializer for seqcount_mutex_t
+ * @s: Pointer to the seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define seqcount_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_ww_mutex_t - sequence count with ww_mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated ww_mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * ww_mutex. The ww_mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_ww_mutex_t.
+ */
+typedef struct seqcount_ww_mutex {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(struct ww_mutex *lock);
+} seqcount_ww_mutex_t;
+
+/**
+ * SEQCNT_WW_MUTEX_ZERO - static initializer for seqcount_ww_mutex_t
+ * @name: Name of the seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define SEQCNT_WW_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_ww_mutex_init - runtime initializer for seqcount_ww_mutex_t
+ * @s: Pointer to the seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define seqcount_ww_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/*
+ * @preempt: Is the associated write serialization lock preemtpible?
+ */
+#define SEQCOUNT_LOCKTYPE(locktype, preempt, lockmember) \
+static inline seqcount_t * \
+__seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
+{ \
+ return &s->seqcount; \
+} \
+ \
+static inline bool \
+__seqcount_##locktype##_preemptible(seqcount_##locktype##_t *s) \
+{ \
+ return preempt; \
+} \
+ \
+static inline void \
+__seqcount_##locktype##_assert(seqcount_##locktype##_t *s) \
+{ \
+ __SEQ_LOCKDEP(lockdep_assert_held(lockmember)); \
+}
+
+/*
+ * Similar hooks, but for plain seqcount_t
+ */
+
+static inline seqcount_t *__seqcount_ptr(seqcount_t *s)
+{
+ return s;
+}
+
+static inline bool __seqcount_preemptible(seqcount_t *s)
+{
+ return false;
+}
+
+static inline void __seqcount_assert(seqcount_t *s)
+{
+ lockdep_assert_preemption_disabled();
+}
+
+/*
+ * @s: Pointer to seqcount_locktype_t, generated hooks first parameter.
+ */
+SEQCOUNT_LOCKTYPE(raw_spinlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(spinlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(rwlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(mutex, true, s->lock)
+SEQCOUNT_LOCKTYPE(ww_mutex, true, &s->lock->base)
+
+#define __seqprop_case(s, locktype, prop) \
+ seqcount_##locktype##_t: __seqcount_##locktype##_##prop((void *)(s))
+
+#define __seqprop(s, prop) _Generic(*(s), \
+ seqcount_t: __seqcount_##prop((void *)(s)), \
+ __seqprop_case((s), raw_spinlock, prop), \
+ __seqprop_case((s), spinlock, prop), \
+ __seqprop_case((s), rwlock, prop), \
+ __seqprop_case((s), mutex, prop), \
+ __seqprop_case((s), ww_mutex, prop))
+
+#define __to_seqcount_t(s) __seqprop(s, ptr)
+#define __associated_lock_exists_and_is_preemptible(s) __seqprop(s, preemptible)
+#define __assert_write_section_is_protected(s) __seqprop(s, assert)
+
/**
* __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -122,7 +388,10 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+#define __read_seqcount_begin(s) \
+ __read_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
{
unsigned ret;
@@ -138,32 +407,38 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
/**
* raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+#define raw_read_seqcount_begin(s) \
+ raw_read_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
+ unsigned ret = __read_seqcount_t_begin(s);
smp_rmb();
return ret;
}
/**
* read_seqcount_begin() - begin a seqcount_t read critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned read_seqcount_begin(const seqcount_t *s)
+#define read_seqcount_begin(s) \
+ read_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
{
seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
+ return raw_read_seqcount_t_begin(s);
}
/**
* raw_read_seqcount() - read the raw seqcount_t counter value
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* raw_read_seqcount opens a read critical section of the given
* seqcount_t, without any lockdep checking, and without checking or
@@ -172,7 +447,10 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
+#define raw_read_seqcount(s) \
+ raw_read_seqcount_t(__to_seqcount_t(s))
+
+static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -183,7 +461,7 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
/**
* raw_seqcount_begin() - begin a seqcount_t read critical section w/o
* lockdep and w/o counter stabilization
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* raw_seqcount_begin opens a read critical section of the given
* seqcount_t. Unlike read_seqcount_begin(), this function will not wait
@@ -197,18 +475,21 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+#define raw_seqcount_begin(s) \
+ raw_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
{
/*
* If the counter is odd, let read_seqcount_retry() fail
* by decrementing the counter.
*/
- return raw_read_seqcount(s) & ~1;
+ return raw_read_seqcount_t(s) & ~1;
}
/**
* __read_seqcount_retry() - end a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin()
*
* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
@@ -221,7 +502,10 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
*
* Return: true if a read section retry is required, else false
*/
-static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define __read_seqcount_retry(s, start) \
+ __read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
kcsan_atomic_next(0);
return unlikely(READ_ONCE(s->sequence) != start);
@@ -229,7 +513,7 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
/**
* read_seqcount_retry() - end a seqcount_t read critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin()
*
* read_seqcount_retry closes the read critical section of given
@@ -238,17 +522,28 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
*
* Return: true if a read section retry is required, else false
*/
-static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define read_seqcount_retry(s, start) \
+ read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return __read_seqcount_t_retry(s, start);
}
/**
* raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*/
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+#define raw_write_seqcount_begin(s) \
+do { \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ raw_write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+static inline void raw_write_seqcount_t_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -257,49 +552,50 @@ static inline void raw_write_seqcount_begin(seqcount_t *s)
/**
* raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*/
-static inline void raw_write_seqcount_end(seqcount_t *s)
+#define raw_write_seqcount_end(s) \
+do { \
+ raw_write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void raw_write_seqcount_t_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
kcsan_nestable_atomic_end();
}
-static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
-}
-
/**
* write_seqcount_begin_nested() - start a seqcount_t write section with
* custom lockdep nesting level
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
* @subclass: lockdep nesting level
*
* See Documentation/locking/lockdep-design.rst
*/
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- lockdep_assert_preemption_disabled();
- __write_seqcount_begin_nested(s, subclass);
-}
+#define write_seqcount_begin_nested(s, subclass) \
+do { \
+ __assert_write_section_is_protected(s); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin_nested(__to_seqcount_t(s), subclass); \
+} while (0)
-/*
- * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
- *
- * Use for internal seqlock.h code where it's known that preemption is
- * already disabled. For example, seqlock_t write side functions.
- */
-static inline void __write_seqcount_begin(seqcount_t *s)
+static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
- __write_seqcount_begin_nested(s, 0);
+ raw_write_seqcount_t_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
/**
* write_seqcount_begin() - start a seqcount_t write side critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* write_seqcount_begin opens a write side critical section of the given
* seqcount_t.
@@ -308,26 +604,44 @@ static inline void __write_seqcount_begin(seqcount_t *s)
* non-preemptible. If readers can be invoked from hardirq or softirq
* context, interrupts or bottom halves must be respectively disabled.
*/
-static inline void write_seqcount_begin(seqcount_t *s)
+#define write_seqcount_begin(s) \
+do { \
+ __assert_write_section_is_protected(s); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+static inline void write_seqcount_t_begin(seqcount_t *s)
{
- write_seqcount_begin_nested(s, 0);
+ write_seqcount_t_begin_nested(s, 0);
}
/**
* write_seqcount_end() - end a seqcount_t write side critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* The write section must've been opened with write_seqcount_begin().
*/
-static inline void write_seqcount_end(seqcount_t *s)
+#define write_seqcount_end(s) \
+do { \
+ write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void write_seqcount_t_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
- raw_write_seqcount_end(s);
+ raw_write_seqcount_t_end(s);
}
/**
* raw_write_seqcount_barrier() - do a seqcount_t write barrier
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* This can be used to provide an ordering guarantee instead of the usual
* consistency guarantee. It is one wmb cheaper, because it can collapse
@@ -366,7 +680,10 @@ static inline void write_seqcount_end(seqcount_t *s)
* WRITE_ONCE(X, false);
* }
*/
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
+#define raw_write_seqcount_barrier(s) \
+ raw_write_seqcount_t_barrier(__to_seqcount_t(s))
+
+static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -378,12 +695,15 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
/**
* write_seqcount_invalidate() - invalidate in-progress seqcount_t read
* side operations
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* After write_seqcount_invalidate, no seqcount_t read side operations
* will complete successfully and see data older than this.
*/
-static inline void write_seqcount_invalidate(seqcount_t *s)
+#define write_seqcount_invalidate(s) \
+ write_seqcount_t_invalidate(__to_seqcount_t(s))
+
+static inline void write_seqcount_t_invalidate(seqcount_t *s)
{
smp_wmb();
kcsan_nestable_atomic_begin();
@@ -393,7 +713,7 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
/**
* raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* Use seqcount_t latching to switch between two storage places protected
* by a sequence counter. Doing so allows having interruptible, preemptible,
@@ -406,7 +726,10 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
* picking which data copy to read. The full counter value must then be
* checked with read_seqcount_retry().
*/
-static inline int raw_read_seqcount_latch(seqcount_t *s)
+#define raw_read_seqcount_latch(s) \
+ raw_read_seqcount_t_latch(__to_seqcount_t(s))
+
+static inline int raw_read_seqcount_t_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
@@ -415,7 +738,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
/**
* raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -494,7 +817,10 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
-static inline void raw_write_seqcount_latch(seqcount_t *s)
+#define raw_write_seqcount_latch(s) \
+ raw_write_seqcount_t_latch(__to_seqcount_t(s))
+
+static inline void raw_write_seqcount_t_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
@@ -592,7 +918,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -604,7 +930,7 @@ static inline void write_seqlock(seqlock_t *sl)
*/
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
@@ -618,7 +944,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -631,7 +957,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
*/
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
@@ -645,7 +971,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -657,7 +983,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
*/
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -666,7 +992,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
return flags;
}
@@ -695,13 +1021,13 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
/**
* read_seqlock_excl() - begin a seqlock_t locking reader section
- * @sl: Pointer to seqlock_t
+ * @sl: Pointer to seqlock_t
*
* read_seqlock_excl opens a seqlock_t locking reader critical section. A
* locking reader exclusively locks out *both* other writers *and* other
--
2.20.1
Proper documentation for the design and usage of sequence counters and
sequential locks does not exist. Complete the seqlock.h documentation as
follows:
- Divide all documentation on a seqcount_t vs. seqlock_t basis. The
description for both mechanisms was intermingled, which is incorrect
since the usage constrains for each type are vastly different.
- Add an introductory paragraph describing the internal design of, and
rationale for, sequence counters.
- Document seqcount_t writer non-preemptibility requirement, which was
not previously documented anywhere, and provide a clear rationale.
- Provide template code for seqcount_t and seqlock_t initialization
and reader/writer critical sections.
- Recommend using seqlock_t by default. It implicitly handles the
serialization and non-preemptibility requirements of writers.
At seqlock.h:
- Remove references to brlocks as they've long been removed from the
kernel.
- Remove references to gcc-3.x since the kernel's minimum supported
gcc version is 4.9.
References: 0f6ed63b1707 ("no need to keep brlock macros anymore...")
References: 6ec4476ac825 ("Raise gcc version requirement to 4.9")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 170 ++++++++++++++++++++++++++++++
include/linux/seqlock.h | 81 +++++++-------
3 files changed, 209 insertions(+), 43 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
index d785878cad65..7003bd5aeff4 100644
--- a/Documentation/locking/index.rst
+++ b/Documentation/locking/index.rst
@@ -14,6 +14,7 @@ locking
mutex-design
rt-mutex-design
rt-mutex
+ seqlock
spinlocks
ww-mutex-design
preempt-locking
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
new file mode 100644
index 000000000000..366dd368d90a
--- /dev/null
+++ b/Documentation/locking/seqlock.rst
@@ -0,0 +1,170 @@
+======================================
+Sequence counters and sequential locks
+======================================
+
+Introduction
+============
+
+Sequence counters are a reader-writer consistency mechanism with
+lockless readers (read-only retry loops), and no writer starvation. They
+are used for data that's rarely written to (e.g. system time), where the
+reader wants a consistent set of information and is willing to retry if
+that information changes.
+
+A data set is consistent when the sequence count at the beginning of the
+read side critical section is even and the same sequence count value is
+read again at the end of the critical section. The data in the set must
+be copied out inside the read side critical section. If the sequence
+count has changed between the start and the end of the critical section,
+the reader must retry.
+
+Writers increment the sequence count at the start and the end of their
+critical section. After starting the critical section the sequence count
+is odd and indicates to the readers that an update is in progress. At
+the end of the write side critical section the sequence count becomes
+even again which lets readers make progress.
+
+A sequence counter write side critical section must never be preempted
+or interrupted by read side sections. Otherwise the reader will spin for
+the entire scheduler tick due to the odd sequence count value and the
+interrupted writer. If that reader belongs to a real-time scheduling
+class, it can spin forever and the kernel will livelock.
+
+This mechanism cannot be used if the protected data contains pointers,
+as the writer can invalidate a pointer that the reader is following.
+
+
+.. _seqcount_t:
+
+Sequence counters (``seqcount_t``)
+==================================
+
+This is the the raw counting mechanism, which does not protect against
+multiple writers. Write side critical sections must thus be serialized
+by an external lock.
+
+If the write serialization primitive is not implicitly disabling
+preemption, preemption must be explicitly disabled before entering the
+write side section. If the read section can be invoked from hardirq or
+softirq contexts, interrupts or bottom halves must also be respectively
+disabled before entering the write section.
+
+If it's desired to automatically handle the sequence counter
+requirements of writer serialization and non-preemptibility, use
+:ref:`seqlock_t` instead.
+
+Initialization::
+
+ /* dynamic */
+ seqcount_t foo_seqcount;
+ seqcount_init(&foo_seqcount);
+
+ /* static */
+ static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_ZERO(foo.seq),
+ } foo;
+
+Write path::
+
+ /* Serialized context with disabled preemption */
+
+ write_seqcount_begin(&foo_seqcount);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_seqcount_end(&foo_seqcount);
+
+Read path::
+
+ do {
+ seq = read_seqcount_begin(&foo_seqcount);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqcount_retry(&foo_seqcount, seq));
+
+
+.. _seqlock_t:
+
+Sequential locks (``seqlock_t``)
+================================
+
+This contains the :ref:`seqcount_t` mechanism earlier discussed, plus an
+embedded spinlock for writer serialization and non-preemptibility.
+
+If the read side section can be invoked from hardirq or softirq context,
+use the write side function variants which disable interrupts or bottom
+halves respectively.
+
+Initialization::
+
+ /* dynamic */
+ seqlock_t foo_seqlock;
+ seqlock_init(&foo_seqlock);
+
+ /* static */
+ static DEFINE_SEQLOCK(foo_seqlock);
+
+ /* C99 struct init */
+ struct {
+ .seql = __SEQLOCK_UNLOCKED(foo.seql)
+ } foo;
+
+Write path::
+
+ write_seqlock(&foo_seqlock);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_sequnlock(&foo_seqlock);
+
+Read path, three categories:
+
+1. Normal Sequence readers which never block a writer but they must
+ retry if a writer is in progress by detecting change in the sequence
+ number. Writers do not wait for a sequence reader::
+
+ do {
+ seq = read_seqbegin(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqretry(&foo_seqlock, seq));
+
+2. Locking readers which will wait if a writer or another locking reader
+ is in progress. A locking reader in progress will also block a writer
+ from entering its critical section. This read lock is
+ exclusive. Unlike rwlock_t, only one locking reader can acquire it::
+
+ read_seqlock_excl(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ read_sequnlock_excl(&foo_seqlock);
+
+3. Conditional lockless reader (as in 1), or locking reader (as in 2),
+ according to a passed marker. This is used to avoid lockless readers
+ starvation (too much retry loops) in case of a sharp spike in write
+ activity. First, a lockless read is tried (even marker passed). If
+ that trial fails (odd sequence counter is returned, which is used as
+ the next iteration marker), the lockless read is transformed to a
+ full locking read and no retry loop is necessary::
+
+ /* marker; even initialization */
+ int seq = 0;
+ do {
+ read_seqbegin_or_lock(&foo_seqlock, &seq);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (need_seqretry(&foo_seqlock, seq));
+ done_seqretry(&foo_seqlock, seq);
+
+
+API documentation
+=================
+
+.. kernel-doc:: include/linux/seqlock.h
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8b97204f35a7..299d68f10325 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
*
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
+ * See Documentation/locking/seqlock.rst
*
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
- *
- *
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/
#include <linux/spinlock.h>
@@ -41,8 +20,8 @@
#include <asm/processor.h>
/*
- * The seqlock interface does not prescribe a precise sequence of read
- * begin/retry/end. For readers, typically there is a call to
+ * The seqlock seqcount_t interface does not prescribe a precise sequence of
+ * read begin/retry/end. For readers, typically there is a call to
* read_seqcount_begin() and read_seqcount_retry(), however, there are more
* esoteric cases which do not follow this pattern.
*
@@ -50,16 +29,30 @@
* via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
* pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
* atomics; if there is a matching read_seqcount_retry() call, no following
- * memory operations are considered atomic. Usage of seqlocks via seqlock_t
- * interface is not affected.
+ * memory operations are considered atomic. Usage of the seqlock_t interface
+ * is not affected.
*/
#define KCSAN_SEQLOCK_REGION_MAX 1000
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * This is the raw counting mechanism, without any writer protection.
+ *
+ * Write side critical sections must be serialized and non-preemptible.
+ *
+ * If readers can be invoked from hardirq or softirq contexts,
+ * interrupts or bottom halves must also be respectively disabled before
+ * entering the write section.
+ *
+ * This mechanism can't be used if the protected data contains pointers,
+ * as the writer can invalidate a pointer that a reader is following.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
typedef struct seqcount {
unsigned sequence;
@@ -398,10 +391,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-/*
- * Sequence counter only version assumes that callers are using their
- * own mutexing.
- */
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
@@ -434,15 +423,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * For more info, see:
+ * - Comments on top of seqcount_t
+ * - Documentation/locking/seqlock.rst
+ */
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
--
2.20.1
On Mon, Jul 20, 2020 at 05:55:06PM +0200, Ahmed S. Darwish wrote:
> Hi,
>
> This is v4 of the seqlock patch series:
>
> [PATCH v1 00/25]
> https://lore.kernel.org/lkml/[email protected]
>
> [PATCH v2 00/06] (bugfixes-only, merged)
> https://lore.kernel.org/lkml/[email protected]
>
> [PATCH v2 00/18]
> https://lore.kernel.org/lkml/[email protected]
>
> [PATCH v3 00/20]
> https://lore.kernel.org/lkml/[email protected]
>
> It is based over:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git :: locking/core
>
Please include an explanation of the patch series in the cover letter. It looks
like you sent it in v1 and then stopped including it. That doesn't work;
reviewers shouldn't have to read all previous versions too and try to guess
which parts are still up-to-date.
- Eric
On Mon, Jul 20, 2020 at 09:49:12AM -0700, Eric Biggers wrote:
> On Mon, Jul 20, 2020 at 05:55:06PM +0200, Ahmed S. Darwish wrote:
> > Hi,
> >
> > This is v4 of the seqlock patch series:
> >
> > [PATCH v1 00/25]
> > https://lore.kernel.org/lkml/[email protected]
> >
> > [PATCH v2 00/06] (bugfixes-only, merged)
> > https://lore.kernel.org/lkml/[email protected]
> >
> > [PATCH v2 00/18]
> > https://lore.kernel.org/lkml/[email protected]
> >
> > [PATCH v3 00/20]
> > https://lore.kernel.org/lkml/[email protected]
> >
> > It is based over:
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git :: locking/core
> >
>
> Please include an explanation of the patch series in the cover letter. It looks
> like you sent it in v1 and then stopped including it. That doesn't work;
> reviewers shouldn't have to read all previous versions too and try to guess
> which parts are still up-to-date.
>
Noted. Thanks for the heads-up.
--
Ahmed S. Darwish
Linutronix GmbH
On Mon, 20 Jul 2020 17:55:07 +0200
"Ahmed S. Darwish" <[email protected]> wrote:
> +Read path, three categories:
> +
> +1. Normal Sequence readers which never block a writer but they must
> + retry if a writer is in progress by detecting change in the sequence
> + number. Writers do not wait for a sequence reader::
> +
> + do {
> + seq = read_seqbegin(&foo_seqlock);
> +
> + /* ... [[read-side critical section]] ... */
> +
> + } while (read_seqretry(&foo_seqlock, seq));
> +
> +2. Locking readers which will wait if a writer or another locking reader
> + is in progress. A locking reader in progress will also block a writer
> + from entering its critical section. This read lock is
> + exclusive. Unlike rwlock_t, only one locking reader can acquire it::
Nit, but I would mention that this acts similar to a normal spin_lock, and even disables preeption (in the non-RT case).
> +
> + read_seqlock_excl(&foo_seqlock);
> +
> + /* ... [[read-side critical section]] ... */
> +
> + read_sequnlock_excl(&foo_seqlock);
> +
> +3. Conditional lockless reader (as in 1), or locking reader (as in 2),
> + according to a passed marker. This is used to avoid lockless readers
> + starvation (too much retry loops) in case of a sharp spike in write
> + activity. First, a lockless read is tried (even marker passed). If
> + that trial fails (odd sequence counter is returned, which is used as
> + the next iteration marker), the lockless read is transformed to a
> + full locking read and no retry loop is necessary::
> +
> + /* marker; even initialization */
> + int seq = 0;
> + do {
> + read_seqbegin_or_lock(&foo_seqlock, &seq);
> +
> + /* ... [[read-side critical section]] ... */
> +
> + } while (need_seqretry(&foo_seqlock, seq));
> + done_seqretry(&foo_seqlock, seq);
> +
> +
> +API documentation
> +=================
> +
> +.. kernel-doc:: include/linux/seqlock.h
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index 8b97204f35a7..299d68f10325 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -1,36 +1,15 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #ifndef __LINUX_SEQLOCK_H
> #define __LINUX_SEQLOCK_H
> +
> /*
> - * Reader/writer consistent mechanism without starving writers. This type of
> - * lock for data where the reader wants a consistent set of information
> - * and is willing to retry if the information changes. There are two types
> - * of readers:
> - * 1. Sequence readers which never block a writer but they may have to retry
> - * if a writer is in progress by detecting change in sequence number.
> - * Writers do not wait for a sequence reader.
> - * 2. Locking readers which will wait if a writer or another locking reader
> - * is in progress. A locking reader in progress will also block a writer
> - * from going forward. Unlike the regular rwlock, the read lock here is
> - * exclusive so that only one locking reader can get it.
> + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
> + * lockless readers (read-only retry loops), and no writer starvation.
> *
> - * This is not as cache friendly as brlock. Also, this may not work well
> - * for data that contains pointers, because any writer could
> - * invalidate a pointer that a reader was following.
> + * See Documentation/locking/seqlock.rst
> *
> - * Expected non-blocking reader usage:
> - * do {
> - * seq = read_seqbegin(&foo);
> - * ...
> - * } while (read_seqretry(&foo, seq));
> - *
> - *
> - * On non-SMP the spin locks disappear but the writer still needs
> - * to increment the sequence variables because an interrupt routine could
> - * change the state of the data.
> - *
> - * Based on x86_64 vsyscall gettimeofday
> - * by Keith Owens and Andrea Arcangeli
> + * Copyrights:
> + * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
> */
>
> #include <linux/spinlock.h>
> @@ -41,8 +20,8 @@
> #include <asm/processor.h>
>
> /*
> - * The seqlock interface does not prescribe a precise sequence of read
> - * begin/retry/end. For readers, typically there is a call to
> + * The seqlock seqcount_t interface does not prescribe a precise sequence of
> + * read begin/retry/end. For readers, typically there is a call to
> * read_seqcount_begin() and read_seqcount_retry(), however, there are more
> * esoteric cases which do not follow this pattern.
> *
> @@ -50,16 +29,30 @@
> * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
> * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
> * atomics; if there is a matching read_seqcount_retry() call, no following
> - * memory operations are considered atomic. Usage of seqlocks via seqlock_t
> - * interface is not affected.
> + * memory operations are considered atomic. Usage of the seqlock_t interface
> + * is not affected.
> */
> #define KCSAN_SEQLOCK_REGION_MAX 1000
>
> /*
> - * Version using sequence counter only.
> - * This can be used when code has its own mutex protecting the
> - * updating starting before the write_seqcountbeqin() and ending
> - * after the write_seqcount_end().
> + * Sequence counters (seqcount_t)
> + *
> + * This is the raw counting mechanism, without any writer protection.
> + *
> + * Write side critical sections must be serialized and non-preemptible.
> + *
> + * If readers can be invoked from hardirq or softirq contexts,
> + * interrupts or bottom halves must also be respectively disabled before
> + * entering the write section.
> + *
> + * This mechanism can't be used if the protected data contains pointers,
> + * as the writer can invalidate a pointer that a reader is following.
> + *
> + * If it's desired to automatically handle the sequence counter writer
> + * serialization and non-preemptibility requirements, use a sequential
> + * lock (seqlock_t) instead.
> + *
> + * See Documentation/locking/seqlock.rst
> */
> typedef struct seqcount {
> unsigned sequence;
> @@ -398,10 +391,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
> smp_wmb(); /* increment "sequence" before following stores */
> }
>
> -/*
> - * Sequence counter only version assumes that callers are using their
> - * own mutexing.
> - */
> static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
> {
> raw_write_seqcount_begin(s);
> @@ -434,15 +423,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
> kcsan_nestable_atomic_end();
> }
>
> +/*
> + * Sequential locks (seqlock_t)
> + *
> + * Sequence counters with an embedded spinlock for writer serialization
> + * and non-preemptibility.
> + *
> + * For more info, see:
> + * - Comments on top of seqcount_t
> + * - Documentation/locking/seqlock.rst
> + */
> typedef struct {
> struct seqcount seqcount;
> spinlock_t lock;
> } seqlock_t;
>
> -/*
> - * These macros triggered gcc-3.x compile-time problems. We think these are
> - * OK now. Be cautious.
> - */
> #define __SEQLOCK_UNLOCKED(lockname) \
> { \
> .seqcount = SEQCNT_ZERO(lockname), \
On Mon, 20 Jul 2020 21:35:51 -0400
Steven Rostedt <[email protected]> wrote:
> On Mon, 20 Jul 2020 17:55:07 +0200
> "Ahmed S. Darwish" <[email protected]> wrote:
> > +Read path, three categories:
> > +
> > +1. Normal Sequence readers which never block a writer but they must
> > + retry if a writer is in progress by detecting change in the sequence
> > + number. Writers do not wait for a sequence reader::
> > +
> > + do {
> > + seq = read_seqbegin(&foo_seqlock);
> > +
> > + /* ... [[read-side critical section]] ... */
> > +
> > + } while (read_seqretry(&foo_seqlock, seq));
> > +
> > +2. Locking readers which will wait if a writer or another locking reader
> > + is in progress. A locking reader in progress will also block a writer
> > + from entering its critical section. This read lock is
> > + exclusive. Unlike rwlock_t, only one locking reader can acquire it::
>
> Nit, but I would mention that this acts similar to a normal spin_lock, and even disables preeption (in the non-RT case).
What I learned today: Ctrl-return on claws mail sends the email :-p
I need to disable that before I send another email before I'm done with it!
I haven't finished reading the rest.
-- Steve
On Mon, 20 Jul 2020 17:55:07 +0200
"Ahmed S. Darwish" <[email protected]> wrote:
> +++ b/include/linux/seqlock.h
> @@ -1,36 +1,15 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #ifndef __LINUX_SEQLOCK_H
> #define __LINUX_SEQLOCK_H
> +
> /*
> - * Reader/writer consistent mechanism without starving writers. This type of
> - * lock for data where the reader wants a consistent set of information
> - * and is willing to retry if the information changes. There are two types
> - * of readers:
> - * 1. Sequence readers which never block a writer but they may have to retry
> - * if a writer is in progress by detecting change in sequence number.
> - * Writers do not wait for a sequence reader.
> - * 2. Locking readers which will wait if a writer or another locking reader
> - * is in progress. A locking reader in progress will also block a writer
> - * from going forward. Unlike the regular rwlock, the read lock here is
> - * exclusive so that only one locking reader can get it.
> + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
> + * lockless readers (read-only retry loops), and no writer starvation.
> *
> - * This is not as cache friendly as brlock. Also, this may not work well
> - * for data that contains pointers, because any writer could
> - * invalidate a pointer that a reader was following.
> + * See Documentation/locking/seqlock.rst
I absolutely hate it when I see this.
I much rather have the documentation next to the code. Because
honestly, I trust that comments next to the code will get updated if
the code changes much more likely than comments buried in the
Documentation directory.
It's also more likely that I wont even bother looking at the doc
(because I wont trust it to be up to date) and just read the code and
try to figure it out. Or look at how others have used it.
-- Steve
> *
> - * Expected non-blocking reader usage:
> - * do {
> - * seq = read_seqbegin(&foo);
> - * ...
> - * } while (read_seqretry(&foo, seq));
> - *
> - *
> - * On non-SMP the spin locks disappear but the writer still needs
> - * to increment the sequence variables because an interrupt routine could
> - * change the state of the data.
> - *
> - * Based on x86_64 vsyscall gettimeofday
> - * by Keith Owens and Andrea Arcangeli
> + * Copyrights:
> + * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
> */
>
> #include <linux/spinlock.h>
> @@ -41,8 +20,8 @@
> #include <asm/processor.h>
>
> /*
> - * The seqlock interface does not prescribe a precise sequence of read
> - * begin/retry/end. For readers, typically there is a call to
> + * The seqlock seqcount_t interface does not prescribe a precise sequence of
> + * read begin/retry/end. For readers, typically there is a call to
> * read_seqcount_begin() and read_seqcount_retry(), however, there are more
> * esoteric cases which do not follow this pattern.
> *
> @@ -50,16 +29,30 @@
> * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
> * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
> * atomics; if there is a matching read_seqcount_retry() call, no following
> - * memory operations are considered atomic. Usage of seqlocks via seqlock_t
> - * interface is not affected.
> + * memory operations are considered atomic. Usage of the seqlock_t interface
> + * is not affected.
> */
> #define KCSAN_SEQLOCK_REGION_MAX 1000
>
> /*
> - * Version using sequence counter only.
> - * This can be used when code has its own mutex protecting the
> - * updating starting before the write_seqcountbeqin() and ending
> - * after the write_seqcount_end().
> + * Sequence counters (seqcount_t)
> + *
> + * This is the raw counting mechanism, without any writer protection.
> + *
> + * Write side critical sections must be serialized and non-preemptible.
> + *
> + * If readers can be invoked from hardirq or softirq contexts,
> + * interrupts or bottom halves must also be respectively disabled before
> + * entering the write section.
> + *
> + * This mechanism can't be used if the protected data contains pointers,
> + * as the writer can invalidate a pointer that a reader is following.
> + *
> + * If it's desired to automatically handle the sequence counter writer
> + * serialization and non-preemptibility requirements, use a sequential
> + * lock (seqlock_t) instead.
> + *
> + * See Documentation/locking/seqlock.rst
> */
> typedef struct seqcount {
> unsigned sequence;
> @@ -398,10 +391,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
> smp_wmb(); /* increment "sequence" before following stores */
> }
>
> -/*
> - * Sequence counter only version assumes that callers are using their
> - * own mutexing.
> - */
> static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
> {
> raw_write_seqcount_begin(s);
> @@ -434,15 +423,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
> kcsan_nestable_atomic_end();
> }
>
> +/*
> + * Sequential locks (seqlock_t)
> + *
> + * Sequence counters with an embedded spinlock for writer serialization
> + * and non-preemptibility.
> + *
> + * For more info, see:
> + * - Comments on top of seqcount_t
> + * - Documentation/locking/seqlock.rst
> + */
> typedef struct {
> struct seqcount seqcount;
> spinlock_t lock;
> } seqlock_t;
>
> -/*
> - * These macros triggered gcc-3.x compile-time problems. We think these are
> - * OK now. Be cautious.
> - */
> #define __SEQLOCK_UNLOCKED(lockname) \
> { \
> .seqcount = SEQCNT_ZERO(lockname), \
On Mon, 20 Jul 2020 21:44:00 -0400
Steven Rostedt <[email protected]> wrote:
> > - * This is not as cache friendly as brlock. Also, this may not work well
> > - * for data that contains pointers, because any writer could
> > - * invalidate a pointer that a reader was following.
> > + * See Documentation/locking/seqlock.rst
>
> I absolutely hate it when I see this.
>
> I much rather have the documentation next to the code. Because
> honestly, I trust that comments next to the code will get updated if
> the code changes much more likely than comments buried in the
> Documentation directory.
>
> It's also more likely that I wont even bother looking at the doc
> (because I wont trust it to be up to date) and just read the code and
> try to figure it out. Or look at how others have used it.
Never mind,
I see that kerneldoc is added in patch 5, which helps.
-- Steve
On Mon, Jul 20, 2020 at 09:35:51PM -0400, Steven Rostedt wrote:
> On Mon, 20 Jul 2020 17:55:07 +0200
> "Ahmed S. Darwish" <[email protected]> wrote:
> > +Read path, three categories:
> > +
> > +1. Normal Sequence readers which never block a writer but they must
> > + retry if a writer is in progress by detecting change in the sequence
> > + number. Writers do not wait for a sequence reader::
> > +
> > + do {
> > + seq = read_seqbegin(&foo_seqlock);
> > +
> > + /* ... [[read-side critical section]] ... */
> > +
> > + } while (read_seqretry(&foo_seqlock, seq));
> > +
> > +2. Locking readers which will wait if a writer or another locking reader
> > + is in progress. A locking reader in progress will also block a writer
> > + from entering its critical section. This read lock is
> > + exclusive. Unlike rwlock_t, only one locking reader can acquire it::
>
> Nit, but I would mention that this acts similar to a normal spin_lock,
> and even disables preeption (in the non-RT case).
will do.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Mon, Jul 20, 2020 at 09:51:15PM -0400, Steven Rostedt wrote:
> On Mon, 20 Jul 2020 21:44:00 -0400
> Steven Rostedt <[email protected]> wrote:
>
> > > - * This is not as cache friendly as brlock. Also, this may not work well
> > > - * for data that contains pointers, because any writer could
> > > - * invalidate a pointer that a reader was following.
> > > + * See Documentation/locking/seqlock.rst
> >
> > I absolutely hate it when I see this.
> >
> > I much rather have the documentation next to the code. Because
> > honestly, I trust that comments next to the code will get updated if
> > the code changes much more likely than comments buried in the
> > Documentation directory.
> >
> > It's also more likely that I wont even bother looking at the doc
> > (because I wont trust it to be up to date) and just read the code and
> > try to figure it out. Or look at how others have used it.
>
> Never mind,
>
> I see that kerneldoc is added in patch 5, which helps.
>
Even looking at the current patch #1 *in isolation*, no relevant
comments were removed from seqlock.h at all. They were just moved closer
to the seqcount_t and seqlock_t structure definitions.
The original comments were horrible. They intermingled seqcount_t and
seqlock_t descriptions in a *very* ambiguous, sometimes even in the same
sentence. It was misleading, as the usage constrains for each data type
(especially on the write side) are vastly different.
Even the new KCSAN comment, which was freshly merged this release cycle,
got tainted by such already-existing incoherence. It kept talking about
the "seqlock interface" while it actually meant the seqcount_t
interface. Then at the end, it realized the semantical ambiguity and
noted how the "seqlock interface" does *not* cover seqlock_t.
Moreover, this seqcount_t/seqlock_t documentation intermingling led to
the critical usage constraint of disabling preemption before entering a
seqcount_t write side critical section never getting explicitly
mentioned. This has (naturally) led to a considerable number of buggy
call sites, and the fixes are now merged.
I've tried to fix the problem at the roots, and basically identified
three major problems:
1. The seqcount_t/seqlock_t intermingling is confusing everyone. That
is, both of call-site developers *and* core kernel engineers.
2. The big picture of seqlock.h was never expressed in a sufficient
detail.
3. The usage constraints for the exported seqlock.h APIs were never
explicitly mentioned.. leading to buggy call sites.
To fix #1, I've changed the all of the existing seqlock.h comments to
explicitly mention either "seqcount_t" or "seqlock_t". Then, divided the
the top seqlock.h comment into a seqcount_t part and a seqlock_t one.
Afterwards, the whole seqlock.h header file code was grouped into
"sections", as seen in patch #4.
To fix #2, Documentation/locking/seqlock.rst was introduced and boldly
referenced at the header-file top comment.
To fix #3, kernel-doc was added for all of the seqlock.h exported APIs.
@Peter, kindly note that all of the above *is exactly why* I've resisted
hiding the new seqcount_LOCKTYPE_t APIs introduced by this patch series
inside generative macros. All the parts that will be referenced by
call-site developers are kept both explicit and kernel-doc commented.
Thanks!
--
Ahmed S. Darwish
Linutronix GmbH
On Mon, Jul 20, 2020 at 8:57 AM Ahmed S. Darwish
<[email protected]> wrote:
>
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. A plain seqcount_t does not
> contain the information of which lock must be held when entering a write
> side critical section.
>
> Use the new seqcount_spinlock_t data type, which allows to associate a
> spinlock with the sequence counter. This enables lockdep to verify that
> the spinlock used for writer serialization is held when the write side
> critical section is entered.
>
> If lockdep is disabled this lock association is compiled out and has
> neither storage size nor runtime overhead.
>
> Signed-off-by: Ahmed S. Darwish <[email protected]>
Acked-by: Song Liu <[email protected]>
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 025e82bcbc34cd071390e72fd0b31593282f9425
Gitweb: https://git.kernel.org/tip/025e82bcbc34cd071390e72fd0b31593282f9425
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:23 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:27 +02:00
timekeeping: Use sequence counter with associated raw spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
kernel/time/timekeeping.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d20d489..05ecfd8 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -39,18 +39,19 @@ enum timekeeping_adv_mode {
TK_ADV_FREQ
};
+static DEFINE_RAW_SPINLOCK(timekeeper_lock);
+
/*
* The most important data for readout fits into a single 64 byte
* cache line.
*/
static struct {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct timekeeper timekeeper;
} tk_core ____cacheline_aligned = {
- .seq = SEQCNT_ZERO(tk_core.seq),
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_core.seq, &timekeeper_lock),
};
-static DEFINE_RAW_SPINLOCK(timekeeper_lock);
static struct timekeeper shadow_timekeeper;
/**
@@ -63,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct tk_read_base base[2];
};
@@ -80,11 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -157,7 +160,7 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
* tk_clock_read - atomic clocksource read() helper
*
* This helper is necessary to use in the read paths because, while the
- * seqlock ensures we don't return a bad value while structures are updated,
+ * seqcount ensures we don't return a bad value while structures are updated,
* it doesn't protect from potential crashes. There is the possibility that
* the tkr's clocksource may change between the read reference, and the
* clock reference passed to the read function. This can cause crashes if
@@ -222,10 +225,10 @@ static inline u64 timekeeping_get_delta(const struct tk_read_base *tkr)
unsigned int seq;
/*
- * Since we're called holding a seqlock, the data may shift
+ * Since we're called holding a seqcount, the data may shift
* under us while we're doing the calculation. This can cause
* false positives, since we'd note a problem but throw the
- * results away. So nest another seqlock here to atomically
+ * results away. So nest another seqcount here to atomically
* grab the points we are checking with.
*/
do {
@@ -486,7 +489,7 @@ EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
*
* To keep it NMI safe since we're accessing from tracing, we're not using a
* separate timekeeper with updates to monotonic clock and boot offset
- * protected with seqlocks. This has the following minor side effects:
+ * protected with seqcounts. This has the following minor side effects:
*
* (1) Its possible that a timestamp be taken after the boot offset is updated
* but before the timekeeper is updated. If this happens, the new boot offset
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 859247d39fb008ea812e8f0c398a58a20c12899e
Gitweb: https://git.kernel.org/tip/859247d39fb008ea812e8f0c398a58a20c12899e
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:14 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:24 +02:00
seqlock: lockdep assert non-preemptibility on seqcount_t write
Preemption must be disabled before entering a sequence count write side
critical section. Failing to do so, the seqcount read side can preempt
the write side section and spin for the entire scheduler tick. If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.
Assert through lockdep that preemption is disabled for seqcount writers.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 29 +++++++++++++++++++++++------
1 file changed, 23 insertions(+), 6 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index e885702..54bc204 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -266,6 +266,12 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
/**
* write_seqcount_begin_nested() - start a seqcount_t write section with
* custom lockdep nesting level
@@ -276,8 +282,19 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
*/
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+ lockdep_assert_preemption_disabled();
+ __write_seqcount_begin_nested(s, subclass);
+}
+
+/*
+ * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption is
+ * already disabled. For example, seqlock_t write side functions.
+ */
+static inline void __write_seqcount_begin(seqcount_t *s)
+{
+ __write_seqcount_begin_nested(s, 0);
}
/**
@@ -575,7 +592,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -601,7 +618,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -628,7 +645,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
}
/**
@@ -649,7 +666,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_begin(&sl->seqcount);
+ __write_seqcount_begin(&sl->seqcount);
return flags;
}
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 8fd8ad5c5dfcb09cf62abadd4043eaf1afbbd0ce
Gitweb: https://git.kernel.org/tip/8fd8ad5c5dfcb09cf62abadd4043eaf1afbbd0ce
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:13 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:24 +02:00
lockdep: Add preemption enabled/disabled assertion APIs
Asserting that preemption is enabled or disabled is a critical sanity
check. Developers are usually reluctant to add such a check in a
fastpath as reading the preemption count can be costly.
Extend the lockdep API with macros asserting that preemption is disabled
or enabled. If lockdep is disabled, or if the underlying architecture
does not support kernel preemption, this assert has no runtime overhead.
References: f54bb2ec02c8 ("locking/lockdep: Add IRQs disabled/enabled assertion APIs: ...")
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/lockdep.h | 19 +++++++++++++++++++
lib/Kconfig.debug | 1 +
2 files changed, 20 insertions(+)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 7aafba0..39a3569 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -549,6 +549,22 @@ do { \
WARN_ON_ONCE(debug_locks && !this_cpu_read(hardirq_context)); \
} while (0)
+#define lockdep_assert_preemption_enabled() \
+do { \
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
+ debug_locks && \
+ (preempt_count() != 0 || \
+ !this_cpu_read(hardirqs_enabled))); \
+} while (0)
+
+#define lockdep_assert_preemption_disabled() \
+do { \
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT) && \
+ debug_locks && \
+ (preempt_count() == 0 && \
+ this_cpu_read(hardirqs_enabled))); \
+} while (0)
+
#else
# define might_lock(lock) do { } while (0)
# define might_lock_read(lock) do { } while (0)
@@ -557,6 +573,9 @@ do { \
# define lockdep_assert_irqs_enabled() do { } while (0)
# define lockdep_assert_irqs_disabled() do { } while (0)
# define lockdep_assert_in_irq() do { } while (0)
+
+# define lockdep_assert_preemption_enabled() do { } while (0)
+# define lockdep_assert_preemption_disabled() do { } while (0)
#endif
#ifdef CONFIG_PROVE_RAW_LOCK_NESTING
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9ad9210..5379931 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1117,6 +1117,7 @@ config PROVE_LOCKING
select DEBUG_RWSEMS
select DEBUG_WW_MUTEX_SLOWPATH
select DEBUG_LOCK_ALLOC
+ select PREEMPT_COUNT if !ARCH_NO_PREEMPT
select TRACE_IRQFLAGS
default n
help
The following commit has been merged into the locking/core branch of tip:
Commit-ID: f4a27cbcec90ac04ee60e04b222e1449dcdba0bd
Gitweb: https://git.kernel.org/tip/f4a27cbcec90ac04ee60e04b222e1449dcdba0bd
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:10 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:23 +02:00
seqlock: Reorder seqcount_t and seqlock_t API definitions
The seqlock.h seqcount_t and seqlock_t API definitions are presented in
the chronological order of their development rather than the order that
makes most sense to readers. This makes it hard to follow and understand
the header file code.
Group and reorder all of the exported seqlock.h functions according to
their function.
First, group together the seqcount_t standard read path functions:
- __read_seqcount_begin()
- raw_read_seqcount_begin()
- read_seqcount_begin()
since each function is implemented exactly in terms of the one above
it. Then, group the special-case seqcount_t readers on their own as:
- raw_read_seqcount()
- raw_seqcount_begin()
since the only difference between the two functions is that the second
one masks the sequence counter LSB while the first one does not. Note
that raw_seqcount_begin() can actually be implemented in terms of
raw_read_seqcount(), which will be done in a follow-up commit.
Then, group the seqcount_t write path functions, instead of injecting
unrelated seqcount_t latch functions between them, and order them as:
- raw_write_seqcount_begin()
- raw_write_seqcount_end()
- write_seqcount_begin_nested()
- write_seqcount_begin()
- write_seqcount_end()
- raw_write_seqcount_barrier()
- write_seqcount_invalidate()
which is the expected natural order. This also isolates the seqcount_t
latch functions into their own area, at the end of the sequence counters
section, and before jumping to the next one: sequential locks
(seqlock_t).
Do a similar grouping and reordering for seqlock_t "locking" readers vs.
the "conditionally locking or lockless" ones.
No implementation code was changed in any of the reordering above.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 158 +++++++++++++++++++--------------------
1 file changed, 78 insertions(+), 80 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d724b5e..4c14560 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -129,23 +129,6 @@ repeat:
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
- */
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
-{
- unsigned ret = READ_ONCE(s->sequence);
- smp_rmb();
- kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
- return ret;
-}
-
-/**
* raw_read_seqcount_begin - start seq-read critical section w/o lockdep
* @s: pointer to seqcount_t
* Returns: count to be passed to read_seqcount_retry
@@ -177,6 +160,23 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
}
/**
+ * raw_read_seqcount - Read the raw seqcount
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * raw_read_seqcount opens a read critical section of the given
+ * seqcount without any lockdep checking and without checking or
+ * masking the LSB. Calling code is responsible for handling that.
+ */
+static inline unsigned raw_read_seqcount(const seqcount_t *s)
+{
+ unsigned ret = READ_ONCE(s->sequence);
+ smp_rmb();
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
+ return ret;
+}
+
+/**
* raw_seqcount_begin - begin a seq-read critical section
* @s: pointer to seqcount_t
* Returns: count to be passed to read_seqcount_retry
@@ -234,8 +234,6 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_retry(s, start);
}
-
-
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
@@ -250,6 +248,23 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+ raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
+static inline void write_seqcount_begin(seqcount_t *s)
+{
+ write_seqcount_begin_nested(s, 0);
+}
+
+static inline void write_seqcount_end(seqcount_t *s)
+{
+ seqcount_release(&s->dep_map, _RET_IP_);
+ raw_write_seqcount_end(s);
+}
+
/**
* raw_write_seqcount_barrier - do a seq write barrier
* @s: pointer to seqcount_t
@@ -300,6 +315,21 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * write_seqcount_invalidate - invalidate in-progress read-side seq operations
+ * @s: pointer to seqcount_t
+ *
+ * After write_seqcount_invalidate, no read-side seq operations will complete
+ * successfully and see data older than this.
+ */
+static inline void write_seqcount_invalidate(seqcount_t *s)
+{
+ smp_wmb();
+ kcsan_nestable_atomic_begin();
+ s->sequence+=2;
+ kcsan_nestable_atomic_end();
+}
+
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
@@ -395,38 +425,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
-}
-
-static inline void write_seqcount_begin(seqcount_t *s)
-{
- write_seqcount_begin_nested(s, 0);
-}
-
-static inline void write_seqcount_end(seqcount_t *s)
-{
- seqcount_release(&s->dep_map, _RET_IP_);
- raw_write_seqcount_end(s);
-}
-
-/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
- *
- * After write_seqcount_invalidate, no read-side seq operations will complete
- * successfully and see data older than this.
- */
-static inline void write_seqcount_invalidate(seqcount_t *s)
-{
- smp_wmb();
- kcsan_nestable_atomic_begin();
- s->sequence+=2;
- kcsan_nestable_atomic_end();
-}
-
/*
* Sequential locks (seqlock_t)
*
@@ -555,35 +553,6 @@ static inline void read_sequnlock_excl(seqlock_t *sl)
spin_unlock(&sl->lock);
}
-/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
- *
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
- */
-static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
-{
- if (!(*seq & 1)) /* Even */
- *seq = read_seqbegin(lock);
- else /* Odd */
- read_seqlock_excl(lock);
-}
-
-static inline int need_seqretry(seqlock_t *lock, int seq)
-{
- return !(seq & 1) && read_seqretry(lock, seq);
-}
-
-static inline void done_seqretry(seqlock_t *lock, int seq)
-{
- if (seq & 1)
- read_sequnlock_excl(lock);
-}
-
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
@@ -621,6 +590,35 @@ read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
spin_unlock_irqrestore(&sl->lock, flags);
}
+/**
+ * read_seqbegin_or_lock - begin a sequence number check or locking block
+ * @lock: sequence lock
+ * @seq : sequence number to be checked
+ *
+ * First try it once optimistically without taking the lock. If that fails,
+ * take the lock. The sequence number is also used as a marker for deciding
+ * whether to be a reader (even) or writer (odd).
+ * N.B. seq must be initialized to an even number to begin with.
+ */
+static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
+{
+ if (!(*seq & 1)) /* Even */
+ *seq = read_seqbegin(lock);
+ else /* Odd */
+ read_seqlock_excl(lock);
+}
+
+static inline int need_seqretry(seqlock_t *lock, int seq)
+{
+ return !(seq & 1) && read_seqretry(lock, seq);
+}
+
+static inline void done_seqretry(seqlock_t *lock, int seq)
+{
+ if (seq & 1)
+ read_sequnlock_excl(lock);
+}
+
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
The following commit has been merged into the locking/core branch of tip:
Commit-ID: d3b35b87f436c1b226a8061bee9c8875ba6658bd
Gitweb: https://git.kernel.org/tip/d3b35b87f436c1b226a8061bee9c8875ba6658bd
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:09 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:23 +02:00
seqlock: seqcount_t latch: End read sections with read_seqcount_retry()
The seqcount_t latch reader example at the raw_write_seqcount_latch()
kernel-doc comment ends the latch read section with a manual smp memory
barrier and sequence counter comparison.
This is technically correct, but it is suboptimal: read_seqcount_retry()
already contains the same logic of an smp memory barrier and sequence
counter comparison.
End the latch read critical section example with read_seqcount_retry().
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 6c4f68e..d724b5e 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -363,8 +363,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes needed smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq));
*
* return entry;
* }
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 55f3560df975f557c48aa6afc636808f31ecb87a
Gitweb: https://git.kernel.org/tip/55f3560df975f557c48aa6afc636808f31ecb87a
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:15 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:25 +02:00
seqlock: Extend seqcount API with associated locks
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the write side critical section.
There is no built-in debugging mechanism to verify that the lock used
for writer serialization is held and preemption is disabled. Some usage
sites like dma-buf have explicit lockdep checks for the writer-side
lock, but this covers only a small portion of the sequence counter usage
in the kernel.
Add new sequence counter types which allows to associate a lock to the
sequence counter at initialization time. The seqcount API functions are
extended to provide appropriate lockdep assertions depending on the
seqcount/lock type.
For sequence counters with associated locks that do not implicitly
disable preemption, preemption protection is enforced in the sequence
counter write side functions. This removes the need to explicitly add
preempt_disable/enable() around the write side critical sections: the
write_begin/end() functions for these new sequence counter types
automatically do this.
Introduce the following seqcount types with associated locks:
seqcount_spinlock_t
seqcount_raw_spinlock_t
seqcount_rwlock_t
seqcount_mutex_t
seqcount_ww_mutex_t
Extend the seqcount read and write functions to branch out to the
specific seqcount_LOCKTYPE_t implementation at compile-time. This avoids
kernel API explosion per each new seqcount_LOCKTYPE_t added. Add such
compile-time type detection logic into a new, internal, seqlock header.
Document the proper seqcount_LOCKTYPE_t usage, and rationale, at
Documentation/locking/seqlock.rst.
If lockdep is disabled, this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
Documentation/locking/seqlock.rst | 52 +++-
include/linux/seqlock.h | 464 ++++++++++++++++++++++++-----
2 files changed, 447 insertions(+), 69 deletions(-)
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index 366dd36..62c5ad9 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -87,6 +87,58 @@ Read path::
} while (read_seqcount_retry(&foo_seqcount, seq));
+.. _seqcount_locktype_t:
+
+Sequence counters with associated locks (``seqcount_LOCKTYPE_t``)
+-----------------------------------------------------------------
+
+As discussed at :ref:`seqcount_t`, sequence count write side critical
+sections must be serialized and non-preemptible. This variant of
+sequence counters associate the lock used for writer serialization at
+initialization time, which enables lockdep to validate that the write
+side critical sections are properly serialized.
+
+This lock association is a NOOP if lockdep is disabled and has neither
+storage nor runtime overhead. If lockdep is enabled, the lock pointer is
+stored in struct seqcount and lockdep's "lock is held" assertions are
+injected at the beginning of the write side critical section to validate
+that it is properly protected.
+
+For lock types which do not implicitly disable preemption, preemption
+protection is enforced in the write side function.
+
+The following sequence counters with associated locks are defined:
+
+ - ``seqcount_spinlock_t``
+ - ``seqcount_raw_spinlock_t``
+ - ``seqcount_rwlock_t``
+ - ``seqcount_mutex_t``
+ - ``seqcount_ww_mutex_t``
+
+The plain seqcount read and write APIs branch out to the specific
+seqcount_LOCKTYPE_t implementation at compile-time. This avoids kernel
+API explosion per each new seqcount LOCKTYPE.
+
+Initialization (replace "LOCKTYPE" with one of the supported locks)::
+
+ /* dynamic */
+ seqcount_LOCKTYPE_t foo_seqcount;
+ seqcount_LOCKTYPE_init(&foo_seqcount, &lock);
+
+ /* static */
+ static seqcount_LOCKTYPE_t foo_seqcount =
+ SEQCNT_LOCKTYPE_ZERO(foo_seqcount, &lock);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_LOCKTYPE_ZERO(foo.seq, &lock),
+ } foo;
+
+Write path: same as in :ref:`seqcount_t`, while running from a context
+with the associated LOCKTYPE lock acquired.
+
+Read path: same as in :ref:`seqcount_t`.
+
.. _seqlock_t:
Sequential locks (``seqlock_t``)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 54bc204..8c16a49 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -10,13 +10,17 @@
*
* Copyrights:
* - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
+ * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
*/
-#include <linux/spinlock.h>
-#include <linux/preempt.h>
-#include <linux/lockdep.h>
#include <linux/compiler.h>
#include <linux/kcsan-checks.h>
+#include <linux/lockdep.h>
+#include <linux/mutex.h>
+#include <linux/preempt.h>
+#include <linux/spinlock.h>
+#include <linux/ww_mutex.h>
+
#include <asm/processor.h>
/*
@@ -48,6 +52,10 @@
* This mechanism can't be used if the protected data contains pointers,
* as the writer can invalidate a pointer that a reader is following.
*
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_t) instead.
+ *
* If it's desired to automatically handle the sequence counter writer
* serialization and non-preemptibility requirements, use a sequential
* lock (seqlock_t) instead.
@@ -108,9 +116,267 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*/
#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
+/*
+ * Sequence counters with associated locks (seqcount_LOCKTYPE_t)
+ *
+ * A sequence counter which associates the lock used for writer
+ * serialization at initialization time. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ *
+ * For associated locks which do not implicitly disable preemption,
+ * preemption protection is enforced in the write side function.
+ *
+ * Lockdep is never used in any for the raw write variants.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
+#ifdef CONFIG_LOCKDEP
+#define __SEQ_LOCKDEP(expr) expr
+#else
+#define __SEQ_LOCKDEP(expr)
+#endif
+
+#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+ __SEQ_LOCKDEP(.lock = (assoc_lock)) \
+}
+
+#define seqcount_locktype_init(s, assoc_lock) \
+do { \
+ seqcount_init(&(s)->seqcount); \
+ __SEQ_LOCKDEP((s)->lock = (assoc_lock)); \
+} while (0)
+
+/**
+ * typedef seqcount_spinlock_t - sequence counter with spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * spinlock. The spinlock is associated to the sequence count in the
+ * static initializer or init function. This enables lockdep to validate
+ * that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_spinlock {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(spinlock_t *lock);
+} seqcount_spinlock_t;
+
+/**
+ * SEQCNT_SPINLOCK_ZERO - static initializer for seqcount_spinlock_t
+ * @name: Name of the seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define SEQCNT_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_spinlock_init - runtime initializer for seqcount_spinlock_t
+ * @s: Pointer to the seqcount_spinlock_t instance
+ * @lock: Pointer to the associated spinlock
+ */
+#define seqcount_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_raw_spinlock_t - sequence count with raw spinlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated raw spinlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * raw spinlock. The raw spinlock is associated to the sequence count in
+ * the static initializer or init function. This enables lockdep to
+ * validate that the write side critical section is properly serialized.
+ */
+typedef struct seqcount_raw_spinlock {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(raw_spinlock_t *lock);
+} seqcount_raw_spinlock_t;
+
+/**
+ * SEQCNT_RAW_SPINLOCK_ZERO - static initializer for seqcount_raw_spinlock_t
+ * @name: Name of the seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_raw_spinlock_init - runtime initializer for seqcount_raw_spinlock_t
+ * @s: Pointer to the seqcount_raw_spinlock_t instance
+ * @lock: Pointer to the associated raw_spinlock
+ */
+#define seqcount_raw_spinlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_rwlock_t - sequence count with rwlock associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated rwlock
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * rwlock. The rwlock is associated to the sequence count in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ */
+typedef struct seqcount_rwlock {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(rwlock_t *lock);
+} seqcount_rwlock_t;
+
+/**
+ * SEQCNT_RWLOCK_ZERO - static initializer for seqcount_rwlock_t
+ * @name: Name of the seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define SEQCNT_RWLOCK_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_rwlock_init - runtime initializer for seqcount_rwlock_t
+ * @s: Pointer to the seqcount_rwlock_t instance
+ * @lock: Pointer to the associated rwlock
+ */
+#define seqcount_rwlock_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_mutex_t - sequence count with mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * mutex. The mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_mutex_t.
+ */
+typedef struct seqcount_mutex {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(struct mutex *lock);
+} seqcount_mutex_t;
+
+/**
+ * SEQCNT_MUTEX_ZERO - static initializer for seqcount_mutex_t
+ * @name: Name of the seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define SEQCNT_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_mutex_init - runtime initializer for seqcount_mutex_t
+ * @s: Pointer to the seqcount_mutex_t instance
+ * @lock: Pointer to the associated mutex
+ */
+#define seqcount_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/**
+ * typedef seqcount_ww_mutex_t - sequence count with ww_mutex associated
+ * @seqcount: The real sequence counter
+ * @lock: Pointer to the associated ww_mutex
+ *
+ * A plain sequence counter with external writer synchronization by a
+ * ww_mutex. The ww_mutex is associated to the sequence counter in the static
+ * initializer or init function. This enables lockdep to validate that
+ * the write side critical section is properly serialized.
+ *
+ * The write side API functions write_seqcount_begin()/end() automatically
+ * disable and enable preemption when used with seqcount_ww_mutex_t.
+ */
+typedef struct seqcount_ww_mutex {
+ seqcount_t seqcount;
+ __SEQ_LOCKDEP(struct ww_mutex *lock);
+} seqcount_ww_mutex_t;
+
+/**
+ * SEQCNT_WW_MUTEX_ZERO - static initializer for seqcount_ww_mutex_t
+ * @name: Name of the seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define SEQCNT_WW_MUTEX_ZERO(name, lock) \
+ SEQCOUNT_LOCKTYPE_ZERO(name, lock)
+
+/**
+ * seqcount_ww_mutex_init - runtime initializer for seqcount_ww_mutex_t
+ * @s: Pointer to the seqcount_ww_mutex_t instance
+ * @lock: Pointer to the associated ww_mutex
+ */
+#define seqcount_ww_mutex_init(s, lock) \
+ seqcount_locktype_init(s, lock)
+
+/*
+ * @preempt: Is the associated write serialization lock preemtpible?
+ */
+#define SEQCOUNT_LOCKTYPE(locktype, preempt, lockmember) \
+static inline seqcount_t * \
+__seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
+{ \
+ return &s->seqcount; \
+} \
+ \
+static inline bool \
+__seqcount_##locktype##_preemptible(seqcount_##locktype##_t *s) \
+{ \
+ return preempt; \
+} \
+ \
+static inline void \
+__seqcount_##locktype##_assert(seqcount_##locktype##_t *s) \
+{ \
+ __SEQ_LOCKDEP(lockdep_assert_held(lockmember)); \
+}
+
+/*
+ * Similar hooks, but for plain seqcount_t
+ */
+
+static inline seqcount_t *__seqcount_ptr(seqcount_t *s)
+{
+ return s;
+}
+
+static inline bool __seqcount_preemptible(seqcount_t *s)
+{
+ return false;
+}
+
+static inline void __seqcount_assert(seqcount_t *s)
+{
+ lockdep_assert_preemption_disabled();
+}
+
+/*
+ * @s: Pointer to seqcount_locktype_t, generated hooks first parameter.
+ */
+SEQCOUNT_LOCKTYPE(raw_spinlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(spinlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(rwlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(mutex, true, s->lock)
+SEQCOUNT_LOCKTYPE(ww_mutex, true, &s->lock->base)
+
+#define __seqprop_case(s, locktype, prop) \
+ seqcount_##locktype##_t: __seqcount_##locktype##_##prop((void *)(s))
+
+#define __seqprop(s, prop) _Generic(*(s), \
+ seqcount_t: __seqcount_##prop((void *)(s)), \
+ __seqprop_case((s), raw_spinlock, prop), \
+ __seqprop_case((s), spinlock, prop), \
+ __seqprop_case((s), rwlock, prop), \
+ __seqprop_case((s), mutex, prop), \
+ __seqprop_case((s), ww_mutex, prop))
+
+#define __to_seqcount_t(s) __seqprop(s, ptr)
+#define __associated_lock_exists_and_is_preemptible(s) __seqprop(s, preemptible)
+#define __assert_write_section_is_protected(s) __seqprop(s, assert)
+
/**
* __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -122,7 +388,10 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+#define __read_seqcount_begin(s) \
+ __read_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
{
unsigned ret;
@@ -138,32 +407,38 @@ repeat:
/**
* raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+#define raw_read_seqcount_begin(s) \
+ raw_read_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
{
- unsigned ret = __read_seqcount_begin(s);
+ unsigned ret = __read_seqcount_t_begin(s);
smp_rmb();
return ret;
}
/**
* read_seqcount_begin() - begin a seqcount_t read critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned read_seqcount_begin(const seqcount_t *s)
+#define read_seqcount_begin(s) \
+ read_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
{
seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
+ return raw_read_seqcount_t_begin(s);
}
/**
* raw_read_seqcount() - read the raw seqcount_t counter value
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* raw_read_seqcount opens a read critical section of the given
* seqcount_t, without any lockdep checking, and without checking or
@@ -172,7 +447,10 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned raw_read_seqcount(const seqcount_t *s)
+#define raw_read_seqcount(s) \
+ raw_read_seqcount_t(__to_seqcount_t(s))
+
+static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
@@ -183,7 +461,7 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
/**
* raw_seqcount_begin() - begin a seqcount_t read critical section w/o
* lockdep and w/o counter stabilization
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* raw_seqcount_begin opens a read critical section of the given
* seqcount_t. Unlike read_seqcount_begin(), this function will not wait
@@ -197,18 +475,21 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
*
* Return: count to be passed to read_seqcount_retry()
*/
-static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+#define raw_seqcount_begin(s) \
+ raw_seqcount_t_begin(__to_seqcount_t(s))
+
+static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
{
/*
* If the counter is odd, let read_seqcount_retry() fail
* by decrementing the counter.
*/
- return raw_read_seqcount(s) & ~1;
+ return raw_read_seqcount_t(s) & ~1;
}
/**
* __read_seqcount_retry() - end a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin()
*
* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
@@ -221,7 +502,10 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
*
* Return: true if a read section retry is required, else false
*/
-static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define __read_seqcount_retry(s, start) \
+ __read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
kcsan_atomic_next(0);
return unlikely(READ_ONCE(s->sequence) != start);
@@ -229,7 +513,7 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
/**
* read_seqcount_retry() - end a seqcount_t read critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
* @start: count, from read_seqcount_begin()
*
* read_seqcount_retry closes the read critical section of given
@@ -238,17 +522,28 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
*
* Return: true if a read section retry is required, else false
*/
-static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+#define read_seqcount_retry(s, start) \
+ read_seqcount_t_retry(__to_seqcount_t(s), start)
+
+static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return __read_seqcount_t_retry(s, start);
}
/**
* raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*/
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+#define raw_write_seqcount_begin(s) \
+do { \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ raw_write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+static inline void raw_write_seqcount_t_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -257,49 +552,50 @@ static inline void raw_write_seqcount_begin(seqcount_t *s)
/**
* raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*/
-static inline void raw_write_seqcount_end(seqcount_t *s)
+#define raw_write_seqcount_end(s) \
+do { \
+ raw_write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void raw_write_seqcount_t_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
kcsan_nestable_atomic_end();
}
-static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
-}
-
/**
* write_seqcount_begin_nested() - start a seqcount_t write section with
* custom lockdep nesting level
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
* @subclass: lockdep nesting level
*
* See Documentation/locking/lockdep-design.rst
*/
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- lockdep_assert_preemption_disabled();
- __write_seqcount_begin_nested(s, subclass);
-}
-
-/*
- * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
- *
- * Use for internal seqlock.h code where it's known that preemption is
- * already disabled. For example, seqlock_t write side functions.
- */
-static inline void __write_seqcount_begin(seqcount_t *s)
+#define write_seqcount_begin_nested(s, subclass) \
+do { \
+ __assert_write_section_is_protected(s); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin_nested(__to_seqcount_t(s), subclass); \
+} while (0)
+
+static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
- __write_seqcount_begin_nested(s, 0);
+ raw_write_seqcount_t_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
/**
* write_seqcount_begin() - start a seqcount_t write side critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* write_seqcount_begin opens a write side critical section of the given
* seqcount_t.
@@ -308,26 +604,44 @@ static inline void __write_seqcount_begin(seqcount_t *s)
* non-preemptible. If readers can be invoked from hardirq or softirq
* context, interrupts or bottom halves must be respectively disabled.
*/
-static inline void write_seqcount_begin(seqcount_t *s)
+#define write_seqcount_begin(s) \
+do { \
+ __assert_write_section_is_protected(s); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_disable(); \
+ \
+ write_seqcount_t_begin(__to_seqcount_t(s)); \
+} while (0)
+
+static inline void write_seqcount_t_begin(seqcount_t *s)
{
- write_seqcount_begin_nested(s, 0);
+ write_seqcount_t_begin_nested(s, 0);
}
/**
* write_seqcount_end() - end a seqcount_t write side critical section
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* The write section must've been opened with write_seqcount_begin().
*/
-static inline void write_seqcount_end(seqcount_t *s)
+#define write_seqcount_end(s) \
+do { \
+ write_seqcount_t_end(__to_seqcount_t(s)); \
+ \
+ if (__associated_lock_exists_and_is_preemptible(s)) \
+ preempt_enable(); \
+} while (0)
+
+static inline void write_seqcount_t_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
- raw_write_seqcount_end(s);
+ raw_write_seqcount_t_end(s);
}
/**
* raw_write_seqcount_barrier() - do a seqcount_t write barrier
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* This can be used to provide an ordering guarantee instead of the usual
* consistency guarantee. It is one wmb cheaper, because it can collapse
@@ -366,7 +680,10 @@ static inline void write_seqcount_end(seqcount_t *s)
* WRITE_ONCE(X, false);
* }
*/
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
+#define raw_write_seqcount_barrier(s) \
+ raw_write_seqcount_t_barrier(__to_seqcount_t(s))
+
+static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
s->sequence++;
@@ -378,12 +695,15 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
/**
* write_seqcount_invalidate() - invalidate in-progress seqcount_t read
* side operations
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* After write_seqcount_invalidate, no seqcount_t read side operations
* will complete successfully and see data older than this.
*/
-static inline void write_seqcount_invalidate(seqcount_t *s)
+#define write_seqcount_invalidate(s) \
+ write_seqcount_t_invalidate(__to_seqcount_t(s))
+
+static inline void write_seqcount_t_invalidate(seqcount_t *s)
{
smp_wmb();
kcsan_nestable_atomic_begin();
@@ -393,7 +713,7 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
/**
* raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* Use seqcount_t latching to switch between two storage places protected
* by a sequence counter. Doing so allows having interruptible, preemptible,
@@ -406,7 +726,10 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
* picking which data copy to read. The full counter value must then be
* checked with read_seqcount_retry().
*/
-static inline int raw_read_seqcount_latch(seqcount_t *s)
+#define raw_read_seqcount_latch(s) \
+ raw_read_seqcount_t_latch(__to_seqcount_t(s))
+
+static inline int raw_read_seqcount_t_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
@@ -415,7 +738,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
/**
* raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to seqcount_t
+ * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -494,7 +817,10 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
-static inline void raw_write_seqcount_latch(seqcount_t *s)
+#define raw_write_seqcount_latch(s) \
+ raw_write_seqcount_t_latch(__to_seqcount_t(s))
+
+static inline void raw_write_seqcount_t_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
@@ -592,7 +918,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -604,7 +930,7 @@ static inline void write_seqlock(seqlock_t *sl)
*/
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
@@ -618,7 +944,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -631,7 +957,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
*/
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
@@ -645,7 +971,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
}
/**
@@ -657,7 +983,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
*/
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -666,7 +992,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount);
return flags;
}
@@ -695,13 +1021,13 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
/**
* read_seqlock_excl() - begin a seqlock_t locking reader section
- * @sl: Pointer to seqlock_t
+ * @sl: Pointer to seqlock_t
*
* read_seqlock_excl opens a seqlock_t locking reader critical section. A
* locking reader exclusively locks out *both* other writers *and* other
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 932e46365226324d2cf26d8bdec8b51ceb296948
Gitweb: https://git.kernel.org/tip/932e46365226324d2cf26d8bdec8b51ceb296948
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:12 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:24 +02:00
seqlock: Implement raw_seqcount_begin() in terms of raw_read_seqcount()
raw_seqcount_begin() has the same code as raw_read_seqcount(), with the
exception of masking the sequence counter's LSB before returning it to
the caller.
Note, raw_seqcount_begin() masks the counter's LSB before returning it
to the caller so that read_seqcount_retry() can fail if the counter is
odd -- without the overhead of an extra branching instruction.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 85fb3ac..e885702 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -199,10 +199,11 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
*/
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
- unsigned ret = READ_ONCE(s->sequence);
- smp_rmb();
- kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
- return ret & ~1;
+ /*
+ * If the counter is odd, let read_seqcount_retry() fail
+ * by decrementing the counter.
+ */
+ return raw_read_seqcount(s) & ~1;
}
/**
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 15cbe67bbd3adeb4854c42713dbeaf2ff876beee
Gitweb: https://git.kernel.org/tip/15cbe67bbd3adeb4854c42713dbeaf2ff876beee
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:08 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:23 +02:00
seqlock: Properly format kernel-doc code samples
Align the code samples and note sections inside kernel-doc comments with
tabs. This way they can be properly parsed and rendered by Sphinx. It
also makes the code samples easier to read from text editors.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 108 ++++++++++++++++++++-------------------
1 file changed, 56 insertions(+), 52 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 299d68f..6c4f68e 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -263,32 +263,32 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* atomically, avoiding compiler optimizations; b) to document which writes are
* meant to propagate to the reader critical section. This is necessary because
* neither writes before and after the barrier are enclosed in a seq-writer
- * critical section that would ensure readers are aware of ongoing writes.
+ * critical section that would ensure readers are aware of ongoing writes::
*
- * seqcount_t seq;
- * bool X = true, Y = false;
+ * seqcount_t seq;
+ * bool X = true, Y = false;
*
- * void read(void)
- * {
- * bool x, y;
+ * void read(void)
+ * {
+ * bool x, y;
*
- * do {
- * int s = read_seqcount_begin(&seq);
+ * do {
+ * int s = read_seqcount_begin(&seq);
*
- * x = X; y = Y;
+ * x = X; y = Y;
*
- * } while (read_seqcount_retry(&seq, s));
+ * } while (read_seqcount_retry(&seq, s));
*
- * BUG_ON(!x && !y);
+ * BUG_ON(!x && !y);
* }
*
* void write(void)
* {
- * WRITE_ONCE(Y, true);
+ * WRITE_ONCE(Y, true);
*
- * raw_write_seqcount_barrier(seq);
+ * raw_write_seqcount_barrier(seq);
*
- * WRITE_ONCE(X, false);
+ * WRITE_ONCE(X, false);
* }
*/
static inline void raw_write_seqcount_barrier(seqcount_t *s)
@@ -325,64 +325,68 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* Very simply put: we first modify one copy and then the other. This ensures
* there is always one copy in a stable state, ready to give us an answer.
*
- * The basic form is a data structure like:
+ * The basic form is a data structure like::
*
- * struct latch_struct {
- * seqcount_t seq;
- * struct data_struct data[2];
- * };
+ * struct latch_struct {
+ * seqcount_t seq;
+ * struct data_struct data[2];
+ * };
*
* Where a modification, which is assumed to be externally serialized, does the
- * following:
+ * following::
*
- * void latch_modify(struct latch_struct *latch, ...)
- * {
- * smp_wmb(); <- Ensure that the last data[1] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * void latch_modify(struct latch_struct *latch, ...)
+ * {
+ * smp_wmb(); // Ensure that the last data[1] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[0], ...);
+ * modify(latch->data[0], ...);
*
- * smp_wmb(); <- Ensure that the data[0] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
+ * smp_wmb(); // Ensure that the data[0] update is visible
+ * latch->seq++;
+ * smp_wmb(); // Ensure that the seqcount update is visible
*
- * modify(latch->data[1], ...);
- * }
+ * modify(latch->data[1], ...);
+ * }
*
- * The query will have a form like:
+ * The query will have a form like::
*
- * struct entry *latch_query(struct latch_struct *latch, ...)
- * {
- * struct entry *entry;
- * unsigned seq, idx;
+ * struct entry *latch_query(struct latch_struct *latch, ...)
+ * {
+ * struct entry *entry;
+ * unsigned seq, idx;
*
- * do {
- * seq = raw_read_seqcount_latch(&latch->seq);
+ * do {
+ * seq = raw_read_seqcount_latch(&latch->seq);
*
- * idx = seq & 0x01;
- * entry = data_query(latch->data[idx], ...);
+ * idx = seq & 0x01;
+ * entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * smp_rmb();
+ * } while (seq != latch->seq);
*
- * return entry;
- * }
+ * return entry;
+ * }
*
* So during the modification, queries are first redirected to data[1]. Then we
* modify data[0]. When that is complete, we redirect queries back to data[0]
* and we can modify data[1].
*
- * NOTE: The non-requirement for atomic modifications does _NOT_ include
- * the publishing of new entries in the case where data is a dynamic
- * data structure.
+ * NOTE:
+ *
+ * The non-requirement for atomic modifications does _NOT_ include
+ * the publishing of new entries in the case where data is a dynamic
+ * data structure.
+ *
+ * An iteration might start in data[0] and get suspended long enough
+ * to miss an entire modification sequence, once it resumes it might
+ * observe the new entry.
*
- * An iteration might start in data[0] and get suspended long enough
- * to miss an entire modification sequence, once it resumes it might
- * observe the new entry.
+ * NOTE:
*
- * NOTE: When data is a dynamic data structure; one should use regular RCU
- * patterns to manage the lifetimes of the objects within.
+ * When data is a dynamic data structure; one should use regular RCU
+ * patterns to manage the lifetimes of the objects within.
*/
static inline void raw_write_seqcount_latch(seqcount_t *s)
{
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 0d24f65e933ca89d55d17f6dbdb2a72ca88f0992
Gitweb: https://git.kernel.org/tip/0d24f65e933ca89d55d17f6dbdb2a72ca88f0992
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:07 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:22 +02:00
Documentation: locking: Describe seqlock design and usage
Proper documentation for the design and usage of sequence counters and
sequential locks does not exist. Complete the seqlock.h documentation as
follows:
- Divide all documentation on a seqcount_t vs. seqlock_t basis. The
description for both mechanisms was intermingled, which is incorrect
since the usage constrains for each type are vastly different.
- Add an introductory paragraph describing the internal design of, and
rationale for, sequence counters.
- Document seqcount_t writer non-preemptibility requirement, which was
not previously documented anywhere, and provide a clear rationale.
- Provide template code for seqcount_t and seqlock_t initialization
and reader/writer critical sections.
- Recommend using seqlock_t by default. It implicitly handles the
serialization and non-preemptibility requirements of writers.
At seqlock.h:
- Remove references to brlocks as they've long been removed from the
kernel.
- Remove references to gcc-3.x since the kernel's minimum supported
gcc version is 4.9.
References: 0f6ed63b1707 ("no need to keep brlock macros anymore...")
References: 6ec4476ac825 ("Raise gcc version requirement to 4.9")
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
Documentation/locking/index.rst | 1 +-
Documentation/locking/seqlock.rst | 170 +++++++++++++++++++++++++++++-
include/linux/seqlock.h | 85 +++++++--------
3 files changed, 211 insertions(+), 45 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst
diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
index d785878..7003bd5 100644
--- a/Documentation/locking/index.rst
+++ b/Documentation/locking/index.rst
@@ -14,6 +14,7 @@ locking
mutex-design
rt-mutex-design
rt-mutex
+ seqlock
spinlocks
ww-mutex-design
preempt-locking
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
new file mode 100644
index 0000000..366dd36
--- /dev/null
+++ b/Documentation/locking/seqlock.rst
@@ -0,0 +1,170 @@
+======================================
+Sequence counters and sequential locks
+======================================
+
+Introduction
+============
+
+Sequence counters are a reader-writer consistency mechanism with
+lockless readers (read-only retry loops), and no writer starvation. They
+are used for data that's rarely written to (e.g. system time), where the
+reader wants a consistent set of information and is willing to retry if
+that information changes.
+
+A data set is consistent when the sequence count at the beginning of the
+read side critical section is even and the same sequence count value is
+read again at the end of the critical section. The data in the set must
+be copied out inside the read side critical section. If the sequence
+count has changed between the start and the end of the critical section,
+the reader must retry.
+
+Writers increment the sequence count at the start and the end of their
+critical section. After starting the critical section the sequence count
+is odd and indicates to the readers that an update is in progress. At
+the end of the write side critical section the sequence count becomes
+even again which lets readers make progress.
+
+A sequence counter write side critical section must never be preempted
+or interrupted by read side sections. Otherwise the reader will spin for
+the entire scheduler tick due to the odd sequence count value and the
+interrupted writer. If that reader belongs to a real-time scheduling
+class, it can spin forever and the kernel will livelock.
+
+This mechanism cannot be used if the protected data contains pointers,
+as the writer can invalidate a pointer that the reader is following.
+
+
+.. _seqcount_t:
+
+Sequence counters (``seqcount_t``)
+==================================
+
+This is the the raw counting mechanism, which does not protect against
+multiple writers. Write side critical sections must thus be serialized
+by an external lock.
+
+If the write serialization primitive is not implicitly disabling
+preemption, preemption must be explicitly disabled before entering the
+write side section. If the read section can be invoked from hardirq or
+softirq contexts, interrupts or bottom halves must also be respectively
+disabled before entering the write section.
+
+If it's desired to automatically handle the sequence counter
+requirements of writer serialization and non-preemptibility, use
+:ref:`seqlock_t` instead.
+
+Initialization::
+
+ /* dynamic */
+ seqcount_t foo_seqcount;
+ seqcount_init(&foo_seqcount);
+
+ /* static */
+ static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_ZERO(foo.seq),
+ } foo;
+
+Write path::
+
+ /* Serialized context with disabled preemption */
+
+ write_seqcount_begin(&foo_seqcount);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_seqcount_end(&foo_seqcount);
+
+Read path::
+
+ do {
+ seq = read_seqcount_begin(&foo_seqcount);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqcount_retry(&foo_seqcount, seq));
+
+
+.. _seqlock_t:
+
+Sequential locks (``seqlock_t``)
+================================
+
+This contains the :ref:`seqcount_t` mechanism earlier discussed, plus an
+embedded spinlock for writer serialization and non-preemptibility.
+
+If the read side section can be invoked from hardirq or softirq context,
+use the write side function variants which disable interrupts or bottom
+halves respectively.
+
+Initialization::
+
+ /* dynamic */
+ seqlock_t foo_seqlock;
+ seqlock_init(&foo_seqlock);
+
+ /* static */
+ static DEFINE_SEQLOCK(foo_seqlock);
+
+ /* C99 struct init */
+ struct {
+ .seql = __SEQLOCK_UNLOCKED(foo.seql)
+ } foo;
+
+Write path::
+
+ write_seqlock(&foo_seqlock);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_sequnlock(&foo_seqlock);
+
+Read path, three categories:
+
+1. Normal Sequence readers which never block a writer but they must
+ retry if a writer is in progress by detecting change in the sequence
+ number. Writers do not wait for a sequence reader::
+
+ do {
+ seq = read_seqbegin(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqretry(&foo_seqlock, seq));
+
+2. Locking readers which will wait if a writer or another locking reader
+ is in progress. A locking reader in progress will also block a writer
+ from entering its critical section. This read lock is
+ exclusive. Unlike rwlock_t, only one locking reader can acquire it::
+
+ read_seqlock_excl(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ read_sequnlock_excl(&foo_seqlock);
+
+3. Conditional lockless reader (as in 1), or locking reader (as in 2),
+ according to a passed marker. This is used to avoid lockless readers
+ starvation (too much retry loops) in case of a sharp spike in write
+ activity. First, a lockless read is tried (even marker passed). If
+ that trial fails (odd sequence counter is returned, which is used as
+ the next iteration marker), the lockless read is transformed to a
+ full locking read and no retry loop is necessary::
+
+ /* marker; even initialization */
+ int seq = 0;
+ do {
+ read_seqbegin_or_lock(&foo_seqlock, &seq);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (need_seqretry(&foo_seqlock, seq));
+ done_seqretry(&foo_seqlock, seq);
+
+
+API documentation
+=================
+
+.. kernel-doc:: include/linux/seqlock.h
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8b97204..299d68f 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
- *
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
- *
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
- *
- *
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
+ *
+ * See Documentation/locking/seqlock.rst
+ *
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/
#include <linux/spinlock.h>
@@ -41,8 +20,8 @@
#include <asm/processor.h>
/*
- * The seqlock interface does not prescribe a precise sequence of read
- * begin/retry/end. For readers, typically there is a call to
+ * The seqlock seqcount_t interface does not prescribe a precise sequence of
+ * read begin/retry/end. For readers, typically there is a call to
* read_seqcount_begin() and read_seqcount_retry(), however, there are more
* esoteric cases which do not follow this pattern.
*
@@ -50,16 +29,30 @@
* via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
* pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
* atomics; if there is a matching read_seqcount_retry() call, no following
- * memory operations are considered atomic. Usage of seqlocks via seqlock_t
- * interface is not affected.
+ * memory operations are considered atomic. Usage of the seqlock_t interface
+ * is not affected.
*/
#define KCSAN_SEQLOCK_REGION_MAX 1000
/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * This is the raw counting mechanism, without any writer protection.
+ *
+ * Write side critical sections must be serialized and non-preemptible.
+ *
+ * If readers can be invoked from hardirq or softirq contexts,
+ * interrupts or bottom halves must also be respectively disabled before
+ * entering the write section.
+ *
+ * This mechanism can't be used if the protected data contains pointers,
+ * as the writer can invalidate a pointer that a reader is following.
+ *
+ * If it's desired to automatically handle the sequence counter writer
+ * serialization and non-preemptibility requirements, use a sequential
+ * lock (seqlock_t) instead.
+ *
+ * See Documentation/locking/seqlock.rst
*/
typedef struct seqcount {
unsigned sequence;
@@ -398,10 +391,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}
-/*
- * Sequence counter only version assumes that callers are using their
- * own mutexing.
- */
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
@@ -434,15 +423,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * For more info, see:
+ * - Comments on top of seqcount_t
+ * - Documentation/locking/seqlock.rst
+ */
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 89b88845e05752b3d684eaf147f457c8dfa99c5f
Gitweb: https://git.kernel.org/tip/89b88845e05752b3d684eaf147f457c8dfa99c5f
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:11 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:23 +02:00
seqlock: Add kernel-doc for seqcount_t and seqlock_t APIs
seqlock.h is now included by kernel's RST documentation, but a small
number of the the exported seqlock.h functions are kernel-doc annotated.
Add kernel-doc for all seqlock.h exported APIs.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 425 +++++++++++++++++++++++++++++++--------
1 file changed, 348 insertions(+), 77 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 4c14560..85fb3ac 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -75,6 +75,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
+/**
+ * seqcount_init() - runtime initializer for seqcount_t
+ * @s: Pointer to the seqcount_t instance
+ */
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
@@ -98,13 +102,15 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
# define seqcount_lockdep_reader_access(x)
#endif
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
-
+/**
+ * SEQCNT_ZERO() - static initializer for seqcount_t
+ * @name: Name of the seqcount_t instance
+ */
+#define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
+ * @s: Pointer to seqcount_t
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -113,6 +119,8 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* Use carefully, only in critical code, and comment how the barrier is
* provided.
+ *
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
@@ -129,13 +137,10 @@ repeat:
}
/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
+ * @s: Pointer to seqcount_t
*
- * raw_read_seqcount_begin opens a read critical section of the given
- * seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
@@ -145,13 +150,10 @@ static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
}
/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * read_seqcount_begin() - begin a seqcount_t read critical section
+ * @s: Pointer to seqcount_t
*
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
@@ -160,13 +162,15 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
}
/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_read_seqcount() - read the raw seqcount_t counter value
+ * @s: Pointer to seqcount_t
*
* raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
+ * seqcount_t, without any lockdep checking, and without checking or
+ * masking the sequence counter LSB. Calling code is responsible for
+ * handling that.
+ *
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned raw_read_seqcount(const seqcount_t *s)
{
@@ -177,18 +181,21 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
}
/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
+ * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
+ * lockdep and w/o counter stabilization
+ * @s: Pointer to seqcount_t
*
- * raw_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
+ * raw_seqcount_begin opens a read critical section of the given
+ * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
+ * for the count to stabilize. If a writer is active when it begins, it
+ * will fail the read_seqcount_retry() at the end of the read critical
+ * section instead of stabilizing at the beginning of it.
*
- * Unlike read_seqcount_begin(), this function will not wait for the count
- * to stabilize. If a writer is active when we begin, we will fail the
- * read_seqcount_retry() instead of stabilizing at the beginning of the
- * critical section.
+ * Use this only in special kernel hot paths where the read section is
+ * small and has a high probability of success through other external
+ * means. It will save a single branching instruction.
+ *
+ * Return: count to be passed to read_seqcount_retry()
*/
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
@@ -199,10 +206,9 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
}
/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * @start: count, from read_seqcount_begin
- * Returns: 1 if retry is required, else 0
+ * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
+ * @s: Pointer to seqcount_t
+ * @start: count, from read_seqcount_begin()
*
* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -211,6 +217,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
*
* Use carefully, only in critical code, and comment how the barrier is
* provided.
+ *
+ * Return: true if a read section retry is required, else false
*/
static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
@@ -219,14 +227,15 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
}
/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
- * @start: count, from read_seqcount_begin
- * Returns: 1 if retry is required, else 0
+ * read_seqcount_retry() - end a seqcount_t read critical section
+ * @s: Pointer to seqcount_t
+ * @start: count, from read_seqcount_begin()
*
- * read_seqcount_retry closes a read critical section of the given seqcount.
- * If the critical section was invalid, it must be ignored (and typically
- * retried).
+ * read_seqcount_retry closes the read critical section of given
+ * seqcount_t. If the critical section was invalid, it must be ignored
+ * (and typically retried).
+ *
+ * Return: true if a read section retry is required, else false
*/
static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
{
@@ -234,6 +243,10 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_retry(s, start);
}
+/**
+ * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
+ * @s: Pointer to seqcount_t
+ */
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
kcsan_nestable_atomic_begin();
@@ -241,6 +254,10 @@ static inline void raw_write_seqcount_begin(seqcount_t *s)
smp_wmb();
}
+/**
+ * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
+ * @s: Pointer to seqcount_t
+ */
static inline void raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
@@ -248,17 +265,42 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * write_seqcount_begin_nested() - start a seqcount_t write section with
+ * custom lockdep nesting level
+ * @s: Pointer to seqcount_t
+ * @subclass: lockdep nesting level
+ *
+ * See Documentation/locking/lockdep-design.rst
+ */
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
+/**
+ * write_seqcount_begin() - start a seqcount_t write side critical section
+ * @s: Pointer to seqcount_t
+ *
+ * write_seqcount_begin opens a write side critical section of the given
+ * seqcount_t.
+ *
+ * Context: seqcount_t write side critical sections must be serialized and
+ * non-preemptible. If readers can be invoked from hardirq or softirq
+ * context, interrupts or bottom halves must be respectively disabled.
+ */
static inline void write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
+/**
+ * write_seqcount_end() - end a seqcount_t write side critical section
+ * @s: Pointer to seqcount_t
+ *
+ * The write section must've been opened with write_seqcount_begin().
+ */
static inline void write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, _RET_IP_);
@@ -266,12 +308,12 @@ static inline void write_seqcount_end(seqcount_t *s)
}
/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_barrier() - do a seqcount_t write barrier
+ * @s: Pointer to seqcount_t
*
- * This can be used to provide an ordering guarantee instead of the
- * usual consistency guarantee. It is one wmb cheaper, because we can
- * collapse the two back-to-back wmb()s.
+ * This can be used to provide an ordering guarantee instead of the usual
+ * consistency guarantee. It is one wmb cheaper, because it can collapse
+ * the two back-to-back wmb()s.
*
* Note that writes surrounding the barrier should be declared atomic (e.g.
* via WRITE_ONCE): a) to ensure the writes become visible to other threads
@@ -316,11 +358,12 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
}
/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
+ * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
+ * side operations
+ * @s: Pointer to seqcount_t
*
- * After write_seqcount_invalidate, no read-side seq operations will complete
- * successfully and see data older than this.
+ * After write_seqcount_invalidate, no seqcount_t read side operations
+ * will complete successfully and see data older than this.
*/
static inline void write_seqcount_invalidate(seqcount_t *s)
{
@@ -330,6 +373,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
+/**
+ * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
+ * @s: Pointer to seqcount_t
+ *
+ * Use seqcount_t latching to switch between two storage places protected
+ * by a sequence counter. Doing so allows having interruptible, preemptible,
+ * seqcount_t write side critical sections.
+ *
+ * Check raw_write_seqcount_latch() for more details and a full reader and
+ * writer usage example.
+ *
+ * Return: sequence counter raw value. Use the lowest bit as an index for
+ * picking which data copy to read. The full counter value must then be
+ * checked with read_seqcount_retry().
+ */
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
@@ -338,8 +396,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
}
/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
+ * raw_write_seqcount_latch() - redirect readers to even/odd copy
+ * @s: Pointer to seqcount_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -446,17 +504,28 @@ typedef struct {
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+/**
+ * seqlock_init() - dynamic initializer for seqlock_t
+ * @sl: Pointer to the seqlock_t instance
+ */
+#define seqlock_init(sl) \
do { \
- seqcount_init(&(x)->seqcount); \
- spin_lock_init(&(x)->lock); \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+/**
+ * DEFINE_SEQLOCK() - Define a statically allocated seqlock_t
+ * @sl: Name of the seqlock_t instance
+ */
+#define DEFINE_SEQLOCK(sl) \
+ seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
-/*
- * Read side functions for starting and finalizing a read side section.
+/**
+ * read_seqbegin() - start a seqlock_t read side critical section
+ * @sl: Pointer to seqlock_t
+ *
+ * Return: count, to be passed to read_seqretry()
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
@@ -467,6 +536,17 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
return ret;
}
+/**
+ * read_seqretry() - end a seqlock_t read side section
+ * @sl: Pointer to seqlock_t
+ * @start: count, from read_seqbegin()
+ *
+ * read_seqretry closes the read side critical section of given seqlock_t.
+ * If the critical section was invalid, it must be ignored (and typically
+ * retried).
+ *
+ * Return: true if a read section retry is required, else false
+ */
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
/*
@@ -478,10 +558,18 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
return read_seqcount_retry(&sl->seqcount, start);
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * write_seqlock() - start a seqlock_t write side critical section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_seqlock opens a write side critical section for the given
+ * seqlock_t. It also implicitly acquires the spinlock_t embedded inside
+ * that sequential lock. All seqlock_t write side sections are thus
+ * automatically serialized and non-preemptible.
+ *
+ * Context: if the seqlock_t read section, or other write side critical
+ * sections, can be invoked from hardirq or softirq contexts, use the
+ * _irqsave or _bh variants of this function instead.
*/
static inline void write_seqlock(seqlock_t *sl)
{
@@ -489,30 +577,66 @@ static inline void write_seqlock(seqlock_t *sl)
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock() - end a seqlock_t write side critical section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_sequnlock closes the (serialized and non-preemptible) write side
+ * critical section of given seqlock_t.
+ */
static inline void write_sequnlock(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
+/**
+ * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * _bh variant of write_seqlock(). Use only if the read side section, or
+ * other write side sections, can be invoked from softirq contexts.
+ */
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_sequnlock_bh closes the serialized, non-preemptible, and
+ * softirqs-disabled, seqlock_t write side critical section opened with
+ * write_seqlock_bh().
+ */
static inline void write_sequnlock_bh(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
+/**
+ * write_seqlock_irq() - start a non-interruptible seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * _irq variant of write_seqlock(). Use only if the read side section, or
+ * other write sections, can be invoked from hardirq contexts.
+ */
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
+/**
+ * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
+ * @sl: Pointer to seqlock_t
+ *
+ * write_sequnlock_irq closes the serialized and non-interruptible
+ * seqlock_t write side section opened with write_seqlock_irq().
+ */
static inline void write_sequnlock_irq(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
@@ -528,9 +652,28 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
+ * section
+ * @lock: Pointer to seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to write_sequnlock_irqrestore().
+ *
+ * _irqsave variant of write_seqlock(). Use it only if the read side
+ * section, or other write sections, can be invoked from hardirq context.
+ */
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
+/**
+ * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
+ * section
+ * @sl: Pointer to seqlock_t
+ * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
+ *
+ * write_sequnlock_irqrestore closes the serialized and non-interruptible
+ * seqlock_t write section previously opened with write_seqlock_irqsave().
+ */
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
@@ -538,36 +681,79 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
+/**
+ * read_seqlock_excl() - begin a seqlock_t locking reader section
+ * @sl: Pointer to seqlock_t
+ *
+ * read_seqlock_excl opens a seqlock_t locking reader critical section. A
+ * locking reader exclusively locks out *both* other writers *and* other
+ * locking readers, but it does not update the embedded sequence number.
+ *
+ * Locking readers act like a normal spin_lock()/spin_unlock().
+ *
+ * Context: if the seqlock_t write section, *or other read sections*, can
+ * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
+ * variant of this function instead.
+ *
+ * The opened read section must be closed with read_sequnlock_excl().
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
+/**
+ * read_sequnlock_excl() - end a seqlock_t locking reader critical section
+ * @sl: Pointer to seqlock_t
+ */
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
+/**
+ * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
+ * softirqs disabled
+ * @sl: Pointer to seqlock_t
+ *
+ * _bh variant of read_seqlock_excl(). Use this variant only if the
+ * seqlock_t write side section, *or other read sections*, can be invoked
+ * from softirq contexts.
+ */
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
+/**
+ * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
+ * reader section
+ * @sl: Pointer to seqlock_t
+ */
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
+/**
+ * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
+ * reader section
+ * @sl: Pointer to seqlock_t
+ *
+ * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
+ * write side section, *or other read sections*, can be invoked from a
+ * hardirq context.
+ */
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
+/**
+ * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
+ * locking reader section
+ * @sl: Pointer to seqlock_t
+ */
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
@@ -581,9 +767,26 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
return flags;
}
+/**
+ * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
+ * locking reader section
+ * @lock: Pointer to seqlock_t
+ * @flags: Stack-allocated storage for saving caller's local interrupt
+ * state, to be passed to read_sequnlock_excl_irqrestore().
+ *
+ * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
+ * write side section, *or other read sections*, can be invoked from a
+ * hardirq context.
+ */
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+/**
+ * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
+ * locking reader section
+ * @sl: Pointer to seqlock_t
+ * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
+ */
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
@@ -591,14 +794,35 @@ read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
}
/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
- *
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
+ * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
+ * @lock: Pointer to seqlock_t
+ * @seq : Marker and return parameter. If the passed value is even, the
+ * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
+ * If the passed value is odd, the reader will become a *locking* reader
+ * as in read_seqlock_excl(). In the first call to this function, the
+ * caller *must* initialize and pass an even value to @seq; this way, a
+ * lockless read can be optimistically tried first.
+ *
+ * read_seqbegin_or_lock is an API designed to optimistically try a normal
+ * lockless seqlock_t read section first. If an odd counter is found, the
+ * lockless read trial has failed, and the next read iteration transforms
+ * itself into a full seqlock_t locking reader.
+ *
+ * This is typically used to avoid seqlock_t lockless readers starvation
+ * (too much retry loops) in the case of a sharp spike in write side
+ * activity.
+ *
+ * Context: if the seqlock_t write section, *or other read sections*, can
+ * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
+ * variant of this function instead.
+ *
+ * Check Documentation/locking/seqlock.rst for template example code.
+ *
+ * Return: the encountered sequence counter value, through the @seq
+ * parameter, which is overloaded as a return parameter. This returned
+ * value must be checked with need_seqretry(). If the read section need to
+ * be retried, this returned value must also be passed as the @seq
+ * parameter of the next read_seqbegin_or_lock() iteration.
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
@@ -608,17 +832,52 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
read_seqlock_excl(lock);
}
+/**
+ * need_seqretry() - validate seqlock_t "locking or lockless" read section
+ * @lock: Pointer to seqlock_t
+ * @seq: sequence count, from read_seqbegin_or_lock()
+ *
+ * Return: true if a read section retry is required, false otherwise
+ */
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
+/**
+ * done_seqretry() - end seqlock_t "locking or lockless" reader section
+ * @lock: Pointer to seqlock_t
+ * @seq: count, from read_seqbegin_or_lock()
+ *
+ * done_seqretry finishes the seqlock_t read side critical section started
+ * with read_seqbegin_or_lock() and validated by need_seqretry().
+ */
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
+/**
+ * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
+ * a non-interruptible locking reader
+ * @lock: Pointer to seqlock_t
+ * @seq: Marker and return parameter. Check read_seqbegin_or_lock().
+ *
+ * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
+ * the seqlock_t write section, *or other read sections*, can be invoked
+ * from hardirq context.
+ *
+ * Note: Interrupts will be disabled only for "locking reader" mode.
+ *
+ * Return:
+ *
+ * 1. The saved local interrupts state in case of a locking reader, to
+ * be passed to done_seqretry_irqrestore().
+ *
+ * 2. The encountered sequence counter value, returned through @seq
+ * overloaded as a return parameter. Check read_seqbegin_or_lock().
+ */
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
@@ -632,6 +891,18 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
return flags;
}
+/**
+ * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
+ * non-interruptible locking reader section
+ * @lock: Pointer to seqlock_t
+ * @seq: Count, from read_seqbegin_or_lock_irqsave()
+ * @flags: Caller's saved local interrupt state in case of a locking
+ * reader, also from read_seqbegin_or_lock_irqsave()
+ *
+ * This is the _irqrestore variant of done_seqretry(). The read section
+ * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
+ * by need_seqretry().
+ */
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
The following commit has been merged into the locking/core branch of tip:
Commit-ID: b901892b51317b321c7bc96e2ccd2f522d1380ee
Gitweb: https://git.kernel.org/tip/b901892b51317b321c7bc96e2ccd2f522d1380ee
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:21 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:26 +02:00
netfilter: nft_set_rbtree: Use sequence counter with associated rwlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_rwlock_t data type, which allows to associate a
rwlock with the sequence counter. This enables lockdep to verify that
the rwlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
net/netfilter/nft_set_rbtree.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/netfilter/nft_set_rbtree.c b/net/netfilter/nft_set_rbtree.c
index b6aad3f..4b2834f 100644
--- a/net/netfilter/nft_set_rbtree.c
+++ b/net/netfilter/nft_set_rbtree.c
@@ -18,7 +18,7 @@
struct nft_rbtree {
struct rb_root root;
rwlock_t lock;
- seqcount_t count;
+ seqcount_rwlock_t count;
struct delayed_work gc_work;
};
@@ -523,7 +523,7 @@ static int nft_rbtree_init(const struct nft_set *set,
struct nft_rbtree *priv = nft_set_priv(set);
rwlock_init(&priv->lock);
- seqcount_init(&priv->count);
+ seqcount_rwlock_init(&priv->count, &priv->lock);
priv->root = RB_ROOT;
INIT_DEFERRABLE_WORK(&priv->gc_work, nft_rbtree_gc);
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 318ce71f3e3ae4108c1665f3860afa8a2a4c9f02
Gitweb: https://git.kernel.org/tip/318ce71f3e3ae4108c1665f3860afa8a2a4c9f02
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:17 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:25 +02:00
dma-buf: Remove custom seqcount lockdep class key
Commit 3c3b177a9369 ("reservation: add support for read-only access
using rcu") introduced a sequence counter to manage updates to
reservations. Back then, the reservation object initializer
reservation_object_init() was always inlined.
Having the sequence counter initialization inlined meant that each of
the call sites would have a different lockdep class key, which would've
broken lockdep's deadlock detection. The aforementioned commit thus
introduced, and exported, a custom seqcount lockdep class key and name.
The commit 8735f16803f00 ("dma-buf: cleanup reservation_object_init...")
transformed the reservation object initializer to a normal non-inlined C
function. seqcount_init(), which automatically defines the seqcount
lockdep class key and must be called non-inlined, can now be safely used.
Remove the seqcount custom lockdep class key, name, and export. Use
seqcount_init() inside the dma reservation object initializer.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
drivers/dma-buf/dma-resv.c | 9 +--------
include/linux/dma-resv.h | 2 --
2 files changed, 1 insertion(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index b45f851..15efa0c 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -51,12 +51,6 @@
DEFINE_WD_CLASS(reservation_ww_class);
EXPORT_SYMBOL(reservation_ww_class);
-struct lock_class_key reservation_seqcount_class;
-EXPORT_SYMBOL(reservation_seqcount_class);
-
-const char reservation_seqcount_string[] = "reservation_seqcount";
-EXPORT_SYMBOL(reservation_seqcount_string);
-
/**
* dma_resv_list_alloc - allocate fence list
* @shared_max: number of fences we need space for
@@ -135,9 +129,8 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
+ seqcount_init(&obj->seq);
- __seqcount_init(&obj->seq, reservation_seqcount_string,
- &reservation_seqcount_class);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
}
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index ee50d10..a6538ae 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -46,8 +46,6 @@
#include <linux/rcupdate.h>
extern struct ww_class reservation_ww_class;
-extern struct lock_class_key reservation_seqcount_class;
-extern const char reservation_seqcount_string[];
/**
* struct dma_resv_list - a list of shared fences
The following commit has been merged into the locking/core branch of tip:
Commit-ID: cd29f22019ec4ab998d2e1e8c831c7c42db4aa7d
Gitweb: https://git.kernel.org/tip/cd29f22019ec4ab998d2e1e8c831c7c42db4aa7d
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:18 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:25 +02:00
dma-buf: Use sequence counter with associated wound/wait mutex
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
The dma-buf reservation subsystem uses plain sequence counters to manage
updates to reservations. Writer serialization is accomplished through a
wound/wait mutex.
Acquiring a wound/wait mutex does not disable preemption, so this needs
to be done manually before and after the write side critical section.
Use the newly-added seqcount_ww_mutex_t instead:
- It associates the ww_mutex with the sequence count, which enables
lockdep to validate that the write side critical section is properly
serialized.
- It removes the need to explicitly add preempt_disable/enable()
around the write side critical section because the write_begin/end()
functions for this new data type automatically do this.
If lockdep is disabled this ww_mutex lock association is compiled out
and has neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
drivers/dma-buf/dma-resv.c | 8 +-------
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 --
include/linux/dma-resv.h | 2 +-
3 files changed, 2 insertions(+), 10 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 15efa0c..a763135 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -129,7 +129,7 @@ subsys_initcall(dma_resv_lockdep);
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
- seqcount_init(&obj->seq);
+ seqcount_ww_mutex_init(&obj->seq, &obj->lock);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
@@ -260,7 +260,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
fobj = dma_resv_get_list(obj);
count = fobj->shared_count;
- preempt_disable();
write_seqcount_begin(&obj->seq);
for (i = 0; i < count; ++i) {
@@ -282,7 +281,6 @@ replace:
smp_store_mb(fobj->shared_count, count);
write_seqcount_end(&obj->seq);
- preempt_enable();
dma_fence_put(old);
}
EXPORT_SYMBOL(dma_resv_add_shared_fence);
@@ -309,14 +307,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
if (fence)
dma_fence_get(fence);
- preempt_disable();
write_seqcount_begin(&obj->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(obj->fence_excl, fence);
if (old)
old->shared_count = 0;
write_seqcount_end(&obj->seq);
- preempt_enable();
/* inplace update, no shared fences */
while (i--)
@@ -394,13 +390,11 @@ retry:
src_list = dma_resv_get_list(dst);
old = dma_resv_get_excl(dst);
- preempt_disable();
write_seqcount_begin(&dst->seq);
/* write_seqcount_begin provides the necessary memory barrier */
RCU_INIT_POINTER(dst->fence_excl, new);
RCU_INIT_POINTER(dst->fence, dst_list);
write_seqcount_end(&dst->seq);
- preempt_enable();
dma_resv_list_free(src_list);
dma_fence_put(old);
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
index b91b517..ff4b583 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
@@ -258,11 +258,9 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
new->shared_count = k;
/* Install the new fence list, seqcount provides the barriers */
- preempt_disable();
write_seqcount_begin(&resv->seq);
RCU_INIT_POINTER(resv->fence, new);
write_seqcount_end(&resv->seq);
- preempt_enable();
/* Drop the references to the removed fences or move them to ef_list */
for (i = j, k = 0; i < old->shared_count; ++i) {
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index a6538ae..d44a77e 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -69,7 +69,7 @@ struct dma_resv_list {
*/
struct dma_resv {
struct ww_mutex lock;
- seqcount_t seq;
+ seqcount_ww_mutex_t seq;
struct dma_fence __rcu *fence_excl;
struct dma_resv_list __rcu *fence;
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 77cc278f7b202e4f16f8596837219d02cb090b96
Gitweb: https://git.kernel.org/tip/77cc278f7b202e4f16f8596837219d02cb090b96
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:22 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:27 +02:00
xfrm: policy: Use sequence counters with associated lock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.
A plain seqcount_t does not contain the information of which lock must
be held when entering a write side critical section.
Use the new seqcount_spinlock_t and seqcount_mutex_t data types instead,
which allow to associate a lock with the sequence counter. This enables
lockdep to verify that the lock used for writer serialization is held
when the write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
net/xfrm/xfrm_policy.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 564aa64..732a940 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -122,7 +122,7 @@ struct xfrm_pol_inexact_bin {
/* list containing '*:*' policies */
struct hlist_head hhead;
- seqcount_t count;
+ seqcount_spinlock_t count;
/* tree sorted by daddr/prefix */
struct rb_root root_d;
@@ -155,7 +155,7 @@ static struct xfrm_policy_afinfo const __rcu *xfrm_policy_afinfo[AF_INET6 + 1]
__read_mostly;
static struct kmem_cache *xfrm_dst_cache __ro_after_init;
-static __read_mostly seqcount_t xfrm_policy_hash_generation;
+static __read_mostly seqcount_mutex_t xfrm_policy_hash_generation;
static struct rhashtable xfrm_policy_inexact_table;
static const struct rhashtable_params xfrm_pol_inexact_params;
@@ -719,7 +719,7 @@ xfrm_policy_inexact_alloc_bin(const struct xfrm_policy *pol, u8 dir)
INIT_HLIST_HEAD(&bin->hhead);
bin->root_d = RB_ROOT;
bin->root_s = RB_ROOT;
- seqcount_init(&bin->count);
+ seqcount_spinlock_init(&bin->count, &net->xfrm.xfrm_policy_lock);
prev = rhashtable_lookup_get_insert_key(&xfrm_policy_inexact_table,
&bin->k, &bin->head,
@@ -1906,7 +1906,7 @@ static int xfrm_policy_match(const struct xfrm_policy *pol,
static struct xfrm_pol_inexact_node *
xfrm_policy_lookup_inexact_addr(const struct rb_root *r,
- seqcount_t *count,
+ seqcount_spinlock_t *count,
const xfrm_address_t *addr, u16 family)
{
const struct rb_node *parent;
@@ -4153,7 +4153,7 @@ void __init xfrm_init(void)
{
register_pernet_subsys(&xfrm_net_ops);
xfrm_dev_init();
- seqcount_init(&xfrm_policy_hash_generation);
+ seqcount_mutex_init(&xfrm_policy_hash_generation, &hash_resize_mutex);
xfrm_input_init();
#ifdef CONFIG_INET_ESPINTCP
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 8201d923f492703a7d6c980cff3034759a452b86
Gitweb: https://git.kernel.org/tip/8201d923f492703a7d6c980cff3034759a452b86
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:20 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:26 +02:00
netfilter: conntrack: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/net/netfilter/nf_conntrack.h | 2 +-
net/netfilter/nf_conntrack_core.c | 5 +++--
2 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 90690e3..ea4e201 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -286,7 +286,7 @@ int nf_conntrack_hash_resize(unsigned int hashsize);
extern struct hlist_nulls_head *nf_conntrack_hash;
extern unsigned int nf_conntrack_htable_size;
-extern seqcount_t nf_conntrack_generation;
+extern seqcount_spinlock_t nf_conntrack_generation;
extern unsigned int nf_conntrack_max;
/* must be called with rcu read lock held */
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index f33d72c..b597b5b 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -180,7 +180,7 @@ EXPORT_SYMBOL_GPL(nf_conntrack_htable_size);
unsigned int nf_conntrack_max __read_mostly;
EXPORT_SYMBOL_GPL(nf_conntrack_max);
-seqcount_t nf_conntrack_generation __read_mostly;
+seqcount_spinlock_t nf_conntrack_generation __read_mostly;
static unsigned int nf_conntrack_hash_rnd __read_mostly;
static u32 hash_conntrack_raw(const struct nf_conntrack_tuple *tuple,
@@ -2600,7 +2600,8 @@ int nf_conntrack_init_start(void)
/* struct nf_ct_ext uses u8 to store offsets/size */
BUILD_BUG_ON(total_extension_size() > 255u);
- seqcount_init(&nf_conntrack_generation);
+ seqcount_spinlock_init(&nf_conntrack_generation,
+ &nf_conntrack_locks_all_lock);
for (i = 0; i < CONNTRACK_LOCKS; i++)
spin_lock_init(&nf_conntrack_locks[i]);
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 5c73b9a2b1b4ecc809a914aa64970157b3d8c936
Gitweb: https://git.kernel.org/tip/5c73b9a2b1b4ecc809a914aa64970157b3d8c936
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:29 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:29 +02:00
kvm/eventfd: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/kvm_irqfd.h | 2 +-
virt/kvm/eventfd.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dc1da02..dac047a 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
wait_queue_entry_t wait;
/* Update side is protected by irqfds.lock */
struct kvm_kernel_irq_routing_entry irq_entry;
- seqcount_t irq_entry_sc;
+ seqcount_spinlock_t irq_entry_sc;
/* Used for level IRQ fast-path */
int gsi;
struct work_struct inject;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index ef7ed91..d6408bb 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
INIT_LIST_HEAD(&irqfd->list);
INIT_WORK(&irqfd->inject, irqfd_inject);
INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
- seqcount_init(&irqfd->irq_entry_sc);
+ seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
f = fdget(args->fd);
if (!f.file) {
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 67b7b641ca69cafb467f7560316b09b8ff0fa5c9
Gitweb: https://git.kernel.org/tip/67b7b641ca69cafb467f7560316b09b8ff0fa5c9
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:26 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:28 +02:00
iocost: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Daniel Wagner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
block/blk-iocost.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 8ac4aad..8e940c2 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -406,7 +406,7 @@ struct ioc {
enum ioc_running running;
atomic64_t vtime_rate;
- seqcount_t period_seqcount;
+ seqcount_spinlock_t period_seqcount;
u32 period_at; /* wallclock starttime */
u64 period_at_vtime; /* vtime starttime */
@@ -873,7 +873,6 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
{
- lockdep_assert_held(&ioc->lock);
WARN_ON_ONCE(ioc->running != IOC_RUNNING);
write_seqcount_begin(&ioc->period_seqcount);
@@ -2001,7 +2000,7 @@ static int blk_iocost_init(struct request_queue *q)
ioc->running = IOC_IDLE;
atomic64_set(&ioc->vtime_rate, VTIME_PER_USEC);
- seqcount_init(&ioc->period_seqcount);
+ seqcount_spinlock_init(&ioc->period_seqcount, &ioc->lock);
ioc->period_at = ktime_to_us(ktime_get());
atomic64_set(&ioc->cur_period, 0);
atomic_set(&ioc->hweight_gen, 0);
The following commit has been merged into the locking/core branch of tip:
Commit-ID: af5a06b582ec3d7b0160b4faaa65f73d8dcf989f
Gitweb: https://git.kernel.org/tip/af5a06b582ec3d7b0160b4faaa65f73d8dcf989f
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:30 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:29 +02:00
hrtimer: Use sequence counter with associated raw spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_raw_spinlock_t data type, which allows to associate
a raw spinlock with the sequence counter. This enables lockdep to verify
that the raw spinlock used for writer serialization is held when the
write side critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/hrtimer.h | 2 +-
kernel/time/hrtimer.c | 13 ++++++++++---
2 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 15c8ac3..25993b8 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -159,7 +159,7 @@ struct hrtimer_clock_base {
struct hrtimer_cpu_base *cpu_base;
unsigned int index;
clockid_t clockid;
- seqcount_t seq;
+ seqcount_raw_spinlock_t seq;
struct hrtimer *running;
struct timerqueue_head active;
ktime_t (*get_time)(void);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index d89da1c..c403851 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -135,7 +135,11 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
* timer->base->cpu_base
*/
static struct hrtimer_cpu_base migration_cpu_base = {
- .clock_base = { { .cpu_base = &migration_cpu_base, }, },
+ .clock_base = { {
+ .cpu_base = &migration_cpu_base,
+ .seq = SEQCNT_RAW_SPINLOCK_ZERO(migration_cpu_base.seq,
+ &migration_cpu_base.lock),
+ }, },
};
#define migration_base migration_cpu_base.clock_base[0]
@@ -1998,8 +2002,11 @@ int hrtimers_prepare_cpu(unsigned int cpu)
int i;
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
- cpu_base->clock_base[i].cpu_base = cpu_base;
- timerqueue_init_head(&cpu_base->clock_base[i].active);
+ struct hrtimer_clock_base *clock_b = &cpu_base->clock_base[i];
+
+ clock_b->cpu_base = cpu_base;
+ seqcount_raw_spinlock_init(&clock_b->seq, &cpu_base->lock);
+ timerqueue_init_head(&clock_b->active);
}
cpu_base->cpu = cpu;
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 2ca97ac8bdcc31fdc868f40c73c017f0a648dc07
Gitweb: https://git.kernel.org/tip/2ca97ac8bdcc31fdc868f40c73c017f0a648dc07
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:28 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:28 +02:00
userfaultfd: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
fs/userfaultfd.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 52de290..26e8b23 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -61,7 +61,7 @@ struct userfaultfd_ctx {
/* waitqueue head for events */
wait_queue_head_t event_wqh;
/* a refile sequence protected by fault_pending_wqh lock */
- struct seqcount refile_seq;
+ seqcount_spinlock_t refile_seq;
/* pseudo fd refcounting */
refcount_t refcount;
/* userfaultfd syscall flags */
@@ -1998,7 +1998,7 @@ static void init_once_userfaultfd_ctx(void *mem)
init_waitqueue_head(&ctx->fault_wqh);
init_waitqueue_head(&ctx->event_wqh);
init_waitqueue_head(&ctx->fd_wqh);
- seqcount_init(&ctx->refile_seq);
+ seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
}
SYSCALL_DEFINE1(userfaultfd, int, flags)
The following commit has been merged into the locking/core branch of tip:
Commit-ID: ec8702da570ebb59f38471007bf71359c51b027b
Gitweb: https://git.kernel.org/tip/ec8702da570ebb59f38471007bf71359c51b027b
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:16 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:25 +02:00
seqlock: Align multi-line macros newline escapes at 72 columns
Parent commit, "seqlock: Extend seqcount API with associated locks",
introduced a big number of multi-line macros that are newline-escaped
at 72 columns.
For overall cohesion, align the earlier-existing macros similarly.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 29 +++++++++++++++--------------
1 file changed, 15 insertions(+), 14 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8c16a49..b487299 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -80,17 +80,18 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
}
#ifdef CONFIG_DEBUG_LOCK_ALLOC
-# define SEQCOUNT_DEP_MAP_INIT(lockname) \
- .dep_map = { .name = #lockname } \
+
+# define SEQCOUNT_DEP_MAP_INIT(lockname) \
+ .dep_map = { .name = #lockname }
/**
* seqcount_init() - runtime initializer for seqcount_t
* @s: Pointer to the seqcount_t instance
*/
-# define seqcount_init(s) \
- do { \
- static struct lock_class_key __key; \
- __seqcount_init((s), #s, &__key); \
+# define seqcount_init(s) \
+ do { \
+ static struct lock_class_key __key; \
+ __seqcount_init((s), #s, &__key); \
} while (0)
static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
@@ -842,20 +843,20 @@ typedef struct {
spinlock_t lock;
} seqlock_t;
-#define __SEQLOCK_UNLOCKED(lockname) \
- { \
- .seqcount = SEQCNT_ZERO(lockname), \
- .lock = __SPIN_LOCK_UNLOCKED(lockname) \
+#define __SEQLOCK_UNLOCKED(lockname) \
+ { \
+ .seqcount = SEQCNT_ZERO(lockname), \
+ .lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
/**
* seqlock_init() - dynamic initializer for seqlock_t
* @sl: Pointer to the seqlock_t instance
*/
-#define seqlock_init(sl) \
- do { \
- seqcount_init(&(sl)->seqcount); \
- spin_lock_init(&(sl)->lock); \
+#define seqlock_init(sl) \
+ do { \
+ seqcount_init(&(sl)->seqcount); \
+ spin_lock_init(&(sl)->lock); \
} while (0)
/**
The following commit has been merged into the locking/core branch of tip:
Commit-ID: b75058614fdd3140074a640b514f6a0b4d485a2d
Gitweb: https://git.kernel.org/tip/b75058614fdd3140074a640b514f6a0b4d485a2d
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:19 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:26 +02:00
sched: tasks: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/sched.h | 2 +-
init/init_task.c | 3 ++-
kernel/fork.c | 2 +-
3 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8d1de02..9a9d826 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1050,7 +1050,7 @@ struct task_struct {
/* Protected by ->alloc_lock: */
nodemask_t mems_allowed;
/* Seqence number to catch updates: */
- seqcount_t mems_allowed_seq;
+ seqcount_spinlock_t mems_allowed_seq;
int cpuset_mem_spread_rotor;
int cpuset_slab_spread_rotor;
#endif
diff --git a/init/init_task.c b/init/init_task.c
index 15089d1..94fe3ba 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -154,7 +154,8 @@ struct task_struct init_task
.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),
#endif
#ifdef CONFIG_CPUSETS
- .mems_allowed_seq = SEQCNT_ZERO(init_task.mems_allowed_seq),
+ .mems_allowed_seq = SEQCNT_SPINLOCK_ZERO(init_task.mems_allowed_seq,
+ &init_task.alloc_lock),
#endif
#ifdef CONFIG_RT_MUTEXES
.pi_waiters = RB_ROOT_CACHED,
diff --git a/kernel/fork.c b/kernel/fork.c
index 70d9d0a..fc72f09 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2032,7 +2032,7 @@ static __latent_entropy struct task_struct *copy_process(
#ifdef CONFIG_CPUSETS
p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
- seqcount_init(&p->mems_allowed_seq);
+ seqcount_spinlock_init(&p->mems_allowed_seq, &p->alloc_lock);
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
p->irq_events = 0;
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 26475371976c69489d3a8e6c8bbf35afbbc25055
Gitweb: https://git.kernel.org/tip/26475371976c69489d3a8e6c8bbf35afbbc25055
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:24 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:27 +02:00
vfs: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
fs/dcache.c | 2 +-
fs/fs_struct.c | 4 ++--
include/linux/dcache.h | 2 +-
include/linux/fs_struct.h | 2 +-
4 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index 361ea7a..ea04858 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1746,7 +1746,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
dentry->d_lockref.count = 1;
dentry->d_flags = 0;
spin_lock_init(&dentry->d_lock);
- seqcount_init(&dentry->d_seq);
+ seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
dentry->d_inode = NULL;
dentry->d_parent = dentry;
dentry->d_sb = sb;
diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index ca639ed..04b3f5b 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -117,7 +117,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
- seqcount_init(&fs->seq);
+ seqcount_spinlock_init(&fs->seq, &fs->lock);
fs->umask = old->umask;
spin_lock(&old->lock);
@@ -163,6 +163,6 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
- .seq = SEQCNT_ZERO(init_fs.seq),
+ .seq = SEQCNT_SPINLOCK_ZERO(init_fs.seq, &init_fs.lock),
.umask = 0022,
};
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index a81f0c3..65d975b 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -89,7 +89,7 @@ extern struct dentry_stat_t dentry_stat;
struct dentry {
/* RCU lookup touched fields */
unsigned int d_flags; /* protected by d_lock */
- seqcount_t d_seq; /* per dentry seqlock */
+ seqcount_spinlock_t d_seq; /* per dentry seqlock */
struct hlist_bl_node d_hash; /* lookup hash list */
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index cf1015a..783b48d 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -9,7 +9,7 @@
struct fs_struct {
int users;
spinlock_t lock;
- seqcount_t seq;
+ seqcount_spinlock_t seq;
int umask;
int in_exec;
struct path root, pwd;
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 0a87b25ff2eb6169403c88b0d5f3c97bdaa3c930
Gitweb: https://git.kernel.org/tip/0a87b25ff2eb6169403c88b0d5f3c97bdaa3c930
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Mon, 20 Jul 2020 17:55:25 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Wed, 29 Jul 2020 16:14:27 +02:00
raid5: Use sequence counter with associated spinlock
A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.
Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.
If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Song Liu <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
drivers/md/raid5.c | 2 +-
drivers/md/raid5.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index ab8067f..892aefe 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6935,7 +6935,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
} else
goto abort;
spin_lock_init(&conf->device_lock);
- seqcount_init(&conf->gen_lock);
+ seqcount_spinlock_init(&conf->gen_lock, &conf->device_lock);
mutex_init(&conf->cache_size_mutex);
init_waitqueue_head(&conf->wait_for_quiescent);
init_waitqueue_head(&conf->wait_for_stripe);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index f90e070..a2c9e9e 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -589,7 +589,7 @@ struct r5conf {
int prev_chunk_sectors;
int prev_algo;
short generation; /* increments with every reshape */
- seqcount_t gen_lock; /* lock against generation changes */
+ seqcount_spinlock_t gen_lock; /* lock against generation changes */
unsigned long reshape_checkpoint; /* Time we last updated
* metadata */
long long min_offset_diff; /* minimum difference between
On Mon, Jul 20, 2020 at 05:55:14PM +0200, Ahmed S. Darwish wrote:
> Preemption must be disabled before entering a sequence count write side
> critical section. Failing to do so, the seqcount read side can preempt
> the write side section and spin for the entire scheduler tick. If that
> reader belongs to a real-time scheduling class, it can spin forever and
> the kernel will livelock.
>
> Assert through lockdep that preemption is disabled for seqcount writers.
>
This patch is causing compile failures for various images (eg arm:allmodconfig,
arm:imx_v6_v7_defconfig, mips:allmodconfig).
In file included from arch/arm/include/asm/bug.h:60,
from include/linux/bug.h:5,
from include/linux/thread_info.h:12,
from include/asm-generic/current.h:5,
from ./arch/arm/include/generated/asm/current.h:1,
from include/linux/sched.h:12,
from arch/arm/kernel/asm-offsets.c:11:
include/linux/seqlock.h: In function 'write_seqcount_begin_nested':
include/asm-generic/percpu.h:31:40: error: implicit declaration of function 'raw_smp_processor_id'
Reverting it fixes the problem. Is this being addressed ?
Guenter
> Signed-off-by: Ahmed S. Darwish <[email protected]>
> ---
> include/linux/seqlock.h | 29 +++++++++++++++++++++++------
> 1 file changed, 23 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index e885702d8b82..54bc20496392 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -266,6 +266,12 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
> kcsan_nestable_atomic_end();
> }
>
> +static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
> +{
> + raw_write_seqcount_begin(s);
> + seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
> +}
> +
> /**
> * write_seqcount_begin_nested() - start a seqcount_t write section with
> * custom lockdep nesting level
> @@ -276,8 +282,19 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
> */
> static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
> {
> - raw_write_seqcount_begin(s);
> - seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
> + lockdep_assert_preemption_disabled();
> + __write_seqcount_begin_nested(s, subclass);
> +}
> +
> +/*
> + * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
> + *
> + * Use for internal seqlock.h code where it's known that preemption is
> + * already disabled. For example, seqlock_t write side functions.
> + */
> +static inline void __write_seqcount_begin(seqcount_t *s)
> +{
> + __write_seqcount_begin_nested(s, 0);
> }
>
> /**
> @@ -575,7 +592,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
> static inline void write_seqlock(seqlock_t *sl)
> {
> spin_lock(&sl->lock);
> - write_seqcount_begin(&sl->seqcount);
> + __write_seqcount_begin(&sl->seqcount);
> }
>
> /**
> @@ -601,7 +618,7 @@ static inline void write_sequnlock(seqlock_t *sl)
> static inline void write_seqlock_bh(seqlock_t *sl)
> {
> spin_lock_bh(&sl->lock);
> - write_seqcount_begin(&sl->seqcount);
> + __write_seqcount_begin(&sl->seqcount);
> }
>
> /**
> @@ -628,7 +645,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
> static inline void write_seqlock_irq(seqlock_t *sl)
> {
> spin_lock_irq(&sl->lock);
> - write_seqcount_begin(&sl->seqcount);
> + __write_seqcount_begin(&sl->seqcount);
> }
>
> /**
> @@ -649,7 +666,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
> unsigned long flags;
>
> spin_lock_irqsave(&sl->lock, flags);
> - write_seqcount_begin(&sl->seqcount);
> + __write_seqcount_begin(&sl->seqcount);
> return flags;
> }
>
> --
> 2.20.1
>
On Sat, Aug 08, 2020 at 04:21:22PM -0700, Guenter Roeck wrote:
> On Mon, Jul 20, 2020 at 05:55:14PM +0200, Ahmed S. Darwish wrote:
> > Preemption must be disabled before entering a sequence count write side
> > critical section. Failing to do so, the seqcount read side can preempt
> > the write side section and spin for the entire scheduler tick. If that
> > reader belongs to a real-time scheduling class, it can spin forever and
> > the kernel will livelock.
> >
> > Assert through lockdep that preemption is disabled for seqcount writers.
> >
>
> This patch is causing compile failures for various images (eg arm:allmodconfig,
> arm:imx_v6_v7_defconfig, mips:allmodconfig).
>
> In file included from arch/arm/include/asm/bug.h:60,
> from include/linux/bug.h:5,
> from include/linux/thread_info.h:12,
> from include/asm-generic/current.h:5,
> from ./arch/arm/include/generated/asm/current.h:1,
> from include/linux/sched.h:12,
> from arch/arm/kernel/asm-offsets.c:11:
> include/linux/seqlock.h: In function 'write_seqcount_begin_nested':
> include/asm-generic/percpu.h:31:40: error: implicit declaration of function 'raw_smp_processor_id'
>
Also:
Building sparc64:allmodconfig ... failed
--------------
Error log:
<stdin>:1511:2: warning: #warning syscall clone3 not implemented [-Wcpp]
In file included from arch/sparc/include/asm/bug.h:25,
from include/linux/bug.h:5,
from include/linux/thread_info.h:12,
from include/asm-generic/preempt.h:5,
from ./arch/sparc/include/generated/asm/preempt.h:1,
from include/linux/preempt.h:78,
from include/linux/spinlock.h:51,
from include/linux/seqlock.h:15,
from include/linux/time.h:6,
from arch/sparc/vdso/vclock_gettime.c:16:
include/linux/seqlock.h: In function 'write_seqcount_begin_nested':
arch/sparc/include/asm/percpu_64.h:19:25: error: '__local_per_cpu_offset' undeclared
Again, reverting this patch fixes the problem.
Guenter
> Reverting it fixes the problem. Is this being addressed ?
>
> Guenter
>
> > Signed-off-by: Ahmed S. Darwish <[email protected]>
> > ---
> > include/linux/seqlock.h | 29 +++++++++++++++++++++++------
> > 1 file changed, 23 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > index e885702d8b82..54bc20496392 100644
> > --- a/include/linux/seqlock.h
> > +++ b/include/linux/seqlock.h
> > @@ -266,6 +266,12 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
> > kcsan_nestable_atomic_end();
> > }
> >
> > +static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
> > +{
> > + raw_write_seqcount_begin(s);
> > + seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
> > +}
> > +
> > /**
> > * write_seqcount_begin_nested() - start a seqcount_t write section with
> > * custom lockdep nesting level
> > @@ -276,8 +282,19 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
> > */
> > static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
> > {
> > - raw_write_seqcount_begin(s);
> > - seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
> > + lockdep_assert_preemption_disabled();
> > + __write_seqcount_begin_nested(s, subclass);
> > +}
> > +
> > +/*
> > + * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
> > + *
> > + * Use for internal seqlock.h code where it's known that preemption is
> > + * already disabled. For example, seqlock_t write side functions.
> > + */
> > +static inline void __write_seqcount_begin(seqcount_t *s)
> > +{
> > + __write_seqcount_begin_nested(s, 0);
> > }
> >
> > /**
> > @@ -575,7 +592,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
> > static inline void write_seqlock(seqlock_t *sl)
> > {
> > spin_lock(&sl->lock);
> > - write_seqcount_begin(&sl->seqcount);
> > + __write_seqcount_begin(&sl->seqcount);
> > }
> >
> > /**
> > @@ -601,7 +618,7 @@ static inline void write_sequnlock(seqlock_t *sl)
> > static inline void write_seqlock_bh(seqlock_t *sl)
> > {
> > spin_lock_bh(&sl->lock);
> > - write_seqcount_begin(&sl->seqcount);
> > + __write_seqcount_begin(&sl->seqcount);
> > }
> >
> > /**
> > @@ -628,7 +645,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
> > static inline void write_seqlock_irq(seqlock_t *sl)
> > {
> > spin_lock_irq(&sl->lock);
> > - write_seqcount_begin(&sl->seqcount);
> > + __write_seqcount_begin(&sl->seqcount);
> > }
> >
> > /**
> > @@ -649,7 +666,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
> > unsigned long flags;
> >
> > spin_lock_irqsave(&sl->lock, flags);
> > - write_seqcount_begin(&sl->seqcount);
> > + __write_seqcount_begin(&sl->seqcount);
> > return flags;
> > }
> >
> > --
> > 2.20.1
> >
On Sat, Aug 08, 2020 at 04:21:22PM -0700, Guenter Roeck wrote:
> On Mon, Jul 20, 2020 at 05:55:14PM +0200, Ahmed S. Darwish wrote:
> > Preemption must be disabled before entering a sequence count write side
> > critical section. Failing to do so, the seqcount read side can preempt
> > the write side section and spin for the entire scheduler tick. If that
> > reader belongs to a real-time scheduling class, it can spin forever and
> > the kernel will livelock.
> >
> > Assert through lockdep that preemption is disabled for seqcount writers.
> >
>
> This patch is causing compile failures for various images (eg arm:allmodconfig,
> arm:imx_v6_v7_defconfig, mips:allmodconfig).
>
> In file included from arch/arm/include/asm/bug.h:60,
> from include/linux/bug.h:5,
> from include/linux/thread_info.h:12,
> from include/asm-generic/current.h:5,
> from ./arch/arm/include/generated/asm/current.h:1,
> from include/linux/sched.h:12,
> from arch/arm/kernel/asm-offsets.c:11:
> include/linux/seqlock.h: In function 'write_seqcount_begin_nested':
> include/asm-generic/percpu.h:31:40: error: implicit declaration of function 'raw_smp_processor_id'
>
> Reverting it fixes the problem. Is this being addressed ?
>
@Peter, I think let's revert this one for now?
Shall I also send you a version of:
[PATCH v4 09/24] seqlock: Extend seqcount API with associated locks
https://lkml.kernel.org/r/[email protected]/
with the problematic "lockdep_assert_preemption_disabled()" removed?
> Guenter
>
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Sun, Aug 09, 2020 at 08:42:51PM +0200, Ahmed S. Darwish wrote:
> On Sat, Aug 08, 2020 at 04:21:22PM -0700, Guenter Roeck wrote:
> > On Mon, Jul 20, 2020 at 05:55:14PM +0200, Ahmed S. Darwish wrote:
> > > Preemption must be disabled before entering a sequence count write side
> > > critical section. Failing to do so, the seqcount read side can preempt
> > > the write side section and spin for the entire scheduler tick. If that
> > > reader belongs to a real-time scheduling class, it can spin forever and
> > > the kernel will livelock.
> > >
> > > Assert through lockdep that preemption is disabled for seqcount writers.
> > >
> >
> > This patch is causing compile failures for various images (eg arm:allmodconfig,
> > arm:imx_v6_v7_defconfig, mips:allmodconfig).
> >
> > In file included from arch/arm/include/asm/bug.h:60,
> > from include/linux/bug.h:5,
> > from include/linux/thread_info.h:12,
> > from include/asm-generic/current.h:5,
> > from ./arch/arm/include/generated/asm/current.h:1,
> > from include/linux/sched.h:12,
> > from arch/arm/kernel/asm-offsets.c:11:
> > include/linux/seqlock.h: In function 'write_seqcount_begin_nested':
> > include/asm-generic/percpu.h:31:40: error: implicit declaration of function 'raw_smp_processor_id'
> >
> > Reverting it fixes the problem. Is this being addressed ?
> >
>
> @Peter, I think let's revert this one for now?
Please do, it's blowing up my local builds as well :(
thanks,
greg k-h
On Mon, Aug 10, 2020 at 10:59:54AM +0200, Greg KH wrote:
> On Sun, Aug 09, 2020 at 08:42:51PM +0200, Ahmed S. Darwish wrote:
> > @Peter, I think let's revert this one for now?
>
> Please do, it's blowing up my local builds as well :(
There's a bunch of patches queued here:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/urgent
That should fix it all, but I'm not exactly sure when the pull request
for that will go out.
This reverts commit 859247d39fb008ea812e8f0c398a58a20c12899e.
Current implementation of lockdep_assert_preemption_disabled() uses
per-CPU variables, which was done to untangle the existing
seqlock.h<=>sched.h 'current->' task_struct circular dependency.
Using per-CPU variables did not fully untangle the dependency for
various non-x86 architectures though, resulting in multiple broken
builds. For the affected architectures, raw_smp_processor_id() led
back to 'current->', thus having the original seqlock.h<=>sched.h
dependency in full-effect.
For now, revert adding lockdep_assert_preemption_disabled() to
seqlock.h.
Reported-by: Guenter Roeck <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
References: Commit a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-cpu variables")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Notes:
My apologies for the mess on this one :-(
include/linux/seqlock.h | 29 ++++++-----------------------
1 file changed, 6 insertions(+), 23 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 54bc20496392..e885702d8b82 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -266,12 +266,6 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
kcsan_nestable_atomic_end();
}
-static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
-{
- raw_write_seqcount_begin(s);
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
-}
-
/**
* write_seqcount_begin_nested() - start a seqcount_t write section with
* custom lockdep nesting level
@@ -282,19 +276,8 @@ static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
*/
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
- lockdep_assert_preemption_disabled();
- __write_seqcount_begin_nested(s, subclass);
-}
-
-/*
- * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
- *
- * Use for internal seqlock.h code where it's known that preemption is
- * already disabled. For example, seqlock_t write side functions.
- */
-static inline void __write_seqcount_begin(seqcount_t *s)
-{
- __write_seqcount_begin_nested(s, 0);
+ raw_write_seqcount_begin(s);
+ seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
/**
@@ -592,7 +575,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_begin(&sl->seqcount);
}
/**
@@ -618,7 +601,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_begin(&sl->seqcount);
}
/**
@@ -645,7 +628,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_begin(&sl->seqcount);
}
/**
@@ -666,7 +649,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- __write_seqcount_begin(&sl->seqcount);
+ write_seqcount_begin(&sl->seqcount);
return flags;
}
--
2.28.0
On Mon, Aug 10, 2020 at 11:48:20AM +0200, [email protected] wrote:
> On Mon, Aug 10, 2020 at 10:59:54AM +0200, Greg KH wrote:
> > On Sun, Aug 09, 2020 at 08:42:51PM +0200, Ahmed S. Darwish wrote:
>
> > > @Peter, I think let's revert this one for now?
> >
> > Please do, it's blowing up my local builds as well :(
>
> There's a bunch of patches queued here:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/urgent
>
> That should fix it all, but I'm not exactly sure when the pull request
> for that will go out.
Before 5.9-rc1 hopefully? :)
On Mon, Aug 10, 2020 at 11:54:28AM +0200, Ahmed S. Darwish wrote:
> This reverts commit 859247d39fb008ea812e8f0c398a58a20c12899e.
>
> Current implementation of lockdep_assert_preemption_disabled() uses
> per-CPU variables, which was done to untangle the existing
> seqlock.h<=>sched.h 'current->' task_struct circular dependency.
>
> Using per-CPU variables did not fully untangle the dependency for
> various non-x86 architectures though, resulting in multiple broken
> builds. For the affected architectures, raw_smp_processor_id() led
> back to 'current->', thus having the original seqlock.h<=>sched.h
> dependency in full-effect.
>
> For now, revert adding lockdep_assert_preemption_disabled() to
> seqlock.h.
>
> Reported-by: Guenter Roeck <[email protected]>
> Link: https://lkml.kernel.org/r/[email protected]
> Link: https://lkml.kernel.org/r/[email protected]
> References: Commit a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-cpu variables")
> Signed-off-by: Ahmed S. Darwish <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Even after this, there are still some build errors on arm32, but I don't
think they are due to this change:
ERROR: modpost: "__aeabi_uldivmod" [drivers/net/ethernet/sfc/sfc.ko] undefined!
ERROR: modpost: "__bad_udelay" [drivers/net/ethernet/aquantia/atlantic/atlantic.ko] undefined!
thanks,
greg k-h
On Mon, Aug 10, 2020 at 12:05:02PM +0200, Greg KH wrote:
> On Mon, Aug 10, 2020 at 11:54:28AM +0200, Ahmed S. Darwish wrote:
> > This reverts commit 859247d39fb008ea812e8f0c398a58a20c12899e.
> >
> > Current implementation of lockdep_assert_preemption_disabled() uses
> > per-CPU variables, which was done to untangle the existing
> > seqlock.h<=>sched.h 'current->' task_struct circular dependency.
> >
> > Using per-CPU variables did not fully untangle the dependency for
> > various non-x86 architectures though, resulting in multiple broken
> > builds. For the affected architectures, raw_smp_processor_id() led
> > back to 'current->', thus having the original seqlock.h<=>sched.h
> > dependency in full-effect.
> >
> > For now, revert adding lockdep_assert_preemption_disabled() to
> > seqlock.h.
> >
> > Reported-by: Guenter Roeck <[email protected]>
> > Link: https://lkml.kernel.org/r/[email protected]
> > Link: https://lkml.kernel.org/r/[email protected]
> > References: Commit a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-cpu variables")
> > Signed-off-by: Ahmed S. Darwish <[email protected]>
>
> Reviewed-by: Greg Kroah-Hartman <[email protected]>
>
> Even after this, there are still some build errors on arm32, but I don't
> think they are due to this change:
>
> ERROR: modpost: "__aeabi_uldivmod" [drivers/net/ethernet/sfc/sfc.ko] undefined!
> ERROR: modpost: "__bad_udelay" [drivers/net/ethernet/aquantia/atlantic/atlantic.ko] undefined!
>
Yes, they are unrelated to the seqlock.h changes.
(I've locally reverted the whole series just to be sure, and the same
linking errors as above were still there for an ARM allyesconfig.)
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On 8/10/20 3:05 AM, Greg KH wrote:
> On Mon, Aug 10, 2020 at 11:54:28AM +0200, Ahmed S. Darwish wrote:
>> This reverts commit 859247d39fb008ea812e8f0c398a58a20c12899e.
>>
>> Current implementation of lockdep_assert_preemption_disabled() uses
>> per-CPU variables, which was done to untangle the existing
>> seqlock.h<=>sched.h 'current->' task_struct circular dependency.
>>
>> Using per-CPU variables did not fully untangle the dependency for
>> various non-x86 architectures though, resulting in multiple broken
>> builds. For the affected architectures, raw_smp_processor_id() led
>> back to 'current->', thus having the original seqlock.h<=>sched.h
>> dependency in full-effect.
>>
>> For now, revert adding lockdep_assert_preemption_disabled() to
>> seqlock.h.
>>
>> Reported-by: Guenter Roeck <[email protected]>
>> Link: https://lkml.kernel.org/r/[email protected]
>> Link: https://lkml.kernel.org/r/[email protected]
>> References: Commit a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-cpu variables")
>> Signed-off-by: Ahmed S. Darwish <[email protected]>
>
> Reviewed-by: Greg Kroah-Hartman <[email protected]>
>
> Even after this, there are still some build errors on arm32, but I don't
> think they are due to this change:
>
> ERROR: modpost: "__aeabi_uldivmod" [drivers/net/ethernet/sfc/sfc.ko] undefined!
This affects every 32 bit architecture.
> ERROR: modpost: "__bad_udelay" [drivers/net/ethernet/aquantia/atlantic/atlantic.ko] undefined!
>
I don't think that is new. If anything, it is surprising that builds don't fail more
widely because of it. AFAICS it was introduced back in 2018 (a hot 50-ms delay loop
really isn't such a good idea).
Guenter
Greg KH <[email protected]> writes:
> On Sun, Aug 09, 2020 at 08:42:51PM +0200, Ahmed S. Darwish wrote:
>> On Sat, Aug 08, 2020 at 04:21:22PM -0700, Guenter Roeck wrote:
>> > Reverting it fixes the problem. Is this being addressed ?
>> >
>>
>> @Peter, I think let's revert this one for now?
>
> Please do, it's blowing up my local builds as well :(
Peter and Ingo sorted the header mess last week and I just sent a pull
request to Linus.
Thanks,
tglx
On Mon, Aug 10, 2020 at 09:55:20PM +0200, Thomas Gleixner wrote:
> Greg KH <[email protected]> writes:
> > On Sun, Aug 09, 2020 at 08:42:51PM +0200, Ahmed S. Darwish wrote:
> >> On Sat, Aug 08, 2020 at 04:21:22PM -0700, Guenter Roeck wrote:
> >> > Reverting it fixes the problem. Is this being addressed ?
> >> >
> >>
> >> @Peter, I think let's revert this one for now?
> >
> > Please do, it's blowing up my local builds as well :(
>
> Peter and Ingo sorted the header mess last week and I just sent a pull
> request to Linus.
Thanks, that looks good and works for my build tests!
greg k-h
On Mon, 10 Aug 2020 07:10:32 -0700, Guenter Roeck said:
> > ERROR: modpost: "__bad_udelay" [drivers/net/ethernet/aquantia/atlantic/atlantic.ko] undefined!
> >
>
> I don't think that is new. If anything, it is surprising that builds don't fail more
> widely because of it. AFAICS it was introduced back in 2018 (a hot 50-ms delay loop
> really isn't such a good idea).
Well...it wasn't broken in next-20200720. A bit of poking with nm,
and building hw_atl/hw_atl_b0.s, it looks like the culprit is this commit:
commit 8dcf2ad39fdb2d183b7bd4307c837713e3150b9a
Author: Mark Starovoytov <[email protected]>
Date: Mon Jul 20 21:32:44 2020 +0300
net: atlantic: add hwmon getter for MAC temperature
specifically this chunk around line 1634 of hw_atl/hw_atl_b0.c:
+ err = readx_poll_timeout_atomic(hw_atl_b0_ts_ready_and_latch_high_get,
+ self, val, val == 1, 10000U, 500000U);
And doing a 'git revert' makes the build work....
On 8/18/20 3:51 PM, Valdis Klētnieks wrote:
> On Mon, 10 Aug 2020 07:10:32 -0700, Guenter Roeck said:
>>> ERROR: modpost: "__bad_udelay" [drivers/net/ethernet/aquantia/atlantic/atlantic.ko] undefined!
>>>
>>
>> I don't think that is new. If anything, it is surprising that builds don't fail more
>> widely because of it. AFAICS it was introduced back in 2018 (a hot 50-ms delay loop
>> really isn't such a good idea).
>
> Well...it wasn't broken in next-20200720. A bit of poking with nm,
> and building hw_atl/hw_atl_b0.s, it looks like the culprit is this commit:
>
> commit 8dcf2ad39fdb2d183b7bd4307c837713e3150b9a
> Author: Mark Starovoytov <[email protected]>
> Date: Mon Jul 20 21:32:44 2020 +0300
>
> net: atlantic: add hwmon getter for MAC temperature
>
> specifically this chunk around line 1634 of hw_atl/hw_atl_b0.c:
>
>
> + err = readx_poll_timeout_atomic(hw_atl_b0_ts_ready_and_latch_high_get,
> + self, val, val == 1, 10000U, 500000U);
>
> And doing a 'git revert' makes the build work....
>
Nice catch. FWIW, there is no obvious reason why this would need to be atomic.
The calling code does not set a lock, meaning there can be two (or more)
callers entering this code. Weird, especially since the code looks like it
would actually need a mutex to work correctly. It might be interesting to
see what happens if there are, say, half a dozen scripts/processes trying
to read the hwmon attribute introduced by this patch at the same time.
Guenter
On 2020-08-18 17:56:49 [-0700], Guenter Roeck wrote:
> Nice catch. FWIW, there is no obvious reason why this would need to be atomic.
> The calling code does not set a lock, meaning there can be two (or more)
> callers entering this code. Weird, especially since the code looks like it
> would actually need a mutex to work correctly. It might be interesting to
> see what happens if there are, say, half a dozen scripts/processes trying
> to read the hwmon attribute introduced by this patch at the same time.
=> https://lkml.kernel.org/r/[email protected]
> Guenter
Sebastian
On Wed, 19 Aug 2020 09:00:22 +0200, Sebastian Andrzej Siewior said:
> On 2020-08-18 17:56:49 [-0700], Guenter Roeck wrote:
> > Nice catch. FWIW, there is no obvious reason why this would need to be atomic.
> > The calling code does not set a lock, meaning there can be two (or more)
> > callers entering this code. Weird, especially since the code looks like it
> > would actually need a mutex to work correctly. It might be interesting to
> > see what happens if there are, say, half a dozen scripts/processes trying
> > to read the hwmon attribute introduced by this patch at the same time.
>
> => https://lkml.kernel.org/r/[email protected]
Looks reasonable to me, though I've not verified that it's preemptible at that
point...
On 8/19/20 12:34 AM, Valdis Klētnieks wrote:
> On Wed, 19 Aug 2020 09:00:22 +0200, Sebastian Andrzej Siewior said:
>> On 2020-08-18 17:56:49 [-0700], Guenter Roeck wrote:
>>> Nice catch. FWIW, there is no obvious reason why this would need to be atomic.
>>> The calling code does not set a lock, meaning there can be two (or more)
>>> callers entering this code. Weird, especially since the code looks like it
>>> would actually need a mutex to work correctly. It might be interesting to
>>> see what happens if there are, say, half a dozen scripts/processes trying
>>> to read the hwmon attribute introduced by this patch at the same time.
>>
>> => https://lkml.kernel.org/r/[email protected]
>
> Looks reasonable to me, though I've not verified that it's preemptible at that
> point...
>
hw_atl_b0_get_mac_temp is called through the .hw_get_mac_temp callback.
This callback is executed from aq_hwmon_read(), which in turn is called
from the hwmon core, more specifically from hwmon_attr_show().
Calls from the hwmon core are unlocked.
Guenter
Latch sequence counters are a multiversion concurrency control mechanism
where the embedded seqcount_t counter even/odd value is used to switch
between two copies of protected data. This allows the sequence counter
read side to be invoked from NMIs and safely interrupt its own write
side critical section.
Initially, latch sequence counters were implemented as a single write
function above plain seqcount_t, raw_write_seqcount_latch(). The read
path was expected to use plain seqcount_t raw_read_seqcount().
A specialized latch read function, raw_read_seqcount_latch(), was later
added. It became the standardized way for latch read paths. Due to the
dependent load, it has one read memory barrier less than the more
generic raw_read_seqcount() API.
Only raw_write_seqcount_latch() and raw_read_seqcount_latch() should be
used with latch sequence counters. Having unique read and write path
APIs means that latch sequence counters are actually a data type of
their own -- just inappropriately overloading plain seqcount_t.
Introduce seqcount_latch_t. This adds type-safety and ensures that only
the correct latch-safe APIs are to be used.
Not to break bisection, let the latch APIs also accept plain seqcount_t
or seqcount_raw_spinlock_t. After converting all call sites to
seqcount_latch_t, (patches #4 => #7), only allow seqcount_latch_t.
Thanks,
8<--------------
Ahmed S. Darwish (8):
time/sched_clock: Use raw_read_seqcount_latch() during suspend
mm/swap: Do not abuse the seqcount_t latching API
seqlock: Introduce seqcount_latch_t
time/sched_clock: Use seqcount_latch_t
timekeeping: Use seqcount_latch_t
x86/tsc: Use seqcount_latch_t
rbtree_latch: Use seqcount_latch_t
seqlock: seqcount latch APIs: Only allow seqcount_latch_t
Documentation/locking/seqlock.rst | 18 ++++++
arch/x86/kernel/tsc.c | 12 ++--
include/linux/rbtree_latch.h | 6 +-
include/linux/seqlock.h | 96 +++++++++++++++++++++----------
kernel/time/sched_clock.c | 6 +-
kernel/time/timekeeping.c | 10 ++--
mm/swap.c | 65 +++++++++++++++++----
7 files changed, 156 insertions(+), 57 deletions(-)
base-commit: d012a7190fc1fd72ed48911e77ca97ba4521bccd
--
2.28.0
Latch sequence counters have unique read and write APIs, and thus
seqcount_latch_t was recently introduced at seqlock.h.
Use that new data type instead of plain seqcount_t. This adds the
necessary type-safety and ensures that only latching-safe seqcount APIs
are to be used.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
arch/x86/kernel/tsc.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 49d925043171..cbf17e9f1d03 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -54,7 +54,7 @@ struct clocksource *art_related_clocksource;
struct cyc2ns {
struct cyc2ns_data data[2]; /* 0 + 2*16 = 32 */
- seqcount_t seq; /* 32 + 4 = 36 */
+ seqcount_latch_t seq; /* 32 + 4 = 36 */
}; /* fits one cacheline */
@@ -68,19 +68,21 @@ early_param("tsc_early_khz", tsc_early_khz_setup);
__always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
{
+ seqcount_latch_t *seqcount;
int seq, idx;
preempt_disable_notrace();
+ seqcount = &this_cpu_ptr(&cyc2ns)->seq;
do {
- seq = this_cpu_read(cyc2ns.seq.sequence);
+ seq = raw_read_seqcount_latch(seqcount);
idx = seq & 1;
data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
- } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
+ } while (read_seqcount_latch_retry(seqcount, seq));
}
__always_inline void cyc2ns_read_end(void)
@@ -186,7 +188,7 @@ static void __init cyc2ns_init_boot_cpu(void)
{
struct cyc2ns *c2n = this_cpu_ptr(&cyc2ns);
- seqcount_init(&c2n->seq);
+ seqcount_latch_init(&c2n->seq);
__set_cyc2ns_scale(tsc_khz, smp_processor_id(), rdtsc());
}
@@ -203,7 +205,7 @@ static void __init cyc2ns_init_secondary_cpus(void)
for_each_possible_cpu(cpu) {
if (cpu != this_cpu) {
- seqcount_init(&c2n->seq);
+ seqcount_latch_init(&c2n->seq);
c2n = per_cpu_ptr(&cyc2ns, cpu);
c2n->data[0] = data[0];
c2n->data[1] = data[1];
--
2.28.0
Latch sequence counters are a multiversion concurrency control mechanism
where the seqcount_t counter even/odd value is used to switch between
two data storage copies. This allows the seqcount_t read path to safely
interrupt its write side critical section (e.g. from NMIs).
Initially, latch sequence counters were implemented as a single write
function, raw_write_seqcount_latch(), above plain seqcount_t. The read
path was expected to use plain seqcount_t raw_read_seqcount().
A specialized read function was later added, raw_read_seqcount_latch(),
and became the standardized way for latch read paths. Having unique read
and write APIs meant that latch sequence counters are basically a data
type of their own -- just inappropriately overloading plain seqcount_t.
The seqcount_latch_t data type was thus introduced at seqlock.h.
Use that new data type instead of seqcount_raw_spinlock_t. This ensures
that only latch-safe APIs are to be used with the sequence counter.
Note that the use of seqcount_raw_spinlock_t was not very useful in the
first place. Only the "raw_" subset of seqcount_t APIs were used at
timekeeping.c. This subset was created for contexts where lockdep cannot
be used. seqcount_LOCKTYPE_t's raison d'être -- verifying that the
seqcount_t writer serialization lock is held -- cannot thus be done.
References: 0c3351d451ae ("seqlock: Use raw_ prefix instead of _no_lockdep")
References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
kernel/time/timekeeping.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4c47f388a83f..999c981ae766 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -64,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_raw_spinlock_t seq;
+ seqcount_latch_t seq;
struct tk_read_base base[2];
};
@@ -81,13 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
- .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
+ .seq = SEQCNT_LATCH_ZERO(tk_fast_mono.seq),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
- .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
+ .seq = SEQCNT_LATCH_ZERO(tk_fast_raw.seq),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -467,7 +467,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
tk_clock_read(tkr),
tkr->cycle_last,
tkr->mask));
- } while (read_seqcount_retry(&tkf->seq, seq));
+ } while (read_seqcount_latch_retry(&tkf->seq, seq));
return now;
}
@@ -533,7 +533,7 @@ static __always_inline u64 __ktime_get_real_fast_ns(struct tk_fast *tkf)
tk_clock_read(tkr),
tkr->cycle_last,
tkr->mask));
- } while (read_seqcount_retry(&tkf->seq, seq));
+ } while (read_seqcount_latch_retry(&tkf->seq, seq));
return now;
}
--
2.28.0
Commit eef1a429f234 ("mm/swap.c: piggyback lru_add_drain_all() calls")
implemented an optimization mechanism to exit the to-be-started LRU
drain operation (name it A) if another drain operation *started and
finished* while (A) was blocked on the LRU draining mutex.
This was done through a seqcount_t latch, which is an abuse of its
semantics:
1. seqcount_t latching should be used for the purpose of switching
between two storage places with sequence protection to allow
interruptible, preemptible, writer sections. The referenced
optimization mechanism has absolutely nothing to do with that.
2. The used raw_write_seqcount_latch() has two SMP write memory
barriers to insure one consistent storage place out of the two
storage places available. A full memory barrier is required
instead: to guarantee that the pagevec counter stores visible by
local CPU are visible to other CPUs -- before loading the current
drain generation.
Beside the seqcount_t API abuse, the semantics of a latch sequence
counter was force-fitted into the referenced optimization. What was
meant is to track "generations" of LRU draining operations, where
"global lru draining generation = x" implies that all generations
0 < n <= x are already *scheduled* for draining -- thus nothing needs
to be done if the current generation number n <= x.
Remove the conceptually-inappropriate seqcount_t latch usage. Manually
implement the referenced optimization using a counter and SMP memory
barriers.
Note, while at it, use the non-atomic variant of cpumask_set_cpu(),
__cpumask_set_cpu(), due to the already existing mutex protection.
Link: https://lkml.kernel.org/r/CALYGNiPSr-cxV9MX9czaVh6Wz_gzSv3H_8KPvgjBTGbJywUJpA@mail.gmail.com
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
mm/swap.c | 65 +++++++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 54 insertions(+), 11 deletions(-)
diff --git a/mm/swap.c b/mm/swap.c
index d16d65d9b4e0..a1ec807e325d 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -763,10 +763,20 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
*/
void lru_add_drain_all(void)
{
- static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
- static DEFINE_MUTEX(lock);
+ /*
+ * lru_drain_gen - Global pages generation number
+ *
+ * (A) Definition: global lru_drain_gen = x implies that all generations
+ * 0 < n <= x are already *scheduled* for draining.
+ *
+ * This is an optimization for the highly-contended use case where a
+ * user space workload keeps constantly generating a flow of pages for
+ * each CPU.
+ */
+ static unsigned int lru_drain_gen;
static struct cpumask has_work;
- int cpu, seq;
+ static DEFINE_MUTEX(lock);
+ unsigned cpu, this_gen;
/*
* Make sure nobody triggers this path before mm_percpu_wq is fully
@@ -775,21 +785,54 @@ void lru_add_drain_all(void)
if (WARN_ON(!mm_percpu_wq))
return;
- seq = raw_read_seqcount_latch(&seqcount);
+ /*
+ * Guarantee pagevec counter stores visible by this CPU are visible to
+ * other CPUs before loading the current drain generation.
+ */
+ smp_mb();
+
+ /*
+ * (B) Locally cache global LRU draining generation number
+ *
+ * The read barrier ensures that the counter is loaded before the mutex
+ * is taken. It pairs with smp_mb() inside the mutex critical section
+ * at (D).
+ */
+ this_gen = smp_load_acquire(&lru_drain_gen);
mutex_lock(&lock);
/*
- * Piggyback on drain started and finished while we waited for lock:
- * all pages pended at the time of our enter were drained from vectors.
+ * (C) Exit the draining operation if a newer generation, from another
+ * lru_add_drain_all(), was already scheduled for draining. Check (A).
*/
- if (__read_seqcount_retry(&seqcount, seq))
+ if (unlikely(this_gen != lru_drain_gen))
goto done;
- raw_write_seqcount_latch(&seqcount);
+ /*
+ * (D) Increment global generation number
+ *
+ * Pairs with smp_load_acquire() at (B), outside of the critical
+ * section. Use a full memory barrier to guarantee that the new global
+ * drain generation number is stored before loading pagevec counters.
+ *
+ * This pairing must be done here, before the for_each_online_cpu loop
+ * below which drains the page vectors.
+ *
+ * Let x, y, and z represent some system CPU numbers, where x < y < z.
+ * Assume CPU #z is is in the middle of the for_each_online_cpu loop
+ * below and has already reached CPU #y's per-cpu data. CPU #x comes
+ * along, adds some pages to its per-cpu vectors, then calls
+ * lru_add_drain_all().
+ *
+ * If the paired barrier is done at any later step, e.g. after the
+ * loop, CPU #x will just exit at (C) and miss flushing out all of its
+ * added pages.
+ */
+ WRITE_ONCE(lru_drain_gen, lru_drain_gen + 1);
+ smp_mb();
cpumask_clear(&has_work);
-
for_each_online_cpu(cpu) {
struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
@@ -801,7 +844,7 @@ void lru_add_drain_all(void)
need_activate_page_drain(cpu)) {
INIT_WORK(work, lru_add_drain_per_cpu);
queue_work_on(cpu, mm_percpu_wq, work);
- cpumask_set_cpu(cpu, &has_work);
+ __cpumask_set_cpu(cpu, &has_work);
}
}
@@ -816,7 +859,7 @@ void lru_add_drain_all(void)
{
lru_add_drain();
}
-#endif
+#endif /* CONFIG_SMP */
/**
* release_pages - batched put_page()
--
2.28.0
Latch sequence counters are a multiversion concurrency control mechanism
where the seqcount_t counter even/odd value is used to switch between
two copies of protected data. This allows the seqcount_t read path to
safely interrupt its write side critical section (e.g. from NMIs).
Initially, latch sequence counters were implemented as a single write
function above plain seqcount_t: raw_write_seqcount_latch(). The read
side was expected to use plain seqcount_t raw_read_seqcount().
A specialized latch read function, raw_read_seqcount_latch(), was later
added. It became the standardized way for latch read paths. Due to the
dependent load, it has one read memory barrier less than the plain
seqcount_t raw_read_seqcount() API.
Only raw_write_seqcount_latch() and raw_read_seqcount_latch() should be
used with latch sequence counters. Having *unique* read and write path
APIs means that latch sequence counters are actually a data type of
their own -- just inappropriately overloading plain seqcount_t.
Introduce seqcount_latch_t. This adds type-safety and ensures that only
the correct latch-safe APIs are to be used.
Not to break bisection, let the latch APIs also accept plain seqcount_t
or seqcount_raw_spinlock_t. After converting all call sites to
seqcount_latch_t, only that new data type will be allowed.
References: 9b0fd802e8c0 ("seqcount: Add raw_write_seqcount_latch()")
References: 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()")
References: aadd6e5caaac ("time/sched_clock: Use raw_read_seqcount_latch()")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
Documentation/locking/seqlock.rst | 18 ++++++
include/linux/seqlock.h | 104 +++++++++++++++++++++---------
2 files changed, 91 insertions(+), 31 deletions(-)
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index 62c5ad98c11c..a334b584f2b3 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -139,6 +139,24 @@ with the associated LOCKTYPE lock acquired.
Read path: same as in :ref:`seqcount_t`.
+
+.. _seqcount_latch_t:
+
+Latch sequence counters (``seqcount_latch_t``)
+----------------------------------------------
+
+Latch sequence counters are a multiversion concurrency control mechanism
+where the embedded seqcount_t counter even/odd value is used to switch
+between two copies of protected data. This allows the sequence counter
+read path to safely interrupt its own write side critical section.
+
+Use seqcount_latch_t when the write side sections cannot be protected
+from interruption by readers. This is typically the case when the read
+side can be invoked from NMI handlers.
+
+Check `raw_write_seqcount_latch()` for more information.
+
+
.. _seqlock_t:
Sequential locks (``seqlock_t``)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 962d9768945f..f83fbf2180db 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -587,34 +587,76 @@ static inline void write_seqcount_t_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
-/**
- * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+/*
+ * Latch sequence counters (seqcount_latch_t)
+ *
+ * A sequence counter variant where the counter even/odd value is used to
+ * switch between two copies of protected data. This allows the read path,
+ * typically NMIs, to safely interrupt the write side critical section.
*
- * Use seqcount_t latching to switch between two storage places protected
- * by a sequence counter. Doing so allows having interruptible, preemptible,
- * seqcount_t write side critical sections.
+ * As the write sections are fully preemptible, no special handling for
+ * PREEMPT_RT is needed.
+ */
+typedef struct {
+ seqcount_t seqcount;
+} seqcount_latch_t;
+
+/**
+ * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
+ * @seq_name: Name of the seqcount_latch_t instance
+ */
+#define SEQCNT_LATCH_ZERO(seq_name) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+}
+
+/**
+ * seqcount_latch_init() - runtime initializer for seqcount_latch_t
+ * @s: Pointer to the seqcount_latch_t instance
+ */
+static inline void seqcount_latch_init(seqcount_latch_t *s)
+{
+ seqcount_init(&s->seqcount);
+}
+
+/**
+ * raw_read_seqcount_latch() - pick even/odd latch data copy
+ * @s: Pointer to seqcount_t, seqcount_raw_spinlock_t, or seqcount_latch_t
*
- * Check raw_write_seqcount_latch() for more details and a full reader and
- * writer usage example.
+ * See raw_write_seqcount_latch() for details and a full reader/writer
+ * usage example.
*
* Return: sequence counter raw value. Use the lowest bit as an index for
- * picking which data copy to read. The full counter value must then be
- * checked with read_seqcount_retry().
+ * picking which data copy to read. The full counter must then be checked
+ * with read_seqcount_latch_retry().
*/
-#define raw_read_seqcount_latch(s) \
- raw_read_seqcount_t_latch(__seqcount_ptr(s))
+#define raw_read_seqcount_latch(s) \
+({ \
+ /* \
+ * Pairs with the first smp_wmb() in raw_write_seqcount_latch(). \
+ * Due to the dependent load, a full smp_rmb() is not needed. \
+ */ \
+ _Generic(*(s), \
+ seqcount_t: READ_ONCE(((seqcount_t *)s)->sequence), \
+ seqcount_raw_spinlock_t: READ_ONCE(((seqcount_raw_spinlock_t *)s)->seqcount.sequence), \
+ seqcount_latch_t: READ_ONCE(((seqcount_latch_t *)s)->seqcount.sequence)); \
+})
-static inline int raw_read_seqcount_t_latch(seqcount_t *s)
+/**
+ * read_seqcount_latch_retry() - end a seqcount_latch_t read section
+ * @s: Pointer to seqcount_latch_t
+ * @start: count, from raw_read_seqcount_latch()
+ *
+ * Return: true if a read section retry is required, else false
+ */
+static inline int
+read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
{
- /* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
- int seq = READ_ONCE(s->sequence); /* ^^^ */
- return seq;
+ return read_seqcount_retry(&s->seqcount, start);
}
/**
- * raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
+ * @s: Pointer to seqcount_t, seqcount_raw_spinlock_t, or seqcount_latch_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -633,7 +675,7 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* The basic form is a data structure like::
*
* struct latch_struct {
- * seqcount_t seq;
+ * seqcount_latch_t seq;
* struct data_struct data[2];
* };
*
@@ -643,13 +685,13 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* void latch_modify(struct latch_struct *latch, ...)
* {
* smp_wmb(); // Ensure that the last data[1] update is visible
- * latch->seq++;
+ * latch->seq.sequence++;
* smp_wmb(); // Ensure that the seqcount update is visible
*
* modify(latch->data[0], ...);
*
* smp_wmb(); // Ensure that the data[0] update is visible
- * latch->seq++;
+ * latch->seq.sequence++;
* smp_wmb(); // Ensure that the seqcount update is visible
*
* modify(latch->data[1], ...);
@@ -668,8 +710,8 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * // read_seqcount_retry() includes needed smp_rmb()
- * } while (read_seqcount_retry(&latch->seq, seq));
+ * // This includes needed smp_rmb()
+ * } while (read_seqcount_latch_retry(&latch->seq, seq));
*
* return entry;
* }
@@ -693,14 +735,14 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
-#define raw_write_seqcount_latch(s) \
- raw_write_seqcount_t_latch(__seqcount_ptr(s))
-
-static inline void raw_write_seqcount_t_latch(seqcount_t *s)
-{
- smp_wmb(); /* prior stores before incrementing "sequence" */
- s->sequence++;
- smp_wmb(); /* increment "sequence" before following stores */
+#define raw_write_seqcount_latch(s) \
+{ \
+ smp_wmb(); /* prior stores before incrementing "sequence" */ \
+ _Generic(*(s), \
+ seqcount_t: ((seqcount_t *)s)->sequence++, \
+ seqcount_raw_spinlock_t:((seqcount_raw_spinlock_t *)s)->seqcount.sequence++, \
+ seqcount_latch_t: ((seqcount_latch_t *)s)->seqcount.sequence++); \
+ smp_wmb(); /* increment "sequence" before following stores */ \
}
/*
--
2.28.0
Latch sequence counters have unique read and write APIs, and thus
seqcount_latch_t was recently introduced at seqlock.h.
Use that new data type instead of plain seqcount_t. This adds the
necessary type-safety and ensures that only latching-safe seqcount APIs
are to be used.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/rbtree_latch.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index 7d012faa509a..3d1a9e716b80 100644
--- a/include/linux/rbtree_latch.h
+++ b/include/linux/rbtree_latch.h
@@ -42,8 +42,8 @@ struct latch_tree_node {
};
struct latch_tree_root {
- seqcount_t seq;
- struct rb_root tree[2];
+ seqcount_latch_t seq;
+ struct rb_root tree[2];
};
/**
@@ -206,7 +206,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
do {
seq = raw_read_seqcount_latch(&root->seq);
node = __lt_find(key, root, seq & 1, ops->comp);
- } while (read_seqcount_retry(&root->seq, seq));
+ } while (read_seqcount_latch_retry(&root->seq, seq));
return node;
}
--
2.28.0
sched_clock uses seqcount_t latching to switch between two storage
places protected by the sequence counter. This allows it to have
interruptible, NMI-safe, seqcount_t write side critical sections.
Since 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()"),
raw_read_seqcount_latch() became the standardized way for seqcount_t
latch read paths. Due to the dependent load, it has one read memory
barrier less than the currently used raw_read_seqcount() API.
Use raw_read_seqcount_latch() for the suspend path.
Commit aadd6e5caaac ("time/sched_clock: Use raw_read_seqcount_latch()")
missed changing that instance of raw_read_seqcount().
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
kernel/time/sched_clock.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 1c03eec6ca9b..8c6b5febd7a0 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -258,7 +258,7 @@ void __init generic_sched_clock_init(void)
*/
static u64 notrace suspended_sched_clock_read(void)
{
- unsigned int seq = raw_read_seqcount(&cd.seq);
+ unsigned int seq = raw_read_seqcount_latch(&cd.seq);
return cd.read_data[seq & 1].epoch_cyc;
}
--
2.28.0
Hi,
Preemption must be disabled before entering a sequence counter write
side critical section. Failing to do so, the read side section can
preempt the write side section and spin for the entire scheduler tick.
If that reader belongs to a real-time scheduling class, it can spin
forever and the kernel will livelock.
Disabling preemption cannot be done for PREEMPT_RT. It can lead to
higher latencies and the write side sections will not be able to acquire
locks which become sleeping locks (e.g. spinlock_t).
To solve this dilemma, do not disable preemption for seqcount_LOCKTYPE_t
writers. Rather, detect if a seqcount_LOCKTYPE_t writer is in progress.
If that is the case, acquire then release the associated LOCKTYPE writer
serialization lock. This will allow any preempted writer to make progress
until the end of its writer serialization lock critical section.
Implement this technique for all of PREEMPT_RT sleeping locks.
Thanks,
8<--------------
Ahmed S. Darwish (5):
seqlock: seqcount_LOCKTYPE_t: Standardize naming convention
seqlock: Use unique prefix for seqcount_t property accessors
seqlock: seqcount_t: Implement all read APIs as statement expressions
seqlock: seqcount_LOCKTYPE_t: Introduce PREEMPT_RT support
seqlock: PREEMPT_RT: Do not starve seqlock_t writers
include/linux/seqlock.h | 277 ++++++++++++++++++++++++----------------
1 file changed, 167 insertions(+), 110 deletions(-)
base-commit: d012a7190fc1fd72ed48911e77ca97ba4521bccd
--
2.28.0
At seqlock.h, sequence counters with associated locks are either called
seqcount_LOCKNAME_t, seqcount_LOCKTYPE_t, or seqcount_locktype_t.
Standardize on "seqcount_LOCKTYPE_t" for all instances in comments,
kernel-doc, and SEQCOUNT_LOCKTYPE() generative macro paramters.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 80 +++++++++++++++++++++--------------------
1 file changed, 41 insertions(+), 39 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 962d9768945f..598af597f74e 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -138,56 +138,58 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
#endif
/**
- * typedef seqcount_LOCKNAME_t - sequence counter with LOCKTYPR associated
+ * typedef seqcount_LOCKTYPE_t - sequence counter with associated lock
* @seqcount: The real sequence counter
* @lock: Pointer to the associated spinlock
*
- * A plain sequence counter with external writer synchronization by a
- * spinlock. The spinlock is associated to the sequence count in the
+ * A plain sequence counter with external writer synchronization by
+ * LOCKTYPE @lock. The lock is associated to the sequence counter in the
* static initializer or init function. This enables lockdep to validate
* that the write side critical section is properly serialized.
+ *
+ * LOCKTYPE: raw_spinlock, spinlock, rwlock, mutex, or ww_mutex.
*/
/**
- * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
- * @s: Pointer to the seqcount_LOCKNAME_t instance
+ * seqcount_LOCKTYPE_init() - runtime initializer for seqcount_LOCKTYPE_t
+ * @s: Pointer to the seqcount_LOCKTYPE_t instance
* @lock: Pointer to the associated LOCKTYPE
*/
/*
- * SEQCOUNT_LOCKTYPE() - Instantiate seqcount_LOCKNAME_t and helpers
- * @locktype: actual typename
- * @lockname: name
+ * SEQCOUNT_LOCKTYPE() - Instantiate seqcount_LOCKTYPE_t and helpers
+ * @locktype: "LOCKTYPE" part of seqcount_LOCKTYPE_t
+ * @locktype_t: canonical/full LOCKTYPE C data type
* @preemptible: preemptibility of above locktype
* @lockmember: argument for lockdep_assert_held()
*/
-#define SEQCOUNT_LOCKTYPE(locktype, lockname, preemptible, lockmember) \
-typedef struct seqcount_##lockname { \
+#define SEQCOUNT_LOCKTYPE(locktype, locktype_t, preemptible, lockmember) \
+typedef struct seqcount_##locktype { \
seqcount_t seqcount; \
- __SEQ_LOCK(locktype *lock); \
-} seqcount_##lockname##_t; \
+ __SEQ_LOCK(locktype_t *lock); \
+} seqcount_##locktype##_t; \
\
static __always_inline void \
-seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock) \
+seqcount_##locktype##_init(seqcount_##locktype##_t *s, locktype_t *lock)\
{ \
seqcount_init(&s->seqcount); \
__SEQ_LOCK(s->lock = lock); \
} \
\
static __always_inline seqcount_t * \
-__seqcount_##lockname##_ptr(seqcount_##lockname##_t *s) \
+__seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
{ \
return &s->seqcount; \
} \
\
static __always_inline bool \
-__seqcount_##lockname##_preemptible(seqcount_##lockname##_t *s) \
+__seqcount_##locktype##_preemptible(seqcount_##locktype##_t *s) \
{ \
return preemptible; \
} \
\
static __always_inline void \
-__seqcount_##lockname##_assert(seqcount_##lockname##_t *s) \
+__seqcount_##locktype##_assert(seqcount_##locktype##_t *s) \
{ \
__SEQ_LOCK(lockdep_assert_held(lockmember)); \
}
@@ -211,15 +213,15 @@ static inline void __seqcount_assert(seqcount_t *s)
lockdep_assert_preemption_disabled();
}
-SEQCOUNT_LOCKTYPE(raw_spinlock_t, raw_spinlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(spinlock_t, spinlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(rwlock_t, rwlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(struct mutex, mutex, true, s->lock)
-SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)
+SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock)
+SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, false, s->lock)
+SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, false, s->lock)
+SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock)
+SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
/**
- * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
- * @name: Name of the seqcount_LOCKNAME_t instance
+ * SEQCNT_LOCKTYPE_ZERO - static initializer for seqcount_LOCKTYPE_t
+ * @name: Name of the seqcount_LOCKTYPE_t instance
* @lock: Pointer to the associated LOCKTYPE
*/
@@ -235,8 +237,8 @@ SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)
#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-#define __seqprop_case(s, lockname, prop) \
- seqcount_##lockname##_t: __seqcount_##lockname##_##prop((void *)(s))
+#define __seqprop_case(s, locktype, prop) \
+ seqcount_##locktype##_t: __seqcount_##locktype##_##prop((void *)(s))
#define __seqprop(s, prop) _Generic(*(s), \
seqcount_t: __seqcount_##prop((void *)(s)), \
@@ -252,7 +254,7 @@ SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)
/**
* __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
@@ -283,7 +285,7 @@ static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
/**
* raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* Return: count to be passed to read_seqcount_retry()
*/
@@ -299,7 +301,7 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
/**
* read_seqcount_begin() - begin a seqcount_t read critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* Return: count to be passed to read_seqcount_retry()
*/
@@ -314,7 +316,7 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
/**
* raw_read_seqcount() - read the raw seqcount_t counter value
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* raw_read_seqcount opens a read critical section of the given
* seqcount_t, without any lockdep checking, and without checking or
@@ -337,7 +339,7 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
/**
* raw_seqcount_begin() - begin a seqcount_t read critical section w/o
* lockdep and w/o counter stabilization
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* raw_seqcount_begin opens a read critical section of the given
* seqcount_t. Unlike read_seqcount_begin(), this function will not wait
@@ -365,7 +367,7 @@ static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
/**
* __read_seqcount_retry() - end a seqcount_t read section w/o barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
* @start: count, from read_seqcount_begin()
*
* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
@@ -389,7 +391,7 @@ static inline int __read_seqcount_t_retry(const seqcount_t *s, unsigned start)
/**
* read_seqcount_retry() - end a seqcount_t read critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
* @start: count, from read_seqcount_begin()
*
* read_seqcount_retry closes the read critical section of given
@@ -409,7 +411,7 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
/**
* raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*/
#define raw_write_seqcount_begin(s) \
do { \
@@ -428,7 +430,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
/**
* raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*/
#define raw_write_seqcount_end(s) \
do { \
@@ -448,7 +450,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
/**
* write_seqcount_begin_nested() - start a seqcount_t write section with
* custom lockdep nesting level
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
* @subclass: lockdep nesting level
*
* See Documentation/locking/lockdep-design.rst
@@ -471,7 +473,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
/**
* write_seqcount_begin() - start a seqcount_t write side critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* write_seqcount_begin opens a write side critical section of the given
* seqcount_t.
@@ -497,7 +499,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
/**
* write_seqcount_end() - end a seqcount_t write side critical section
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* The write section must've been opened with write_seqcount_begin().
*/
@@ -517,7 +519,7 @@ static inline void write_seqcount_t_end(seqcount_t *s)
/**
* raw_write_seqcount_barrier() - do a seqcount_t write barrier
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* This can be used to provide an ordering guarantee instead of the usual
* consistency guarantee. It is one wmb cheaper, because it can collapse
@@ -571,7 +573,7 @@ static inline void raw_write_seqcount_t_barrier(seqcount_t *s)
/**
* write_seqcount_invalidate() - invalidate in-progress seqcount_t read
* side operations
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*
* After write_seqcount_invalidate, no seqcount_t read side operations
* will complete successfully and see data older than this.
--
2.28.0
At seqlock.h, the following set of functions:
- __seqcount_ptr()
- __seqcount_preemptible()
- __seqcount_assert()
act as plain seqcount_t "property" accessors. Meanwhile, the following
group:
- __seqcount_ptr()
- __seqcount_lock_preemptible()
- __seqcount_assert_lock_held()
act as the equivalent set, but in the generic form, taking either
seqcount_t or any of the seqcount_LOCKTYPE_t variants.
This is quite confusing, especially the first member where it is called
exactly the same in both groups.
Differentiate the first group by using "__seqcount_t_" as prefix. This
also conforms with the rest of seqlock.h naming conventions.
While at it, also constify the property accessors first parameter where
appropriate.
References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 598af597f74e..5470e9cd52ce 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -183,13 +183,13 @@ __seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
} \
\
static __always_inline bool \
-__seqcount_##locktype##_preemptible(seqcount_##locktype##_t *s) \
+__seqcount_##locktype##_preemptible(const seqcount_##locktype##_t *s) \
{ \
return preemptible; \
} \
\
static __always_inline void \
-__seqcount_##locktype##_assert(seqcount_##locktype##_t *s) \
+__seqcount_##locktype##_assert(const seqcount_##locktype##_t *s) \
{ \
__SEQ_LOCK(lockdep_assert_held(lockmember)); \
}
@@ -198,17 +198,17 @@ __seqcount_##locktype##_assert(seqcount_##locktype##_t *s) \
* __seqprop() for seqcount_t
*/
-static inline seqcount_t *__seqcount_ptr(seqcount_t *s)
+static inline seqcount_t *__seqcount_t_ptr(seqcount_t *s)
{
return s;
}
-static inline bool __seqcount_preemptible(seqcount_t *s)
+static inline bool __seqcount_t_preemptible(const seqcount_t *s)
{
return false;
}
-static inline void __seqcount_assert(seqcount_t *s)
+static inline void __seqcount_t_assert(const seqcount_t *s)
{
lockdep_assert_preemption_disabled();
}
@@ -236,12 +236,11 @@ SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKTYPE_ZERO(name, lock)
#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKTYPE_ZERO(name, lock)
-
#define __seqprop_case(s, locktype, prop) \
seqcount_##locktype##_t: __seqcount_##locktype##_##prop((void *)(s))
#define __seqprop(s, prop) _Generic(*(s), \
- seqcount_t: __seqcount_##prop((void *)(s)), \
+ seqcount_t: __seqcount_t_##prop((void *)(s)), \
__seqprop_case((s), raw_spinlock, prop), \
__seqprop_case((s), spinlock, prop), \
__seqprop_case((s), rwlock, prop), \
--
2.28.0
The sequence counters read APIs are implemented as CPP macros, so they
can take either seqcount_t or any of the seqcount_LOCKTYPE_t variants.
Such macros then get *directly* transformed to internal C functions that
only take plain seqcount_t.
Further commits need access to seqcount_LOCKTYPE_t inside of the actual
read APIs code. Thus transform all of the seqcount read APIs to pure GCC
statement expressions instead.
This will not break type-safety: all of the transformed APIs resolve to
a _Generic() selection that does not have a "default" case.
This will also not affect the transformed APIs readability: previously
added kernel-doc above all of seqlock.h functions makes the expectations
quite clear for call-site developers.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 98 ++++++++++++++++++++---------------------
1 file changed, 49 insertions(+), 49 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5470e9cd52ce..d114a9f4e9d9 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -182,6 +182,12 @@ __seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
return &s->seqcount; \
} \
\
+static __always_inline unsigned \
+__seqcount_##locktype##_sequence(const seqcount_##locktype##_t *s) \
+{ \
+ return READ_ONCE(s->seqcount.sequence); \
+} \
+ \
static __always_inline bool \
__seqcount_##locktype##_preemptible(const seqcount_##locktype##_t *s) \
{ \
@@ -203,6 +209,11 @@ static inline seqcount_t *__seqcount_t_ptr(seqcount_t *s)
return s;
}
+static inline unsigned __seqcount_t_sequence(const seqcount_t *s)
+{
+ return READ_ONCE(s->sequence);
+}
+
static inline bool __seqcount_t_preemptible(const seqcount_t *s)
{
return false;
@@ -248,6 +259,7 @@ SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
__seqprop_case((s), ww_mutex, prop))
#define __seqcount_ptr(s) __seqprop(s, ptr)
+#define __seqcount_sequence(s) __seqprop(s, sequence)
#define __seqcount_lock_preemptible(s) __seqprop(s, preemptible)
#define __seqcount_assert_lock_held(s) __seqprop(s, assert)
@@ -266,21 +278,19 @@ SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
* Return: count to be passed to read_seqcount_retry()
*/
#define __read_seqcount_begin(s) \
- __read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
-{
- unsigned ret;
-
-repeat:
- ret = READ_ONCE(s->sequence);
- if (unlikely(ret & 1)) {
- cpu_relax();
- goto repeat;
- }
- kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
- return ret;
-}
+({ \
+ unsigned seq; \
+ \
+ do { \
+ seq = __seqcount_sequence(s); \
+ if (likely(! (seq & 1))) \
+ break; \
+ cpu_relax(); \
+ } while (true); \
+ \
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
+ seq; \
+})
/**
* raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
@@ -289,14 +299,12 @@ static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
* Return: count to be passed to read_seqcount_retry()
*/
#define raw_read_seqcount_begin(s) \
- raw_read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
-{
- unsigned ret = __read_seqcount_t_begin(s);
- smp_rmb();
- return ret;
-}
+({ \
+ unsigned seq = __read_seqcount_begin(s); \
+ \
+ smp_rmb(); \
+ seq; \
+})
/**
* read_seqcount_begin() - begin a seqcount_t read critical section
@@ -305,13 +313,10 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
* Return: count to be passed to read_seqcount_retry()
*/
#define read_seqcount_begin(s) \
- read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
-{
- seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_t_begin(s);
-}
+({ \
+ seqcount_lockdep_reader_access(__seqcount_ptr(s)); \
+ raw_read_seqcount_begin(s); \
+})
/**
* raw_read_seqcount() - read the raw seqcount_t counter value
@@ -325,15 +330,13 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
* Return: count to be passed to read_seqcount_retry()
*/
#define raw_read_seqcount(s) \
- raw_read_seqcount_t(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
-{
- unsigned ret = READ_ONCE(s->sequence);
- smp_rmb();
- kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
- return ret;
-}
+({ \
+ unsigned seq = __seqcount_sequence(s); \
+ \
+ smp_rmb(); \
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
+ seq; \
+})
/**
* raw_seqcount_begin() - begin a seqcount_t read critical section w/o
@@ -353,16 +356,13 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
* Return: count to be passed to read_seqcount_retry()
*/
#define raw_seqcount_begin(s) \
- raw_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
-{
- /*
- * If the counter is odd, let read_seqcount_retry() fail
- * by decrementing the counter.
- */
- return raw_read_seqcount_t(s) & ~1;
-}
+({ \
+ /* \
+ * If the counter is odd, let read_seqcount_retry() fail \
+ * by decrementing the counter. \
+ */ \
+ raw_read_seqcount(s) & ~1; \
+})
/**
* __read_seqcount_retry() - end a seqcount_t read section w/o barrier
--
2.28.0
Preemption must be disabled before entering a sequence counter write
side critical section. Failing to do so, the read side section can
preempt the write side section and spin for the entire scheduler tick.
If that reader belongs to a real-time scheduling class, it can spin
forever and the kernel will livelock.
Disabling preemption cannot be done for PREEMPT_RT. It can lead to
higher latencies and the write side sections will not be able to acquire
locks which become sleeping locks (e.g. spinlock_t).
To solve this dilemma, do not disable preemption for seqcount_LOCKTYPE_t
writers. Rather, detect if a seqcount_LOCKTYPE_t writer is in progress.
If that is the case, acquire then release the associated LOCKTYPE writer
serialization lock. This will allow any preempted writer to make progress
until the end of its writer serialization lock critical section.
Implement this technique for all of PREEMPT_RT sleeping locks.
Link: https://lkml.kernel.org/r/159708609435.2571.13948681727529247231.tglx@nanos
Link: https://lkml.kernel.org/r/[email protected]
References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 72 +++++++++++++++++++++++++++++++++--------
1 file changed, 59 insertions(+), 13 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d114a9f4e9d9..8d4bf12272ba 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -17,6 +17,7 @@
#include <linux/kcsan-checks.h>
#include <linux/lockdep.h>
#include <linux/mutex.h>
+#include <linux/ww_mutex.h>
#include <linux/preempt.h>
#include <linux/spinlock.h>
@@ -131,7 +132,23 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* See Documentation/locking/seqlock.rst
*/
-#ifdef CONFIG_LOCKDEP
+/*
+ * For PREEMPT_RT, seqcount_LOCKTYPE_t write side critical sections cannot
+ * disable preemption. It can lead to higher latencies, and the write side
+ * sections will not be able to acquire locks which become sleeping locks
+ * (e.g. spinlock_t).
+ *
+ * To remain preemptible while avoiding a possible livelock caused by the
+ * read side preempting the write side, use a different technique: detect
+ * if a seqcount_LOCKTYPE_t writer is in progress. If that is the case,
+ * acquire then release the associated LOCKTYPE writer serialization
+ * lock. This will allow any preempted writer to make progress until the
+ * end of its writer serialization lock critical section.
+ *
+ * This lock-unlock technique must be implemented for all of PREEMPT_RT
+ * sleeping locks. See Documentation/locking/locktypes.rst
+ */
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
#define __SEQ_LOCK(expr) expr
#else
#define __SEQ_LOCK(expr)
@@ -162,8 +179,10 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* @locktype_t: canonical/full LOCKTYPE C data type
* @preemptible: preemptibility of above locktype
* @lockmember: argument for lockdep_assert_held()
+ * @lockbase: associated lock release function (prefix only)
+ * @lock_acquire: associated lock acquisition function (full call)
*/
-#define SEQCOUNT_LOCKTYPE(locktype, locktype_t, preemptible, lockmember) \
+#define SEQCOUNT_LOCKTYPE(locktype, locktype_t, preemptible, lockmember, lockbase, lock_acquire) \
typedef struct seqcount_##locktype { \
seqcount_t seqcount; \
__SEQ_LOCK(locktype_t *lock); \
@@ -185,7 +204,23 @@ __seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
static __always_inline unsigned \
__seqcount_##locktype##_sequence(const seqcount_##locktype##_t *s) \
{ \
- return READ_ONCE(s->seqcount.sequence); \
+ unsigned seq = READ_ONCE(s->seqcount.sequence); \
+ \
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
+ return seq; \
+ \
+ if (preemptible && unlikely(seq & 1)) { \
+ __SEQ_LOCK(lock_acquire); \
+ __SEQ_LOCK(lockbase##_unlock(s->lock)); \
+ \
+ /* \
+ * Re-read the sequence counter since the (possibly \
+ * preempted) writer made progress. \
+ */ \
+ seq = READ_ONCE(s->seqcount.sequence); \
+ } \
+ \
+ return seq; \
} \
\
static __always_inline bool \
@@ -224,11 +259,13 @@ static inline void __seqcount_t_assert(const seqcount_t *s)
lockdep_assert_preemption_disabled();
}
-SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock)
-SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, false, s->lock)
-SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, false, s->lock)
-SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock)
-SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
+#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
+SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
+SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
+SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
+SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
/**
* SEQCNT_LOCKTYPE_ZERO - static initializer for seqcount_LOCKTYPE_t
@@ -408,13 +445,22 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_t_retry(s, start);
}
+/*
+ * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
+ * associated lock does not implicitly disable preemption.
+ *
+ * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
+ */
+#define __seq_enforce_preemption_protection(s) \
+ (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
+
/**
* raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
* @s: Pointer to seqcount_t or any of the seqcount_LOCKTYPE_t variants
*/
#define raw_write_seqcount_begin(s) \
do { \
- if (__seqcount_lock_preemptible(s)) \
+ if (__seq_enforce_preemption_protection(s)) \
preempt_disable(); \
\
raw_write_seqcount_t_begin(__seqcount_ptr(s)); \
@@ -435,7 +481,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s)
do { \
raw_write_seqcount_t_end(__seqcount_ptr(s)); \
\
- if (__seqcount_lock_preemptible(s)) \
+ if (__seq_enforce_preemption_protection(s)) \
preempt_enable(); \
} while (0)
@@ -458,7 +504,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s)
do { \
__seqcount_assert_lock_held(s); \
\
- if (__seqcount_lock_preemptible(s)) \
+ if (__seq_enforce_preemption_protection(s)) \
preempt_disable(); \
\
write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass); \
@@ -485,7 +531,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
do { \
__seqcount_assert_lock_held(s); \
\
- if (__seqcount_lock_preemptible(s)) \
+ if (__seq_enforce_preemption_protection(s)) \
preempt_disable(); \
\
write_seqcount_t_begin(__seqcount_ptr(s)); \
@@ -506,7 +552,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s)
do { \
write_seqcount_t_end(__seqcount_ptr(s)); \
\
- if (__seqcount_lock_preemptible(s)) \
+ if (__seq_enforce_preemption_protection(s)) \
preempt_enable(); \
} while (0)
--
2.28.0
On PREEMPT_RT, seqlock_t is transformed to a sleeping lock that do not
disable preemption. A seqlock_t reader can thus preempt the write side
section and spin for the enter scheduler tick. If that reader belongs to
a real-time scheduling class, it can spin forever and the kernel will
livelock.
To break such a possible livelock on PREEMPT_RT, implement seqlock_t in
terms of "seqcount_spinlock_t" instead of plain "seqcount_t".
Beside the pure annotational value, this will leverage the already
existing seqcount_LOCKTYPE_T anti-livelock mechanisms -- without adding
any extra code.
Signed-off-by: Ahmed S. Darwish <[email protected]>
---
include/linux/seqlock.h | 32 +++++++++++++++++++++-----------
1 file changed, 21 insertions(+), 11 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 8d4bf12272ba..151e7a18fd7b 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -761,13 +761,17 @@ static inline void raw_write_seqcount_t_latch(seqcount_t *s)
* - Documentation/locking/seqlock.rst
*/
typedef struct {
- struct seqcount seqcount;
+ /*
+ * Make sure that readers don't starve writers on PREEMPT_RT: use
+ * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
+ */
+ seqcount_spinlock_t seqcount;
spinlock_t lock;
} seqlock_t;
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
- .seqcount = SEQCNT_ZERO(lockname), \
+ .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
@@ -777,8 +781,8 @@ typedef struct {
*/
#define seqlock_init(sl) \
do { \
- seqcount_init(&(sl)->seqcount); \
spin_lock_init(&(sl)->lock); \
+ seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock); \
} while (0)
/**
@@ -825,6 +829,12 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
return read_seqcount_retry(&sl->seqcount, start);
}
+/*
+ * For all seqlock_t write side functions, use write_seqcount_*t*_begin()
+ * instead of the generic write_seqcount_begin(). This way, no redundant
+ * lockdep_assert_held() checks are added.
+ */
+
/**
* write_seqlock() - start a seqlock_t write side critical section
* @sl: Pointer to seqlock_t
@@ -841,7 +851,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
static inline void write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
- write_seqcount_t_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount.seqcount);
}
/**
@@ -853,7 +863,7 @@ static inline void write_seqlock(seqlock_t *sl)
*/
static inline void write_sequnlock(seqlock_t *sl)
{
- write_seqcount_t_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount.seqcount);
spin_unlock(&sl->lock);
}
@@ -867,7 +877,7 @@ static inline void write_sequnlock(seqlock_t *sl)
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
- write_seqcount_t_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount.seqcount);
}
/**
@@ -880,7 +890,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
*/
static inline void write_sequnlock_bh(seqlock_t *sl)
{
- write_seqcount_t_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount.seqcount);
spin_unlock_bh(&sl->lock);
}
@@ -894,7 +904,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
- write_seqcount_t_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount.seqcount);
}
/**
@@ -906,7 +916,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
*/
static inline void write_sequnlock_irq(seqlock_t *sl)
{
- write_seqcount_t_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount.seqcount);
spin_unlock_irq(&sl->lock);
}
@@ -915,7 +925,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- write_seqcount_t_begin(&sl->seqcount);
+ write_seqcount_t_begin(&sl->seqcount.seqcount);
return flags;
}
@@ -944,7 +954,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
- write_seqcount_t_end(&sl->seqcount);
+ write_seqcount_t_end(&sl->seqcount.seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
--
2.28.0
On Fri, Aug 28, 2020 at 03:07:06AM +0200, Ahmed S. Darwish wrote:
> At seqlock.h, sequence counters with associated locks are either called
> seqcount_LOCKNAME_t, seqcount_LOCKTYPE_t, or seqcount_locktype_t.
>
> Standardize on "seqcount_LOCKTYPE_t" for all instances in comments,
> kernel-doc, and SEQCOUNT_LOCKTYPE() generative macro paramters.
> +#define SEQCOUNT_LOCKTYPE(locktype, locktype_t, preemptible, lockmember) \
> +typedef struct seqcount_##locktype { \
> + __SEQ_LOCK(locktype_t *lock); \
> +} seqcount_##locktype##_t; \
Hurmph, so my thinking was that the above 'locktype' is not actually a
type and therefore a misnomer.
But I see your point about it being a bit of a mess.
Would:
s/LOCKTYPE/LOCKNAME/g
s/seqcount_locktype_t/seqcount_LOCKNAME_t/g
help? Then we're consistently at: seqcount_LOCKNAME_t, which is a type.
Hi :)
On Fri, Aug 28, 2020 at 10:18:44AM +0200, [email protected] wrote:
> On Fri, Aug 28, 2020 at 03:07:06AM +0200, Ahmed S. Darwish wrote:
> > At seqlock.h, sequence counters with associated locks are either called
> > seqcount_LOCKNAME_t, seqcount_LOCKTYPE_t, or seqcount_locktype_t.
> >
> > Standardize on "seqcount_LOCKTYPE_t" for all instances in comments,
> > kernel-doc, and SEQCOUNT_LOCKTYPE() generative macro paramters.
>
> > +#define SEQCOUNT_LOCKTYPE(locktype, locktype_t, preemptible, lockmember) \
> > +typedef struct seqcount_##locktype { \
> > + __SEQ_LOCK(locktype_t *lock); \
> > +} seqcount_##locktype##_t; \
>
> Hurmph, so my thinking was that the above 'locktype' is not actually a
> type and therefore a misnomer.
>
> But I see your point about it being a bit of a mess.
>
> Would:
>
> s/LOCKTYPE/LOCKNAME/g
> s/seqcount_locktype_t/seqcount_LOCKNAME_t/g
>
> help? Then we're consistently at: seqcount_LOCKNAME_t, which is a type.
>
Sounds good, will do.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Fri, Aug 28, 2020 at 03:07:07AM +0200, Ahmed S. Darwish wrote:
> Differentiate the first group by using "__seqcount_t_" as prefix. This
> also conforms with the rest of seqlock.h naming conventions.
> #define __seqprop_case(s, locktype, prop) \
> seqcount_##locktype##_t: __seqcount_##locktype##_##prop((void *)(s))
>
> #define __seqprop(s, prop) _Generic(*(s), \
> - seqcount_t: __seqcount_##prop((void *)(s)), \
> + seqcount_t: __seqcount_t_##prop((void *)(s)), \
> __seqprop_case((s), raw_spinlock, prop), \
> __seqprop_case((s), spinlock, prop), \
> __seqprop_case((s), rwlock, prop), \
If instead you do:
#define __seqprop_case(s, _lockname, prop) \
seqcount##_lockname##_t: __seqcount##_lockname##_##prop((void *)(s))
You can have:
__seqprop_case((s), , prop),
__seqprop_case((s), _raw_spinlock, prop),
__seqprop_case((s), _spinlock, prop),
__seqprop_case((s), _rwlock, prop),
__seqprop_case((s), _mutex, prop),
__seqprop_case((s), _ww_mutex, prop),
And it's all good again.
Although arguably we should do something like s/__seqcount/__seqprop/
over this lot.
On Fri, Aug 28, 2020 at 03:07:08AM +0200, Ahmed S. Darwish wrote:
> #define __read_seqcount_begin(s) \
> +({ \
> + unsigned seq; \
> + \
> + do { \
> + seq = __seqcount_sequence(s); \
> + if (likely(! (seq & 1))) \
> + break; \
> + cpu_relax(); \
> + } while (true); \
> + \
> + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
> + seq; \
> +})
Since we're there anyway, does it make sense to (re)write this like:
while ((seq = __seqcount_sequence(s)) & 1)
cpu_relax();
?
On Fri, Aug 28, 2020 at 10:30:22AM +0200, [email protected] wrote:
> On Fri, Aug 28, 2020 at 03:07:08AM +0200, Ahmed S. Darwish wrote:
> > #define __read_seqcount_begin(s) \
> > +({ \
> > + unsigned seq; \
> > + \
> > + do { \
> > + seq = __seqcount_sequence(s); \
> > + if (likely(! (seq & 1))) \
> > + break; \
> > + cpu_relax(); \
> > + } while (true); \
> > + \
> > + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
> > + seq; \
> > +})
>
> Since we're there anyway, does it make sense to (re)write this like:
>
> while ((seq = __seqcount_sequence(s)) & 1)
> cpu_relax();
>
> ?
>
Yeah, much better of course; will do.
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Fri, Aug 28, 2020 at 10:27:54AM +0200, [email protected] wrote:
> On Fri, Aug 28, 2020 at 03:07:07AM +0200, Ahmed S. Darwish wrote:
> > Differentiate the first group by using "__seqcount_t_" as prefix. This
> > also conforms with the rest of seqlock.h naming conventions.
>
> > #define __seqprop_case(s, locktype, prop) \
> > seqcount_##locktype##_t: __seqcount_##locktype##_##prop((void *)(s))
> >
> > #define __seqprop(s, prop) _Generic(*(s), \
> > - seqcount_t: __seqcount_##prop((void *)(s)), \
> > + seqcount_t: __seqcount_t_##prop((void *)(s)), \
> > __seqprop_case((s), raw_spinlock, prop), \
> > __seqprop_case((s), spinlock, prop), \
> > __seqprop_case((s), rwlock, prop), \
>
> If instead you do:
>
> #define __seqprop_case(s, _lockname, prop) \
> seqcount##_lockname##_t: __seqcount##_lockname##_##prop((void *)(s))
>
> You can have:
>
> __seqprop_case((s), , prop),
> __seqprop_case((s), _raw_spinlock, prop),
> __seqprop_case((s), _spinlock, prop),
> __seqprop_case((s), _rwlock, prop),
> __seqprop_case((s), _mutex, prop),
> __seqprop_case((s), _ww_mutex, prop),
>
> And it's all good again.
>
> Although arguably we should do something like s/__seqcount/__seqprop/
> over this lot.
>
ACK.
On Fri, Aug 28, 2020 at 03:07:09AM +0200, Ahmed S. Darwish wrote:
> +#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
> +
> +SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
> +SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
> +SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
> +SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
> +SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
Ooh, reading is hard, but ^^^^ you already have that.
> +/*
> + * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
> + * associated lock does not implicitly disable preemption.
> + *
> + * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
> + */
> +#define __seq_enforce_preemption_protection(s) \
> + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
Then what is this doing ?!? I'm confused now.
On Fri, Aug 28, 2020 at 03:07:09AM +0200, Ahmed S. Darwish wrote:
> +/*
> + * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
> + * associated lock does not implicitly disable preemption.
> + *
> + * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
> + */
> +#define __seq_enforce_preemption_protection(s) \
> + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
Hurph, so basically you want to make __seqcount_lock_preemptible()
return true PREEMPT_RT ? Should we then not muck about with the propery
instead of this?
ISTR I had something like the below, would that not be the same but much
clearer ?
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 300cbf312546..3b5ad026ddfb 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -211,11 +211,13 @@ static inline void __seqcount_assert(seqcount_t *s)
lockdep_assert_preemption_disabled();
}
-SEQCOUNT_LOCKTYPE(raw_spinlock_t, raw_spinlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(spinlock_t, spinlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(rwlock_t, rwlock, false, s->lock)
-SEQCOUNT_LOCKTYPE(struct mutex, mutex, true, s->lock)
-SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)
+#define __PREEMPT_RT IS_BUILTIN(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKTYPE(raw_spinlock_t, raw_spinlock, false, s->lock)
+SEQCOUNT_LOCKTYPE(spinlock_t, spinlock, __PREEMPT_RT, s->lock)
+SEQCOUNT_LOCKTYPE(rwlock_t, rwlock, __PREEMPT_RT, s->lock)
+SEQCOUNT_LOCKTYPE(struct mutex, mutex, true, s->lock)
+SEQCOUNT_LOCKTYPE(struct ww_mutex, ww_mutex, true, &s->lock->base)
/*
* SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
On Fri, Aug 28, 2020 at 10:59:38AM +0200, [email protected] wrote:
> On Fri, Aug 28, 2020 at 03:07:09AM +0200, Ahmed S. Darwish wrote:
> > +#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
> > +
> > +SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
> > +SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
> > +SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
> > +SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
> > +SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
>
> Ooh, reading is hard, but ^^^^ you already have that.
>
Haha, I was just answering the other mail :)
> > +/*
> > + * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
> > + * associated lock does not implicitly disable preemption.
> > + *
> > + * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
> > + */
> > +#define __seq_enforce_preemption_protection(s) \
> > + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
>
> Then what is this doing ?!? I'm confused now.
There were a number of call sites (at DRM mainly) that protected their
seqcount_t write sections with mutex and ww_mutex. So, they manually
disabled preemption before entering the seqcount_t write sections.
Unfortunately these write sections were big, and spinlocks were also
acquired inside them. This was all very bad for RT...
So the idea was to relieve call sites from the responsibility of
disabling/enabling preemption upon entering a seqcount_LOCKNAME_t write
section, and let seqlock.h automatically handle it if LOCKNAME_t is
preemptible (the macro's condition, second part).
Having seqlock.h control the preempt disable/enable allows us to never
disable preemption for RT, which is exactly what is needed since we
already handle any possible livelock through the mechanisms described at
__SEQ_LOCK (the macro's condition test, first part).
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Fri, Aug 28, 2020 at 11:31:32AM +0200, Ahmed S. Darwish wrote:
> On Fri, Aug 28, 2020 at 10:59:38AM +0200, [email protected] wrote:
> > On Fri, Aug 28, 2020 at 03:07:09AM +0200, Ahmed S. Darwish wrote:
> > > +#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
> > > +
> > > +SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
> >
> > Ooh, reading is hard, but ^^^^ you already have that.
> >
>
> Haha, I was just answering the other mail :)
>
> > > +/*
> > > + * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
> > > + * associated lock does not implicitly disable preemption.
> > > + *
> > > + * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
> > > + */
> > > +#define __seq_enforce_preemption_protection(s) \
> > > + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
> >
> > Then what is this doing ?!? I'm confused now.
>
> There were a number of call sites (at DRM mainly) that protected their
> seqcount_t write sections with mutex and ww_mutex. So, they manually
> disabled preemption before entering the seqcount_t write sections.
>
> Unfortunately these write sections were big, and spinlocks were also
> acquired inside them. This was all very bad for RT...
>
> So the idea was to relieve call sites from the responsibility of
> disabling/enabling preemption upon entering a seqcount_LOCKNAME_t write
> section, and let seqlock.h automatically handle it if LOCKNAME_t is
> preemptible (the macro's condition, second part).
>
> Having seqlock.h control the preempt disable/enable allows us to never
> disable preemption for RT, which is exactly what is needed since we
> already handle any possible livelock through the mechanisms described at
> __SEQ_LOCK (the macro's condition test, first part).
>
So to summarize, __seqcount_lock_preemptible() has two use cases:
1. For !PREEMPT_RT, to automatically disable preemption on the write
side when the seqcount associated lock is preemptible.
2. For PREEMPT_RT, to lock/unlock the seqcount associated lock on
the read side if it is RT-preemptible (sleeping lock).
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
FWIW, can you please start a new thread with ever posting? I lost this
one because it got stuck onto some ancient thread.
On Fri, Sep 04, 2020 at 08:52:45AM +0200, [email protected] wrote:
>
> FWIW, can you please start a new thread with ever posting? I lost this
> one because it got stuck onto some ancient thread.
>
Yeah, I used an old send-mail script, sorry.
On Thu, Aug 27, 2020 at 01:40:42PM +0200, Ahmed S. Darwish wrote:
> __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
> {
> + seqcount_latch_t *seqcount;
> int seq, idx;
>
> preempt_disable_notrace();
>
> + seqcount = &this_cpu_ptr(&cyc2ns)->seq;
> do {
> - seq = this_cpu_read(cyc2ns.seq.sequence);
> + seq = raw_read_seqcount_latch(seqcount);
> idx = seq & 1;
>
> data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
>
> - } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
> + } while (read_seqcount_latch_retry(seqcount, seq));
> }
So I worried about this change, it obviously generates worse code. But I
was not expecting this:
Before:
196: 0000000000000110 189 FUNC GLOBAL DEFAULT 1 native_sched_clock
After:
195: 0000000000000110 399 FUNC GLOBAL DEFAULT 1 native_sched_clock
That's _210_ bytes extra!!
If you look at the disassembly of the thing after it's a complete
trainwreck.
On Fri, Sep 04, 2020 at 09:41:42AM +0200, [email protected] wrote:
> On Thu, Aug 27, 2020 at 01:40:42PM +0200, Ahmed S. Darwish wrote:
>
> > __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
> > {
> > + seqcount_latch_t *seqcount;
> > int seq, idx;
> >
> > preempt_disable_notrace();
> >
> > + seqcount = &this_cpu_ptr(&cyc2ns)->seq;
> > do {
> > - seq = this_cpu_read(cyc2ns.seq.sequence);
> > + seq = raw_read_seqcount_latch(seqcount);
> > idx = seq & 1;
> >
> > data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> > data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> > data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
> >
> > - } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
> > + } while (read_seqcount_latch_retry(seqcount, seq));
> > }
>
> So I worried about this change, it obviously generates worse code. But I
> was not expecting this:
>
> Before:
>
> 196: 0000000000000110 189 FUNC GLOBAL DEFAULT 1 native_sched_clock
>
> After:
>
> 195: 0000000000000110 399 FUNC GLOBAL DEFAULT 1 native_sched_clock
>
> That's _210_ bytes extra!!
>
> If you look at the disassembly of the thing after it's a complete
> trainwreck.
The below delta fixes it again.
---
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -68,21 +68,19 @@ early_param("tsc_early_khz", tsc_early_k
__always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
{
- seqcount_latch_t *seqcount;
int seq, idx;
preempt_disable_notrace();
- seqcount = &this_cpu_ptr(&cyc2ns)->seq;
do {
- seq = raw_read_seqcount_latch(seqcount);
+ seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
idx = seq & 1;
data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
- } while (read_seqcount_latch_retry(seqcount, seq));
+ } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
}
__always_inline void cyc2ns_read_end(void)
Hi,
On Fri, Sep 04, 2020 at 10:03:12AM +0200, [email protected] wrote:
> On Fri, Sep 04, 2020 at 09:41:42AM +0200, [email protected] wrote:
> > On Thu, Aug 27, 2020 at 01:40:42PM +0200, Ahmed S. Darwish wrote:
> >
> > > __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
> > > {
> > > + seqcount_latch_t *seqcount;
> > > int seq, idx;
> > >
> > > preempt_disable_notrace();
> > >
> > > + seqcount = &this_cpu_ptr(&cyc2ns)->seq;
> > > do {
> > > - seq = this_cpu_read(cyc2ns.seq.sequence);
> > > + seq = raw_read_seqcount_latch(seqcount);
> > > idx = seq & 1;
> > >
> > > data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> > > data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> > > data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
> > >
> > > - } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
> > > + } while (read_seqcount_latch_retry(seqcount, seq));
> > > }
> >
> > So I worried about this change, it obviously generates worse code. But I
> > was not expecting this:
> >
> > Before:
> >
> > 196: 0000000000000110 189 FUNC GLOBAL DEFAULT 1 native_sched_clock
> >
> > After:
> >
> > 195: 0000000000000110 399 FUNC GLOBAL DEFAULT 1 native_sched_clock
> >
> > That's _210_ bytes extra!!
> >
> > If you look at the disassembly of the thing after it's a complete
> > trainwreck.
>
> The below delta fixes it again.
>
> ---
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -68,21 +68,19 @@ early_param("tsc_early_khz", tsc_early_k
>
> __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
> {
> - seqcount_latch_t *seqcount;
> int seq, idx;
>
> preempt_disable_notrace();
>
> - seqcount = &this_cpu_ptr(&cyc2ns)->seq;
> do {
> - seq = raw_read_seqcount_latch(seqcount);
> + seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
> idx = seq & 1;
>
> data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
>
> - } while (read_seqcount_latch_retry(seqcount, seq));
> + } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
> }
>
> __always_inline void cyc2ns_read_end(void)
I've been unsuccessful in reproducing this huge, 200+ bytes, difference.
Can I please get the defconfig and GCC version?
Here are the two competing implementations:
noinline void cyc2ns_read_begin_v1(struct cyc2ns_data *data)
{
seqcount_latch_t *seqcount;
int seq, idx;
preempt_disable_notrace();
seqcount = &this_cpu_ptr(&cyc2ns)->seq;
do {
seq = raw_read_seqcount_latch(seqcount);
idx = seq & 1;
data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
} while (read_seqcount_latch_retry(seqcount, seq));
}
noinline void cyc2ns_read_begin_v2(struct cyc2ns_data *data)
{
int seq, idx;
preempt_disable_notrace();
do {
seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
idx = seq & 1;
data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
} while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
}
And here are their resulting gcc-7/8/4.9 binary output (DEBUG_PREEMPT=y,
but setting DEBUG_KERNEL/DEBUG_PREEMPT=n also doesn't show any big _v1
vs. _v2 difference; similarly when using gcc-10):
gcc-7
=====
ffffffff81028890 <cyc2ns_read_begin_v1>:
ffffffff81028890: 65 ff 05 a9 e6 fe 7e incl %gs:0x7efee6a9(%rip) # 16f40 <__preempt_count>
ffffffff81028893: R_X86_64_PC32 __preempt_count-0x4
ffffffff81028897: 48 c7 c6 40 a5 1f 00 mov $0x1fa540,%rsi
ffffffff8102889a: R_X86_64_32S .data..percpu+0x1fa540
ffffffff8102889e: 48 89 f2 mov %rsi,%rdx
ffffffff810288a1: 65 48 03 15 bf 8a fe add %gs:0x7efe8abf(%rip),%rdx # 11368 <this_cpu_off>
ffffffff810288a8: 7e
ffffffff810288a5: R_X86_64_PC32 this_cpu_off-0x4
ffffffff810288a9: 8b 4a 20 mov 0x20(%rdx),%ecx
ffffffff810288ac: 89 c8 mov %ecx,%eax
ffffffff810288ae: 83 e0 01 and $0x1,%eax
ffffffff810288b1: 48 c1 e0 04 shl $0x4,%rax
ffffffff810288b5: 48 01 f0 add %rsi,%rax
ffffffff810288b8: 65 4c 8b 40 08 mov %gs:0x8(%rax),%r8
ffffffff810288bd: 4c 89 47 08 mov %r8,0x8(%rdi)
ffffffff810288c1: 65 44 8b 00 mov %gs:(%rax),%r8d
ffffffff810288c5: 44 89 07 mov %r8d,(%rdi)
ffffffff810288c8: 65 8b 40 04 mov %gs:0x4(%rax),%eax
ffffffff810288cc: 89 47 04 mov %eax,0x4(%rdi)
ffffffff810288cf: 8b 42 20 mov 0x20(%rdx),%eax
ffffffff810288d2: 39 c8 cmp %ecx,%eax
ffffffff810288d4: 75 d3 jne ffffffff810288a9 <cyc2ns_read_begin_v1+0x19>
ffffffff810288d6: f3 c3 repz retq
ffffffff810288d8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
ffffffff810288df: 00
ffffffff810288e0 <cyc2ns_read_begin_v2>:
ffffffff810288e0: 65 ff 05 59 e6 fe 7e incl %gs:0x7efee659(%rip) # 16f40 <__preempt_count>
ffffffff810288e3: R_X86_64_PC32 __preempt_count-0x4
ffffffff810288e7: 48 c7 c1 40 a5 1f 00 mov $0x1fa540,%rcx
ffffffff810288ea: R_X86_64_32S .data..percpu+0x1fa540
ffffffff810288ee: 48 89 c8 mov %rcx,%rax
ffffffff810288f1: 65 48 03 05 6f 8a fe add %gs:0x7efe8a6f(%rip),%rax # 11368 <this_cpu_off>
ffffffff810288f8: 7e
ffffffff810288f5: R_X86_64_PC32 this_cpu_off-0x4
ffffffff810288f9: 65 8b 15 60 1c 1d 7f mov %gs:0x7f1d1c60(%rip),%edx # 1fa560 <cyc2ns+0x20>
ffffffff810288fc: R_X86_64_PC32 .data..percpu+0x1fa55c
ffffffff81028900: 89 d0 mov %edx,%eax
ffffffff81028902: 83 e0 01 and $0x1,%eax
ffffffff81028905: 48 c1 e0 04 shl $0x4,%rax
ffffffff81028909: 48 01 c8 add %rcx,%rax
ffffffff8102890c: 65 48 8b 70 08 mov %gs:0x8(%rax),%rsi
ffffffff81028911: 48 89 77 08 mov %rsi,0x8(%rdi)
ffffffff81028915: 65 8b 30 mov %gs:(%rax),%esi
ffffffff81028918: 89 37 mov %esi,(%rdi)
ffffffff8102891a: 65 8b 40 04 mov %gs:0x4(%rax),%eax
ffffffff8102891e: 89 47 04 mov %eax,0x4(%rdi)
ffffffff81028921: 65 8b 05 38 1c 1d 7f mov %gs:0x7f1d1c38(%rip),%eax # 1fa560 <cyc2ns+0x20>
ffffffff81028924: R_X86_64_PC32 .data..percpu+0x1fa55c
ffffffff81028928: 39 c2 cmp %eax,%edx
ffffffff8102892a: 75 cd jne ffffffff810288f9 <cyc2ns_read_begin_v2+0x19>
ffffffff8102892c: f3 c3 repz retq
ffffffff8102892e: 66 90 xchg %ax,%ax
gcc-8
=====
ffffffff810281a0 <cyc2ns_read_begin_v1>:
ffffffff810281a0: 65 ff 05 99 ed fe 7e incl %gs:0x7efeed99(%rip) # 16f40 <__preempt_count>
ffffffff810281a3: R_X86_64_PC32 __preempt_count-0x4
ffffffff810281a7: 49 c7 c0 40 a5 1f 00 mov $0x1fa540,%r8
ffffffff810281aa: R_X86_64_32S .data..percpu+0x1fa540
ffffffff810281ae: 4c 89 c1 mov %r8,%rcx
ffffffff810281b1: 65 48 03 0d af 91 fe add %gs:0x7efe91af(%rip),%rcx # 11368 <this_cpu_off>
ffffffff810281b8: 7e
ffffffff810281b5: R_X86_64_PC32 this_cpu_off-0x4
ffffffff810281b9: 8b 51 20 mov 0x20(%rcx),%edx
ffffffff810281bc: 89 d0 mov %edx,%eax
ffffffff810281be: 83 e0 01 and $0x1,%eax
ffffffff810281c1: 48 c1 e0 04 shl $0x4,%rax
ffffffff810281c5: 4c 01 c0 add %r8,%rax
ffffffff810281c8: 65 48 8b 70 08 mov %gs:0x8(%rax),%rsi
ffffffff810281cd: 48 89 77 08 mov %rsi,0x8(%rdi)
ffffffff810281d1: 65 8b 30 mov %gs:(%rax),%esi
ffffffff810281d4: 89 37 mov %esi,(%rdi)
ffffffff810281d6: 65 8b 40 04 mov %gs:0x4(%rax),%eax
ffffffff810281da: 89 47 04 mov %eax,0x4(%rdi)
ffffffff810281dd: 8b 41 20 mov 0x20(%rcx),%eax
ffffffff810281e0: 39 d0 cmp %edx,%eax
ffffffff810281e2: 75 d5 jne ffffffff810281b9 <cyc2ns_read_begin_v1+0x19>
ffffffff810281e4: c3 retq
ffffffff810281e5: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
ffffffff810281ec: 00 00 00 00
ffffffff810281f0 <cyc2ns_read_begin_v2>:
ffffffff810281f0: 65 ff 05 49 ed fe 7e incl %gs:0x7efeed49(%rip) # 16f40 <__preempt_count>
ffffffff810281f3: R_X86_64_PC32 __preempt_count-0x4
ffffffff810281f7: 48 c7 c1 40 a5 1f 00 mov $0x1fa540,%rcx
ffffffff810281fa: R_X86_64_32S .data..percpu+0x1fa540
ffffffff810281fe: 48 89 c8 mov %rcx,%rax
ffffffff81028201: 65 48 03 05 5f 91 fe add %gs:0x7efe915f(%rip),%rax # 11368 <this_cpu_off>
ffffffff81028208: 7e
ffffffff81028205: R_X86_64_PC32 this_cpu_off-0x4
ffffffff81028209: 65 8b 15 50 23 1d 7f mov %gs:0x7f1d2350(%rip),%edx # 1fa560 <cyc2ns+0x20>
ffffffff8102820c: R_X86_64_PC32 .data..percpu+0x1fa55c
ffffffff81028210: 89 d0 mov %edx,%eax
ffffffff81028212: 83 e0 01 and $0x1,%eax
ffffffff81028215: 48 c1 e0 04 shl $0x4,%rax
ffffffff81028219: 48 01 c8 add %rcx,%rax
ffffffff8102821c: 65 48 8b 70 08 mov %gs:0x8(%rax),%rsi
ffffffff81028221: 48 89 77 08 mov %rsi,0x8(%rdi)
ffffffff81028225: 65 8b 30 mov %gs:(%rax),%esi
ffffffff81028228: 89 37 mov %esi,(%rdi)
ffffffff8102822a: 65 8b 40 04 mov %gs:0x4(%rax),%eax
ffffffff8102822e: 89 47 04 mov %eax,0x4(%rdi)
ffffffff81028231: 65 8b 05 28 23 1d 7f mov %gs:0x7f1d2328(%rip),%eax # 1fa560 <cyc2ns+0x20>
ffffffff81028234: R_X86_64_PC32 .data..percpu+0x1fa55c
ffffffff81028238: 39 c2 cmp %eax,%edx
ffffffff8102823a: 75 cd jne ffffffff81028209 <cyc2ns_read_begin_v2+0x19>
ffffffff8102823c: c3 retq
ffffffff8102823d: 0f 1f 00 nopl (%rax)
gcc-4.9
=======
ffffffff810ab900 <cyc2ns_read_begin_v1>:
ffffffff810ab900: 55 push %rbp
ffffffff810ab901: 48 89 fd mov %rdi,%rbp
ffffffff810ab904: 53 push %rbx
ffffffff810ab905: 65 ff 05 f4 b6 f6 7e incl %gs:0x7ef6b6f4(%rip) # 17000 <__preempt_count>
ffffffff810ab908: R_X86_64_PC32 __preempt_count-0x4
ffffffff810ab90c: e8 df da c9 00 callq ffffffff81d493f0 <debug_smp_processor_id>
ffffffff810ab90d: R_X86_64_PC32 debug_smp_processor_id-0x4
ffffffff810ab911: 89 c0 mov %eax,%eax
ffffffff810ab913: 48 c7 c3 00 ab 02 00 mov $0x2ab00,%rbx
ffffffff810ab916: R_X86_64_32S .data..percpu+0x2ab00
ffffffff810ab91a: 48 03 1c c5 80 87 65 add -0x7d9a7880(,%rax,8),%rbx
ffffffff810ab921: 82
ffffffff810ab91e: R_X86_64_32S __per_cpu_offset
ffffffff810ab922: 8b 53 20 mov 0x20(%rbx),%edx
ffffffff810ab925: 89 d0 mov %edx,%eax
ffffffff810ab927: 83 e0 01 and $0x1,%eax
ffffffff810ab92a: 48 c1 e0 04 shl $0x4,%rax
ffffffff810ab92e: 65 48 8b 88 08 ab 02 mov %gs:0x2ab08(%rax),%rcx
ffffffff810ab935: 00
ffffffff810ab932: R_X86_64_32S .data..percpu+0x2ab08
ffffffff810ab936: 48 89 4d 08 mov %rcx,0x8(%rbp)
ffffffff810ab93a: 65 8b 88 00 ab 02 00 mov %gs:0x2ab00(%rax),%ecx
ffffffff810ab93d: R_X86_64_32S .data..percpu+0x2ab00
ffffffff810ab941: 89 4d 00 mov %ecx,0x0(%rbp)
ffffffff810ab944: 65 8b 80 04 ab 02 00 mov %gs:0x2ab04(%rax),%eax
ffffffff810ab947: R_X86_64_32S .data..percpu+0x2ab04
ffffffff810ab94b: 89 45 04 mov %eax,0x4(%rbp)
ffffffff810ab94e: 8b 43 20 mov 0x20(%rbx),%eax
ffffffff810ab951: 39 c2 cmp %eax,%edx
ffffffff810ab953: 75 cd jne ffffffff810ab922 <cyc2ns_read_begin_v1+0x22>
ffffffff810ab955: 5b pop %rbx
ffffffff810ab956: 5d pop %rbp
ffffffff810ab957: c3 retq
ffffffff810ab958: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
ffffffff810ab95f: 00
ffffffff810ab960 <cyc2ns_read_begin_v2>:
ffffffff810ab960: 65 ff 05 99 b6 f6 7e incl %gs:0x7ef6b699(%rip) # 17000 <__preempt_count>
ffffffff810ab963: R_X86_64_PC32 __preempt_count-0x4
ffffffff810ab967: 65 8b 15 b2 f1 f7 7e mov %gs:0x7ef7f1b2(%rip),%edx # 2ab20 <cyc2ns+0x20>
ffffffff810ab96a: R_X86_64_PC32 .data..percpu+0x2ab1c
ffffffff810ab96e: 89 d0 mov %edx,%eax
ffffffff810ab970: 83 e0 01 and $0x1,%eax
ffffffff810ab973: 48 c1 e0 04 shl $0x4,%rax
ffffffff810ab977: 65 48 8b 88 08 ab 02 mov %gs:0x2ab08(%rax),%rcx
ffffffff810ab97e: 00
ffffffff810ab97b: R_X86_64_32S .data..percpu+0x2ab08
ffffffff810ab97f: 48 89 4f 08 mov %rcx,0x8(%rdi)
ffffffff810ab983: 65 8b 88 00 ab 02 00 mov %gs:0x2ab00(%rax),%ecx
ffffffff810ab986: R_X86_64_32S .data..percpu+0x2ab00
ffffffff810ab98a: 89 0f mov %ecx,(%rdi)
ffffffff810ab98c: 65 8b 80 04 ab 02 00 mov %gs:0x2ab04(%rax),%eax
ffffffff810ab98f: R_X86_64_32S .data..percpu+0x2ab04
ffffffff810ab993: 89 47 04 mov %eax,0x4(%rdi)
ffffffff810ab996: 65 8b 05 83 f1 f7 7e mov %gs:0x7ef7f183(%rip),%eax # 2ab20 <cyc2ns+0x20>
ffffffff810ab999: R_X86_64_PC32 .data..percpu+0x2ab1c
ffffffff810ab99d: 39 c2 cmp %eax,%edx
ffffffff810ab99f: 75 c6 jne ffffffff810ab967 <cyc2ns_read_begin_v2+0x7>
ffffffff810ab9a1: f3 c3 repz retq
ffffffff810ab9a3: 66 66 66 66 2e 0f 1f data16 data16 data16 nopw %cs:0x0(%rax,%rax,1)
ffffffff810ab9aa: 84 00 00 00 00 00
Thanks,
--
Ahmed S. Darwish
Linutronix GmbH
On Mon, Sep 07, 2020 at 06:29:13PM +0200, Ahmed S. Darwish wrote:
> I've been unsuccessful in reproducing this huge, 200+ bytes, difference.
> Can I please get the defconfig and GCC version?
I think I lost the config and it's either gcc-9.3 or gcc-10, I can't
remember.
I just tried with:
make defconfig
./scripts/config --enable PREEMPT --enable DEBUG_ATOMIC_SLEEP
make oldconfig
And that reproduces things a little, but nowhere near as horrible as I
reported. Clearly I had something mad enabled by accident.
> Here are the two competing implementations:
>
> noinline void cyc2ns_read_begin_v1(struct cyc2ns_data *data)
> {
> seqcount_latch_t *seqcount;
> int seq, idx;
>
> preempt_disable_notrace();
>
> seqcount = &this_cpu_ptr(&cyc2ns)->seq;
> do {
> seq = raw_read_seqcount_latch(seqcount);
> idx = seq & 1;
>
> data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
>
> } while (read_seqcount_latch_retry(seqcount, seq));
> }
>
> noinline void cyc2ns_read_begin_v2(struct cyc2ns_data *data)
> {
> int seq, idx;
>
> preempt_disable_notrace();
>
> do {
> seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
> idx = seq & 1;
>
> data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
>
> } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
> }
Don't look at this function in isolation, look at native_sched_clock()
where it's used as a whole.
What happened (afaict) is that the change caused it to use more
registers and ended up spiling crap on the stack.
GCC-9.3 gives me:
(this_cpu variant)
0000 0000000000000c00 <native_sched_clock>:
0000 c00: e9 65 00 00 00 jmpq c6a <native_sched_clock+0x6a>
0005 c05: 0f 31 rdtsc
0007 c07: 48 c1 e2 20 shl $0x20,%rdx
000b c0b: 48 09 c2 or %rax,%rdx
000e c0e: 65 ff 05 00 00 00 00 incl %gs:0x0(%rip) # c15 <native_sched_clock+0x15>
0011 c11: R_X86_64_PC32 __preempt_count-0x4
0015 c15: 65 44 8b 05 00 00 00 mov %gs:0x0(%rip),%r8d # c1d <native_sched_clock+0x1d>
001c c1c: 00
0019 c19: R_X86_64_PC32 .data..percpu..shared_aligned+0x1c
001d c1d: 44 89 c0 mov %r8d,%eax
0020 c20: 83 e0 01 and $0x1,%eax
0023 c23: 48 c1 e0 04 shl $0x4,%rax
0027 c27: 48 8d 88 00 00 00 00 lea 0x0(%rax),%rcx
002a c2a: R_X86_64_32S .data..percpu..shared_aligned
002e c2e: 65 48 8b 79 08 mov %gs:0x8(%rcx),%rdi
0033 c33: 65 8b b0 00 00 00 00 mov %gs:0x0(%rax),%esi
0036 c36: R_X86_64_32S .data..percpu..shared_aligned
003a c3a: 65 8b 49 04 mov %gs:0x4(%rcx),%ecx
003e c3e: 65 8b 05 00 00 00 00 mov %gs:0x0(%rip),%eax # c45 <native_sched_clock+0x45>
0041 c41: R_X86_64_PC32 .data..percpu..shared_aligned+0x1c
0045 c45: 41 39 c0 cmp %eax,%r8d
0048 c48: 75 cb jne c15 <native_sched_clock+0x15>
004a c4a: 89 f0 mov %esi,%eax
004c c4c: 48 f7 e2 mul %rdx
004f c4f: 48 0f ad d0 shrd %cl,%rdx,%rax
0053 c53: 48 d3 ea shr %cl,%rdx
0056 c56: f6 c1 40 test $0x40,%cl
0059 c59: 48 0f 45 c2 cmovne %rdx,%rax
005d c5d: 48 01 f8 add %rdi,%rax
0060 c60: 65 ff 0d 00 00 00 00 decl %gs:0x0(%rip) # c67 <native_sched_clock+0x67>
0063 c63: R_X86_64_PC32 __preempt_count-0x4
0067 c67: 74 1a je c83 <native_sched_clock+0x83>
0069 c69: c3 retq
006a c6a: 48 69 05 00 00 00 00 imul $0xf4240,0x0(%rip),%rax # c75 <native_sched_clock+0x75>
0071 c71: 40 42 0f 00
006d c6d: R_X86_64_PC32 jiffies_64-0x8
0075 c75: 48 ba 00 b8 64 d9 05 movabs $0xfff0be05d964b800,%rdx
007c c7c: be f0 ff
007f c7f: 48 01 d0 add %rdx,%rax
0082 c82: c3 retq
0083 c83: 55 push %rbp
0084 c84: 48 89 e5 mov %rsp,%rbp
0087 c87: e8 00 00 00 00 callq c8c <native_sched_clock+0x8c>
0088 c88: R_X86_64_PLT32 preempt_schedule_notrace_thunk-0x4
008c c8c: 5d pop %rbp
008d c8d: c3 retq
(seqcount_latch variant)
0000 0000000000000c20 <native_sched_clock>:
0000 c20: e9 89 00 00 00 jmpq cae <native_sched_clock+0x8e>
0005 c25: 55 push %rbp
0006 c26: 48 89 e5 mov %rsp,%rbp
0009 c29: 41 54 push %r12
000b c2b: 53 push %rbx
000c c2c: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
0010 c30: 0f 31 rdtsc
0012 c32: 48 c1 e2 20 shl $0x20,%rdx
0016 c36: 48 89 d3 mov %rdx,%rbx
0019 c39: 48 09 c3 or %rax,%rbx
001c c3c: 65 ff 05 00 00 00 00 incl %gs:0x0(%rip) # c43 <native_sched_clock+0x23>
001f c3f: R_X86_64_PC32 __preempt_count-0x4
0023 c43: e8 00 00 00 00 callq c48 <native_sched_clock+0x28>
0024 c44: R_X86_64_PLT32 debug_smp_processor_id-0x4
0028 c48: 49 c7 c4 00 00 00 00 mov $0x0,%r12
002b c4b: R_X86_64_32S .data..percpu..shared_aligned+0x20
002f c4f: 89 c0 mov %eax,%eax
0031 c51: 4c 03 24 c5 00 00 00 add 0x0(,%rax,8),%r12
0038 c58: 00
0035 c55: R_X86_64_32S __per_cpu_offset
0039 c59: 4c 89 e0 mov %r12,%rax
003c c5c: 8b 30 mov (%rax),%esi
003e c5e: 89 f1 mov %esi,%ecx
0040 c60: 83 e1 01 and $0x1,%ecx
0043 c63: 48 c1 e1 04 shl $0x4,%rcx
0047 c67: 48 8d b9 00 00 00 00 lea 0x0(%rcx),%rdi
004a c6a: R_X86_64_32S .data..percpu..shared_aligned
004e c6e: 65 4c 8b 47 08 mov %gs:0x8(%rdi),%r8
0053 c73: 65 44 8b 89 00 00 00 mov %gs:0x0(%rcx),%r9d
005a c7a: 00
0057 c77: R_X86_64_32S .data..percpu..shared_aligned
005b c7b: 65 8b 4f 04 mov %gs:0x4(%rdi),%ecx
005f c7f: 8b 10 mov (%rax),%edx
0061 c81: 39 d6 cmp %edx,%esi
0063 c83: 75 d7 jne c5c <native_sched_clock+0x3c>
0065 c85: 44 89 c8 mov %r9d,%eax
0068 c88: 48 f7 e3 mul %rbx
006b c8b: 48 0f ad d0 shrd %cl,%rdx,%rax
006f c8f: 48 d3 ea shr %cl,%rdx
0072 c92: f6 c1 40 test $0x40,%cl
0075 c95: 48 0f 45 c2 cmovne %rdx,%rax
0079 c99: 4c 01 c0 add %r8,%rax
007c c9c: 65 ff 0d 00 00 00 00 decl %gs:0x0(%rip) # ca3 <native_sched_clock+0x83>
007f c9f: R_X86_64_PC32 __preempt_count-0x4
0083 ca3: 74 22 je cc7 <native_sched_clock+0xa7>
0085 ca5: 48 8d 65 f0 lea -0x10(%rbp),%rsp
0089 ca9: 5b pop %rbx
008a caa: 41 5c pop %r12
008c cac: 5d pop %rbp
008d cad: c3 retq
008e cae: 48 69 05 00 00 00 00 imul $0xf4240,0x0(%rip),%rax # cb9 <native_sched_clock+0x99>
0095 cb5: 40 42 0f 00
0091 cb1: R_X86_64_PC32 jiffies_64-0x8
0099 cb9: 49 b8 00 b8 64 d9 05 movabs $0xfff0be05d964b800,%r8
00a0 cc0: be f0 ff
00a3 cc3: 4c 01 c0 add %r8,%rax
00a6 cc6: c3 retq
00a7 cc7: e8 00 00 00 00 callq ccc <native_sched_clock+0xac>
00a8 cc8: R_X86_64_PLT32 preempt_schedule_notrace_thunk-0x4
00ac ccc: eb d7 jmp ca5 <native_sched_clock+0x85>
And you see it starting to spill stuff.
When we disable DEBUG_ATOMIC_SLEEP it becomes much saner again:
0000 0000000000000b40 <native_sched_clock>:
0000 b40: e9 6b 00 00 00 jmpq bb0 <native_sched_clock+0x70>
0005 b45: 0f 31 rdtsc
0007 b47: 48 c1 e2 20 shl $0x20,%rdx
000b b4b: 48 09 c2 or %rax,%rdx
000e b4e: 65 ff 05 00 00 00 00 incl %gs:0x0(%rip) # b55 <native_sched_clock+0x15>
0011 b51: R_X86_64_PC32 __preempt_count-0x4
0015 b55: 49 c7 c0 00 00 00 00 mov $0x0,%r8
0018 b58: R_X86_64_32S .data..percpu..shared_aligned+0x20
001c b5c: 65 4c 03 05 00 00 00 add %gs:0x0(%rip),%r8 # b64 <native_sched_clock+0x24>
0023 b63: 00
0020 b60: R_X86_64_PC32 this_cpu_off-0x4
0024 b64: 41 8b 30 mov (%r8),%esi
0027 b67: 89 f1 mov %esi,%ecx
0029 b69: 83 e1 01 and $0x1,%ecx
002c b6c: 48 c1 e1 04 shl $0x4,%rcx
0030 b70: 48 8d b9 00 00 00 00 lea 0x0(%rcx),%rdi
0033 b73: R_X86_64_32S .data..percpu..shared_aligned
0037 b77: 65 4c 8b 57 08 mov %gs:0x8(%rdi),%r10
003c b7c: 65 44 8b 89 00 00 00 mov %gs:0x0(%rcx),%r9d
0043 b83: 00
0040 b80: R_X86_64_32S .data..percpu..shared_aligned
0044 b84: 65 8b 4f 04 mov %gs:0x4(%rdi),%ecx
0048 b88: 41 8b 38 mov (%r8),%edi
004b b8b: 39 fe cmp %edi,%esi
004d b8d: 75 d5 jne b64 <native_sched_clock+0x24>
004f b8f: 44 89 c8 mov %r9d,%eax
0052 b92: 48 f7 e2 mul %rdx
0055 b95: 48 0f ad d0 shrd %cl,%rdx,%rax
0059 b99: 48 d3 ea shr %cl,%rdx
005c b9c: f6 c1 40 test $0x40,%cl
005f b9f: 48 0f 45 c2 cmovne %rdx,%rax
0063 ba3: 4c 01 d0 add %r10,%rax
0066 ba6: 65 ff 0d 00 00 00 00 decl %gs:0x0(%rip) # bad <native_sched_clock+0x6d>
0069 ba9: R_X86_64_PC32 __preempt_count-0x4
006d bad: 74 1a je bc9 <native_sched_clock+0x89>
006f baf: c3 retq
0070 bb0: 48 69 05 00 00 00 00 imul $0xf4240,0x0(%rip),%rax # bbb <native_sched_clock+0x7b>
0077 bb7: 40 42 0f 00
0073 bb3: R_X86_64_PC32 jiffies_64-0x8
007b bbb: 49 ba 00 b8 64 d9 05 movabs $0xfff0be05d964b800,%r10
0082 bc2: be f0 ff
0085 bc5: 4c 01 d0 add %r10,%rax
0088 bc8: c3 retq
0089 bc9: 55 push %rbp
008a bca: 48 89 e5 mov %rsp,%rbp
008d bcd: e8 00 00 00 00 callq bd2 <native_sched_clock+0x92>
008e bce: R_X86_64_PLT32 preempt_schedule_notrace_thunk-0x4
0092 bd2: 5d pop %rbp
0093 bd3: c3 retq
But that's still slightly larger.
Anyway, I frobbed the patch to use the this_cpu variant, and I've queued
the lot.
On Mon, Sep 07, 2020 at 07:30:47PM +0200, [email protected] wrote:
...
>
> Don't look at this function in isolation, look at native_sched_clock()
> where it's used as a whole.
>
...
> What happened (afaict) is that the change caused it to use more
> registers and ended up spiling crap on the stack.
>
...
>
> Anyway, I frobbed the patch to use the this_cpu variant, and I've queued
> the lot.
Perfect. Thanks a lot ;-)
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 80793c3471d90d4dc2b48deadb6413bdfe39500f
Gitweb: https://git.kernel.org/tip/80793c3471d90d4dc2b48deadb6413bdfe39500f
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Thu, 27 Aug 2020 13:40:39 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 10 Sep 2020 11:19:28 +02:00
seqlock: Introduce seqcount_latch_t
Latch sequence counters are a multiversion concurrency control mechanism
where the seqcount_t counter even/odd value is used to switch between
two copies of protected data. This allows the seqcount_t read path to
safely interrupt its write side critical section (e.g. from NMIs).
Initially, latch sequence counters were implemented as a single write
function above plain seqcount_t: raw_write_seqcount_latch(). The read
side was expected to use plain seqcount_t raw_read_seqcount().
A specialized latch read function, raw_read_seqcount_latch(), was later
added. It became the standardized way for latch read paths. Due to the
dependent load, it has one read memory barrier less than the plain
seqcount_t raw_read_seqcount() API.
Only raw_write_seqcount_latch() and raw_read_seqcount_latch() should be
used with latch sequence counters. Having *unique* read and write path
APIs means that latch sequence counters are actually a data type of
their own -- just inappropriately overloading plain seqcount_t.
Introduce seqcount_latch_t. This adds type-safety and ensures that only
the correct latch-safe APIs are to be used.
Not to break bisection, let the latch APIs also accept plain seqcount_t
or seqcount_raw_spinlock_t. After converting all call sites to
seqcount_latch_t, only that new data type will be allowed.
References: 9b0fd802e8c0 ("seqcount: Add raw_write_seqcount_latch()")
References: 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()")
References: aadd6e5caaac ("time/sched_clock: Use raw_read_seqcount_latch()")
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
Documentation/locking/seqlock.rst | 18 +++++-
include/linux/seqlock.h | 104 ++++++++++++++++++++---------
2 files changed, 91 insertions(+), 31 deletions(-)
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index 62c5ad9..a334b58 100644
--- a/Documentation/locking/seqlock.rst
+++ b/Documentation/locking/seqlock.rst
@@ -139,6 +139,24 @@ with the associated LOCKTYPE lock acquired.
Read path: same as in :ref:`seqcount_t`.
+
+.. _seqcount_latch_t:
+
+Latch sequence counters (``seqcount_latch_t``)
+----------------------------------------------
+
+Latch sequence counters are a multiversion concurrency control mechanism
+where the embedded seqcount_t counter even/odd value is used to switch
+between two copies of protected data. This allows the sequence counter
+read path to safely interrupt its own write side critical section.
+
+Use seqcount_latch_t when the write side sections cannot be protected
+from interruption by readers. This is typically the case when the read
+side can be invoked from NMI handlers.
+
+Check `raw_write_seqcount_latch()` for more information.
+
+
.. _seqlock_t:
Sequential locks (``seqlock_t``)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 300cbf3..88b917d 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -587,34 +587,76 @@ static inline void write_seqcount_t_invalidate(seqcount_t *s)
kcsan_nestable_atomic_end();
}
-/**
- * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+/*
+ * Latch sequence counters (seqcount_latch_t)
*
- * Use seqcount_t latching to switch between two storage places protected
- * by a sequence counter. Doing so allows having interruptible, preemptible,
- * seqcount_t write side critical sections.
+ * A sequence counter variant where the counter even/odd value is used to
+ * switch between two copies of protected data. This allows the read path,
+ * typically NMIs, to safely interrupt the write side critical section.
*
- * Check raw_write_seqcount_latch() for more details and a full reader and
- * writer usage example.
+ * As the write sections are fully preemptible, no special handling for
+ * PREEMPT_RT is needed.
+ */
+typedef struct {
+ seqcount_t seqcount;
+} seqcount_latch_t;
+
+/**
+ * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
+ * @seq_name: Name of the seqcount_latch_t instance
+ */
+#define SEQCNT_LATCH_ZERO(seq_name) { \
+ .seqcount = SEQCNT_ZERO(seq_name.seqcount), \
+}
+
+/**
+ * seqcount_latch_init() - runtime initializer for seqcount_latch_t
+ * @s: Pointer to the seqcount_latch_t instance
+ */
+static inline void seqcount_latch_init(seqcount_latch_t *s)
+{
+ seqcount_init(&s->seqcount);
+}
+
+/**
+ * raw_read_seqcount_latch() - pick even/odd latch data copy
+ * @s: Pointer to seqcount_t, seqcount_raw_spinlock_t, or seqcount_latch_t
+ *
+ * See raw_write_seqcount_latch() for details and a full reader/writer
+ * usage example.
*
* Return: sequence counter raw value. Use the lowest bit as an index for
- * picking which data copy to read. The full counter value must then be
- * checked with read_seqcount_retry().
+ * picking which data copy to read. The full counter must then be checked
+ * with read_seqcount_latch_retry().
*/
-#define raw_read_seqcount_latch(s) \
- raw_read_seqcount_t_latch(__seqcount_ptr(s))
+#define raw_read_seqcount_latch(s) \
+({ \
+ /* \
+ * Pairs with the first smp_wmb() in raw_write_seqcount_latch(). \
+ * Due to the dependent load, a full smp_rmb() is not needed. \
+ */ \
+ _Generic(*(s), \
+ seqcount_t: READ_ONCE(((seqcount_t *)s)->sequence), \
+ seqcount_raw_spinlock_t: READ_ONCE(((seqcount_raw_spinlock_t *)s)->seqcount.sequence), \
+ seqcount_latch_t: READ_ONCE(((seqcount_latch_t *)s)->seqcount.sequence)); \
+})
-static inline int raw_read_seqcount_t_latch(seqcount_t *s)
+/**
+ * read_seqcount_latch_retry() - end a seqcount_latch_t read section
+ * @s: Pointer to seqcount_latch_t
+ * @start: count, from raw_read_seqcount_latch()
+ *
+ * Return: true if a read section retry is required, else false
+ */
+static inline int
+read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
{
- /* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
- int seq = READ_ONCE(s->sequence); /* ^^^ */
- return seq;
+ return read_seqcount_retry(&s->seqcount, start);
}
/**
- * raw_write_seqcount_latch() - redirect readers to even/odd copy
- * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
+ * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
+ * @s: Pointer to seqcount_t, seqcount_raw_spinlock_t, or seqcount_latch_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
@@ -633,7 +675,7 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* The basic form is a data structure like::
*
* struct latch_struct {
- * seqcount_t seq;
+ * seqcount_latch_t seq;
* struct data_struct data[2];
* };
*
@@ -643,13 +685,13 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* void latch_modify(struct latch_struct *latch, ...)
* {
* smp_wmb(); // Ensure that the last data[1] update is visible
- * latch->seq++;
+ * latch->seq.sequence++;
* smp_wmb(); // Ensure that the seqcount update is visible
*
* modify(latch->data[0], ...);
*
* smp_wmb(); // Ensure that the data[0] update is visible
- * latch->seq++;
+ * latch->seq.sequence++;
* smp_wmb(); // Ensure that the seqcount update is visible
*
* modify(latch->data[1], ...);
@@ -668,8 +710,8 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * // read_seqcount_retry() includes needed smp_rmb()
- * } while (read_seqcount_retry(&latch->seq, seq));
+ * // This includes needed smp_rmb()
+ * } while (read_seqcount_latch_retry(&latch->seq, seq));
*
* return entry;
* }
@@ -693,14 +735,14 @@ static inline int raw_read_seqcount_t_latch(seqcount_t *s)
* When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
-#define raw_write_seqcount_latch(s) \
- raw_write_seqcount_t_latch(__seqcount_ptr(s))
-
-static inline void raw_write_seqcount_t_latch(seqcount_t *s)
-{
- smp_wmb(); /* prior stores before incrementing "sequence" */
- s->sequence++;
- smp_wmb(); /* increment "sequence" before following stores */
+#define raw_write_seqcount_latch(s) \
+{ \
+ smp_wmb(); /* prior stores before incrementing "sequence" */ \
+ _Generic(*(s), \
+ seqcount_t: ((seqcount_t *)s)->sequence++, \
+ seqcount_raw_spinlock_t:((seqcount_raw_spinlock_t *)s)->seqcount.sequence++, \
+ seqcount_latch_t: ((seqcount_latch_t *)s)->seqcount.sequence++); \
+ smp_wmb(); /* increment "sequence" before following stores */ \
}
/*
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 249d053835320cb3e7c00066cf085a6ba9b1f126
Gitweb: https://git.kernel.org/tip/249d053835320cb3e7c00066cf085a6ba9b1f126
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Thu, 27 Aug 2020 13:40:41 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 10 Sep 2020 11:19:29 +02:00
timekeeping: Use seqcount_latch_t
Latch sequence counters are a multiversion concurrency control mechanism
where the seqcount_t counter even/odd value is used to switch between
two data storage copies. This allows the seqcount_t read path to safely
interrupt its write side critical section (e.g. from NMIs).
Initially, latch sequence counters were implemented as a single write
function, raw_write_seqcount_latch(), above plain seqcount_t. The read
path was expected to use plain seqcount_t raw_read_seqcount().
A specialized read function was later added, raw_read_seqcount_latch(),
and became the standardized way for latch read paths. Having unique read
and write APIs meant that latch sequence counters are basically a data
type of their own -- just inappropriately overloading plain seqcount_t.
The seqcount_latch_t data type was thus introduced at seqlock.h.
Use that new data type instead of seqcount_raw_spinlock_t. This ensures
that only latch-safe APIs are to be used with the sequence counter.
Note that the use of seqcount_raw_spinlock_t was not very useful in the
first place. Only the "raw_" subset of seqcount_t APIs were used at
timekeeping.c. This subset was created for contexts where lockdep cannot
be used. seqcount_LOCKTYPE_t's raison d'être -- verifying that the
seqcount_t writer serialization lock is held -- cannot thus be done.
References: 0c3351d451ae ("seqlock: Use raw_ prefix instead of _no_lockdep")
References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
kernel/time/timekeeping.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4c47f38..999c981 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -64,7 +64,7 @@ static struct timekeeper shadow_timekeeper;
* See @update_fast_timekeeper() below.
*/
struct tk_fast {
- seqcount_raw_spinlock_t seq;
+ seqcount_latch_t seq;
struct tk_read_base base[2];
};
@@ -81,13 +81,13 @@ static struct clocksource dummy_clock = {
};
static struct tk_fast tk_fast_mono ____cacheline_aligned = {
- .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_mono.seq, &timekeeper_lock),
+ .seq = SEQCNT_LATCH_ZERO(tk_fast_mono.seq),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
static struct tk_fast tk_fast_raw ____cacheline_aligned = {
- .seq = SEQCNT_RAW_SPINLOCK_ZERO(tk_fast_raw.seq, &timekeeper_lock),
+ .seq = SEQCNT_LATCH_ZERO(tk_fast_raw.seq),
.base[0] = { .clock = &dummy_clock, },
.base[1] = { .clock = &dummy_clock, },
};
@@ -467,7 +467,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
tk_clock_read(tkr),
tkr->cycle_last,
tkr->mask));
- } while (read_seqcount_retry(&tkf->seq, seq));
+ } while (read_seqcount_latch_retry(&tkf->seq, seq));
return now;
}
@@ -533,7 +533,7 @@ static __always_inline u64 __ktime_get_real_fast_ns(struct tk_fast *tkf)
tk_clock_read(tkr),
tkr->cycle_last,
tkr->mask));
- } while (read_seqcount_retry(&tkf->seq, seq));
+ } while (read_seqcount_latch_retry(&tkf->seq, seq));
return now;
}
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 24bf401cebfd630cc9e2c3746e43945e836626f9
Gitweb: https://git.kernel.org/tip/24bf401cebfd630cc9e2c3746e43945e836626f9
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Thu, 27 Aug 2020 13:40:43 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 10 Sep 2020 11:19:29 +02:00
rbtree_latch: Use seqcount_latch_t
Latch sequence counters have unique read and write APIs, and thus
seqcount_latch_t was recently introduced at seqlock.h.
Use that new data type instead of plain seqcount_t. This adds the
necessary type-safety and ensures that only latching-safe seqcount APIs
are to be used.
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/rbtree_latch.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index 7d012fa..3d1a9e7 100644
--- a/include/linux/rbtree_latch.h
+++ b/include/linux/rbtree_latch.h
@@ -42,8 +42,8 @@ struct latch_tree_node {
};
struct latch_tree_root {
- seqcount_t seq;
- struct rb_root tree[2];
+ seqcount_latch_t seq;
+ struct rb_root tree[2];
};
/**
@@ -206,7 +206,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
do {
seq = raw_read_seqcount_latch(&root->seq);
node = __lt_find(key, root, seq & 1, ops->comp);
- } while (read_seqcount_retry(&root->seq, seq));
+ } while (read_seqcount_latch_retry(&root->seq, seq));
return node;
}
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 8117ab508f9c476e0a10b9db7f4818f784cf3176
Gitweb: https://git.kernel.org/tip/8117ab508f9c476e0a10b9db7f4818f784cf3176
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Fri, 04 Sep 2020 17:32:30 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 10 Sep 2020 11:19:31 +02:00
seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
Preemption must be disabled before entering a sequence counter write
side critical section. Otherwise the read side section can preempt the
write side section and spin for the entire scheduler tick. If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.
Disabling preemption cannot be done for PREEMPT_RT though: it can lead
to higher latencies, and the write side sections will not be able to
acquire locks which become sleeping locks (e.g. spinlock_t).
To remain preemptible, while avoiding a possible livelock caused by the
reader preempting the writer, use a different technique: let the reader
detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
case, acquire then release the associated LOCKNAME writer serialization
lock. This will allow any possibly-preempted writer to make progress
until the end of its writer serialization lock critical section.
Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
an associated (PREEMPT_RT) sleeping lock.
References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/seqlock.h | 61 +++++++++++++++++++++++++++++++++-------
1 file changed, 51 insertions(+), 10 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f3b7827..2bc9510 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -17,6 +17,7 @@
#include <linux/kcsan-checks.h>
#include <linux/lockdep.h>
#include <linux/mutex.h>
+#include <linux/ww_mutex.h>
#include <linux/preempt.h>
#include <linux/spinlock.h>
@@ -131,7 +132,23 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* See Documentation/locking/seqlock.rst
*/
-#ifdef CONFIG_LOCKDEP
+/*
+ * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
+ * disable preemption. It can lead to higher latencies, and the write side
+ * sections will not be able to acquire locks which become sleeping locks
+ * (e.g. spinlock_t).
+ *
+ * To remain preemptible while avoiding a possible livelock caused by the
+ * reader preempting the writer, use a different technique: let the reader
+ * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
+ * case, acquire then release the associated LOCKNAME writer serialization
+ * lock. This will allow any possibly-preempted writer to make progress
+ * until the end of its writer serialization lock critical section.
+ *
+ * This lock-unlock technique must be implemented for all of PREEMPT_RT
+ * sleeping locks. See Documentation/locking/locktypes.rst
+ */
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
#define __SEQ_LOCK(expr) expr
#else
#define __SEQ_LOCK(expr)
@@ -162,10 +179,12 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
* @locktype: LOCKNAME canonical C data type
- * @preemptible: preemptibility of above lockname
+ * @preemptible: preemptibility of above locktype
* @lockmember: argument for lockdep_assert_held()
+ * @lockbase: associated lock release function (prefix only)
+ * @lock_acquire: associated lock acquisition function (full call)
*/
-#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember) \
+#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
typedef struct seqcount_##lockname { \
seqcount_t seqcount; \
__SEQ_LOCK(locktype *lock); \
@@ -187,13 +206,33 @@ __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
static __always_inline unsigned \
__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
{ \
- return READ_ONCE(s->seqcount.sequence); \
+ unsigned seq = READ_ONCE(s->seqcount.sequence); \
+ \
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
+ return seq; \
+ \
+ if (preemptible && unlikely(seq & 1)) { \
+ __SEQ_LOCK(lock_acquire); \
+ __SEQ_LOCK(lockbase##_unlock(s->lock)); \
+ \
+ /* \
+ * Re-read the sequence counter since the (possibly \
+ * preempted) writer made progress. \
+ */ \
+ seq = READ_ONCE(s->seqcount.sequence); \
+ } \
+ \
+ return seq; \
} \
\
static __always_inline bool \
__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
{ \
- return preemptible; \
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
+ return preemptible; \
+ \
+ /* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
+ return false; \
} \
\
static __always_inline void \
@@ -226,11 +265,13 @@ static inline void __seqprop_assert(const seqcount_t *s)
lockdep_assert_preemption_disabled();
}
-SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock)
-SEQCOUNT_LOCKNAME(spinlock, spinlock_t, false, s->lock)
-SEQCOUNT_LOCKNAME(rwlock, rwlock_t, false, s->lock)
-SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock)
-SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base)
+#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
+SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
+SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
+SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
+SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
/*
* SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
The following commit has been merged into the locking/core branch of tip:
Commit-ID: a1f1066133d85d5f42217cc72a2490bb7aa889c5
Gitweb: https://git.kernel.org/tip/a1f1066133d85d5f42217cc72a2490bb7aa889c5
Author: Ahmed S. Darwish <[email protected]>
AuthorDate: Thu, 27 Aug 2020 13:40:42 +02:00
Committer: Peter Zijlstra <[email protected]>
CommitterDate: Thu, 10 Sep 2020 11:19:29 +02:00
x86/tsc: Use seqcount_latch_t
Latch sequence counters have unique read and write APIs, and thus
seqcount_latch_t was recently introduced at seqlock.h.
Use that new data type instead of plain seqcount_t. This adds the
necessary type-safety and ensures that only latching-safe seqcount APIs
are to be used.
Signed-off-by: Ahmed S. Darwish <[email protected]>
[peterz: unwreck cyc2ns_read_begin()]
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
arch/x86/kernel/tsc.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 49d9250..f70dffc 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -54,7 +54,7 @@ struct clocksource *art_related_clocksource;
struct cyc2ns {
struct cyc2ns_data data[2]; /* 0 + 2*16 = 32 */
- seqcount_t seq; /* 32 + 4 = 36 */
+ seqcount_latch_t seq; /* 32 + 4 = 36 */
}; /* fits one cacheline */
@@ -73,14 +73,14 @@ __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
preempt_disable_notrace();
do {
- seq = this_cpu_read(cyc2ns.seq.sequence);
+ seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
idx = seq & 1;
data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
- } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
+ } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
}
__always_inline void cyc2ns_read_end(void)
@@ -186,7 +186,7 @@ static void __init cyc2ns_init_boot_cpu(void)
{
struct cyc2ns *c2n = this_cpu_ptr(&cyc2ns);
- seqcount_init(&c2n->seq);
+ seqcount_latch_init(&c2n->seq);
__set_cyc2ns_scale(tsc_khz, smp_processor_id(), rdtsc());
}
@@ -203,7 +203,7 @@ static void __init cyc2ns_init_secondary_cpus(void)
for_each_possible_cpu(cpu) {
if (cpu != this_cpu) {
- seqcount_init(&c2n->seq);
+ seqcount_latch_init(&c2n->seq);
c2n = per_cpu_ptr(&cyc2ns, cpu);
c2n->data[0] = data[0];
c2n->data[1] = data[1];