2020-05-19 21:49:25

by Ahmed S. Darwish

Subject: [PATCH v1 00/25] seqlock: Extend seqcount API with associated locks

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

2020-05-19 21:49:37

by Ahmed S. Darwish

Subject: [PATCH v1 09/25] 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 4.6.

- 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

2020-05-19 21:49:39

by Ahmed S. Darwish

Subject: [PATCH v1 14/25] 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]>
---
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

2020-05-19 21:49:41

by Ahmed S. Darwish

Subject: [PATCH v1 13/25] 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]>
---
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

2020-05-19 21:50:08

by Ahmed S. Darwish

Subject: [PATCH v1 11/25] seqlock: Add missing kernel-doc annotations

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

2020-05-19 21:50:09

by Ahmed S. Darwish

Subject: [PATCH v1 20/25] 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]>
---
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

2020-05-19 21:50:18

by Ahmed S. Darwish

Subject: [PATCH v1 18/25] 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]>
---
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

2020-05-19 21:50:33

by Ahmed S. Darwish

Subject: [PATCH v1 17/25] 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]>
---
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

2020-05-19 21:50:40

by Ahmed S. Darwish

Subject: [PATCH v1 24/25] 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]>
---
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

2020-05-19 21:51:01

by Ahmed S. Darwish

Subject: [PATCH v1 08/25] 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]>
---
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

2020-05-19 21:51:16

by Ahmed S. Darwish

Subject: [PATCH v1 06/25] 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]>
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

2020-05-19 21:51:40

by Ahmed S. Darwish

Subject: [PATCH v1 15/25] 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]>
---
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

2020-05-19 21:51:44

by Ahmed S. Darwish

Subject: [PATCH v1 12/25] 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]>
---
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

2020-05-19 21:52:09

by Ahmed S. Darwish

Subject: [PATCH v1 19/25] 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]>
---
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

2020-05-19 21:52:18

by Ahmed S. Darwish

Subject: [PATCH v1 16/25] 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]>
---
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

2020-05-19 21:52:23

by Ahmed S. Darwish

Subject: [PATCH v1 23/25] 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]>
---
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

2020-05-19 21:52:30

by Ahmed S. Darwish

Subject: [PATCH v1 25/25] 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]>
---
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

2020-05-19 21:52:36

by Ahmed S. Darwish

Subject: [PATCH v1 21/25] 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]>
---
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

2020-05-20 10:50:37

by Christian König

Subject: Re: [PATCH v1 13/25] dma-buf: Use sequence counter with associated wound/wait mutex

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;

2020-05-21 00:12:50

by Ahmed S. Darwish

Subject: Re: [PATCH v1 13/25] dma-buf: Use sequence counter with associated wound/wait mutex

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

2020-05-21 13:22:29

by Christian König

Subject: Re: [PATCH v1 13/25] dma-buf: Use sequence counter with associated wound/wait mutex

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

2020-05-22 18:04:06

by Peter Zijlstra

Subject: Re: [PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage

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.

2020-05-22 22:26:15

by Steven Rostedt

Subject: Re: [PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage

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

2020-05-25 10:55:30

by Ahmed S. Darwish

Subject: Re: [PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage

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

2020-05-25 18:07:39

by Ahmed S. Darwish

Subject: Re: [PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage

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

2020-06-08 01:00:52

by Ahmed S. Darwish

Subject: [PATCH v2 00/18] seqlock: Extend seqcount API with associated locks

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

2020-06-08 01:01:09

by Ahmed S. Darwish

Subject: [PATCH v2 08/18] 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]>
---
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

2020-06-08 01:01:22

by Ahmed S. Darwish

Subject: [PATCH v2 12/18] 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]>
---
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

2020-06-08 01:01:25

by Ahmed S. Darwish

Subject: [PATCH v2 14/18] 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]>
---
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

2020-06-08 01:02:03

by Ahmed S. Darwish

Subject: [PATCH v2 17/18] 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]>
---
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

2020-06-08 01:02:12

by Ahmed S. Darwish

Subject: [PATCH v2 18/18] 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]>
---
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

2020-06-08 01:02:26

by Ahmed S. Darwish

Subject: [PATCH v2 13/18] 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]>
---
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

2020-06-08 01:02:31

by Ahmed S. Darwish

Subject: [PATCH v2 11/18] 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]>
---
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

2020-06-08 01:03:20

by Ahmed S. Darwish

Subject: [PATCH v2 10/18] 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]>
---
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

2020-06-08 01:03:50

by Ahmed S. Darwish

Subject: [PATCH v2 16/18] 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]>
---
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

2020-06-08 01:04:35

by Ahmed S. Darwish

Subject: [PATCH v2 09/18] 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]>
---
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

2020-06-08 13:01:42

by Paolo Bonzini

Subject: Re: [PATCH v2 17/18] kvm/eventfd: Use sequence counter with associated spinlock

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]>

2020-06-30 05:45:38

by Ahmed S. Darwish

Subject: [PATCH v3 00/20] seqlock: Extend seqcount API with associated locks

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

2020-06-30 05:45:49

by Ahmed S. Darwish

Subject: [PATCH v3 01/20] 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 4.6.

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

2020-06-30 05:45:55

by Ahmed S. Darwish

Subject: [PATCH v3 02/20] 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]>
---
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

2020-06-30 05:46:06

by Ahmed S. Darwish

Subject: [PATCH v3 03/20] seqlock: Add missing kernel-doc annotations

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

2020-06-30 05:46:22

by Ahmed S. Darwish

Subject: [PATCH v3 05/20] 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]>
---
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

2020-06-30 05:46:30

by Ahmed S. Darwish

Subject: [PATCH v3 06/20] 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]>
---
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

2020-06-30 05:46:33

by Ahmed S. Darwish

Subject: [PATCH v3 07/20] 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]>
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

2020-06-30 05:46:39

by Ahmed S. Darwish

Subject: [PATCH v3 08/20] 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]>
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

2020-06-30 05:46:45

by Ahmed S. Darwish

Subject: [PATCH v3 09/20] 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]>
---
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

2020-06-30 05:46:55

by Ahmed S. Darwish

Subject: [PATCH v3 10/20] 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]>
---
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

2020-06-30 05:47:05

by Ahmed S. Darwish

Subject: [PATCH v3 11/20] 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]>
---
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

2020-06-30 05:47:32

by Ahmed S. Darwish

Subject: [PATCH v3 12/20] 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]>
---
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

2020-06-30 05:47:54

by Ahmed S. Darwish

Subject: [PATCH v3 17/20] NFSv4: 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]>
---
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

2020-06-30 05:48:04

by Ahmed S. Darwish

Subject: [PATCH v3 20/20] 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]>
---
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