This new form allows using hardware assisted waiting.
Requested-by: Will Deacon <[email protected]>
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
---
include/linux/compiler.h | 25 +++++++++++++++++++------
kernel/locking/qspinlock.c | 12 ++++++------
kernel/sched/core.c | 8 ++++----
kernel/sched/sched.h | 2 +-
kernel/smp.c | 2 +-
5 files changed, 31 insertions(+), 18 deletions(-)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -305,21 +305,34 @@ static __always_inline void __write_once
})
/**
- * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable to wait on
* @cond: boolean expression to wait for
*
* Equivalent to using smp_load_acquire() on the condition variable but employs
* the control dependency of the wait to reduce the barrier on many platforms.
*
+ * Due to C lacking lambda expressions we load the value of *ptr into a
+ * pre-named variable @VAL to be used in @cond.
+ *
* The control dependency provides a LOAD->STORE order, the additional RMB
* provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
* aka. ACQUIRE.
*/
-#define smp_cond_acquire(cond) do { \
- while (!(cond)) \
- cpu_relax(); \
- smp_rmb(); /* ctrl + rmb := acquire */ \
-} while (0)
+#ifndef smp_cond_load_acquire
+#define smp_cond_load_acquire(ptr, cond_expr) ({ \
+ typeof(ptr) __PTR = (ptr); \
+ typeof(*ptr) VAL; \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cpu_relax(); \
+ } \
+ smp_rmb(); /* ctrl + rmb := acquire */ \
+ VAL; \
+})
+#endif
#endif /* __KERNEL__ */
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -358,7 +358,7 @@ void queued_spin_lock_slowpath(struct qs
* sequentiality; this is because not all clear_pending_set_locked()
* implementations imply full barriers.
*/
- smp_cond_acquire(!(atomic_read(&lock->val) & _Q_LOCKED_MASK));
+ smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
/*
* take ownership and clear the pending bit.
@@ -434,7 +434,7 @@ void queued_spin_lock_slowpath(struct qs
*
* The PV pv_wait_head_or_lock function, if active, will acquire
* the lock and return a non-zero value. So we have to skip the
- * smp_cond_acquire() call. As the next PV queue head hasn't been
+ * smp_cond_load_acquire() call. As the next PV queue head hasn't been
* designated yet, there is no way for the locked value to become
* _Q_SLOW_VAL. So both the set_locked() and the
* atomic_cmpxchg_relaxed() calls will be safe.
@@ -445,7 +445,7 @@ void queued_spin_lock_slowpath(struct qs
if ((val = pv_wait_head_or_lock(lock, node)))
goto locked;
- smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
+ val = smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_PENDING_MASK));
locked:
/*
@@ -465,9 +465,9 @@ void queued_spin_lock_slowpath(struct qs
break;
}
/*
- * The smp_cond_acquire() call above has provided the necessary
- * acquire semantics required for locking. At most two
- * iterations of this loop may be ran.
+ * The smp_cond_load_acquire() call above has provided the
+ * necessary acquire semantics required for locking. At most
+ * two iterations of this loop may be ran.
*/
old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1843,7 +1843,7 @@ static void ttwu_queue(struct task_struc
* chain to provide order. Instead we do:
*
* 1) smp_store_release(X->on_cpu, 0)
- * 2) smp_cond_acquire(!X->on_cpu)
+ * 2) smp_cond_load_acquire(!X->on_cpu)
*
* Example:
*
@@ -1854,7 +1854,7 @@ static void ttwu_queue(struct task_struc
* sched-out X
* smp_store_release(X->on_cpu, 0);
*
- * smp_cond_acquire(!X->on_cpu);
+ * smp_cond_load_acquire(&X->on_cpu, !VAL);
* X->state = WAKING
* set_task_cpu(X,2)
*
@@ -1880,7 +1880,7 @@ static void ttwu_queue(struct task_struc
* This means that any means of doing remote wakeups must order the CPU doing
* the wakeup against the CPU the task is going to end up running on. This,
* however, is already required for the regular Program-Order guarantee above,
- * since the waking CPU is the one issueing the ACQUIRE (smp_cond_acquire).
+ * since the waking CPU is the one issueing the ACQUIRE (smp_cond_load_acquire).
*
*/
@@ -1953,7 +1953,7 @@ try_to_wake_up(struct task_struct *p, un
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
*/
- smp_cond_acquire(!p->on_cpu);
+ smp_cond_load_acquire(&p->on_cpu, !VAL);
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1104,7 +1104,7 @@ static inline void finish_lock_switch(st
* In particular, the load of prev->state in finish_task_switch() must
* happen before this.
*
- * Pairs with the smp_cond_acquire() in try_to_wake_up().
+ * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
*/
smp_store_release(&prev->on_cpu, 0);
#endif
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -107,7 +107,7 @@ void __init call_function_init(void)
*/
static __always_inline void csd_lock_wait(struct call_single_data *csd)
{
- smp_cond_acquire(!(csd->flags & CSD_FLAG_LOCK));
+ smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));
}
static __always_inline void csd_lock(struct call_single_data *csd)
On 04/04/2016 08:22 AM, Peter Zijlstra wrote:
> This new form allows using hardware assisted waiting.
>
> Requested-by: Will Deacon<[email protected]>
> Suggested-by: Linus Torvalds<[email protected]>
> Signed-off-by: Peter Zijlstra (Intel)<[email protected]>
> ---
> include/linux/compiler.h | 25 +++++++++++++++++++------
> kernel/locking/qspinlock.c | 12 ++++++------
> kernel/sched/core.c | 8 ++++----
> kernel/sched/sched.h | 2 +-
> kernel/smp.c | 2 +-
> 5 files changed, 31 insertions(+), 18 deletions(-)
>
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -305,21 +305,34 @@ static __always_inline void __write_once
> })
>
> /**
> - * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
> + * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
> + * @ptr: pointer to the variable to wait on
> * @cond: boolean expression to wait for
> *
> * Equivalent to using smp_load_acquire() on the condition variable but employs
> * the control dependency of the wait to reduce the barrier on many platforms.
> *
> + * Due to C lacking lambda expressions we load the value of *ptr into a
> + * pre-named variable @VAL to be used in @cond.
> + *
> * The control dependency provides a LOAD->STORE order, the additional RMB
> * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
> * aka. ACQUIRE.
> */
> -#define smp_cond_acquire(cond) do { \
> - while (!(cond)) \
> - cpu_relax(); \
> - smp_rmb(); /* ctrl + rmb := acquire */ \
> -} while (0)
> +#ifndef smp_cond_load_acquire
> +#define smp_cond_load_acquire(ptr, cond_expr) ({ \
> + typeof(ptr) __PTR = (ptr); \
> + typeof(*ptr) VAL; \
> + for (;;) { \
> + VAL = READ_ONCE(*__PTR); \
> + if (cond_expr) \
> + break; \
> + cpu_relax(); \
> + } \
> + smp_rmb(); /* ctrl + rmb := acquire */ \
> + VAL; \
> +})
> +#endif
Using a predefined VAR seems a bit awkward as a reader of the code may
not know where VAR comes from. How about passing in a variable to hold
the latest value of (*ptr), e.g.
#ifndef smp_cond_load_acquire
#define smp_cond_load_acquire(ptr, var, cond_expr) do { \
typeof(ptr) __PTR = (ptr); \
for (;;) { \
var = READ_ONCE(*__PTR); \
if (cond_expr) \
break; \
cpu_relax(); \
} \
smp_rmb(); /* ctrl + rmb := acquire */ \
} while (0)
#endif
Cheers,
Longman