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locking/osq: Relax atomic semantics 

... by using acquire/release for ops around the lock->tail. As such,
weakly ordered archs can benefit from more relaxed use of barriers
when issuing atomics.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <Waiman.Long@hpe.com>
Link: http://lkml.kernel.org/r/1442216244-4409-3-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Davidlohr Bueso 2015-09-14 00:37:24 -07:00 committed by Ingo Molnar
parent 6e1e519697
commit c55a6ffa62
1 changed files with 8 additions and 3 deletions
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11
kernel/locking/osq_lock.c
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@ -50,7 +50,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
for (;;) { for (;;) {
if (atomic_read(&lock->tail) == curr && if (atomic_read(&lock->tail) == curr &&
atomic_cmpxchg(&lock->tail, curr, old) == curr) { atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {
/* /*
* We were the last queued, we moved @lock back. @prev * We were the last queued, we moved @lock back. @prev
* will now observe @lock and will complete its * will now observe @lock and will complete its
@ -92,7 +92,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
node->next = NULL; node->next = NULL;
node->cpu = curr; node->cpu = curr;
old = atomic_xchg(&lock->tail, curr); /*
* ACQUIRE semantics, pairs with corresponding RELEASE
* in unlock() uncontended, or fastpath.
*/
old = atomic_xchg_acquire(&lock->tail, curr);
if (old == OSQ_UNLOCKED_VAL) if (old == OSQ_UNLOCKED_VAL)
return true; return true;
@ -184,7 +188,8 @@ void osq_unlock(struct optimistic_spin_queue *lock)
/* /*
* Fast path for the uncontended case. * Fast path for the uncontended case.
*/ */
if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr)) if (likely(atomic_cmpxchg_release(&lock->tail, curr,
OSQ_UNLOCKED_VAL) == curr))
return; return;
/* /*