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locking/osq_lock: Use atomic_try_cmpxchg_release() in osq_unlock()

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Replace this pattern in osq_unlock():

    atomic_cmpxchg(*ptr, old, new) == old

... with the simpler and faster:

    atomic_try_cmpxchg(*ptr, &old, new)

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after the CMPXCHG.  The code
in the fast path of osq_unlock() improves from:

 11b:	31 c9                	xor    %ecx,%ecx
 11d:	8d 50 01             	lea    0x1(%rax),%edx
 120:	89 d0                	mov    %edx,%eax
 122:	f0 0f b1 0f          	lock cmpxchg %ecx,(%rdi)
 126:	39 c2                	cmp    %eax,%edx
 128:	75 05                	jne    12f <...>

to:

 12b:	31 d2                	xor    %edx,%edx
 12d:	83 c0 01             	add    $0x1,%eax
 130:	f0 0f b1 17          	lock cmpxchg %edx,(%rdi)
 134:	75 05                	jne    13b <...>

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20241001114606.820277-1-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2024-10-01 13:45:57 +02:00 committed by Peter Zijlstra
parent f730fd535f
commit 0d75e0c420
1 changed files with 1 additions and 2 deletions
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3
kernel/locking/osq_lock.c
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@ -215,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
/*
* Fast path for the uncontended case.
*/
if (likely(atomic_cmpxchg_release(&lock->tail, curr,
OSQ_UNLOCKED_VAL) == curr))
if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
return;
/*