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

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

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 75a6f6133866d81ab4bd94f128d3f20f85c7c15f..b4233dc2c2b04c13add7d5ac1395705307ea9383 100644 (file)
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -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;
 
        /*