The following commit has been merged into the locking/core branch of tip: Commit-ID: 4fbf8b136ded943f8661cf48270482ad1f5ce7bd Gitweb: https://git.kernel.org/tip/4fbf8b136ded943f8661cf48270482ad1f5ce7bd Author: Uros Bizjak <ubizjak@xxxxxxxxx> AuthorDate: Tue, 09 May 2023 17:02:55 +02:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Tue, 10 Oct 2023 10:14:27 +02:00 locking/atomics: Use atomic_try_cmpxchg_release() to micro-optimize rcuref_put_slowpath() Use atomic_try_cmpxchg() instead of atomic_cmpxchg(*ptr, old, new) == old in rcuref_put_slowpath(). On x86 the CMPXCHG instruction returns success in the ZF flag, so this change saves a compare after CMPXCHG. Additionaly, the compiler reorders some code blocks to follow likely/unlikely annotations in the atomic_try_cmpxchg() macro, improving the code from: 9a: f0 0f b1 0b lock cmpxchg %ecx,(%rbx) 9e: 83 f8 ff cmp $0xffffffff,%eax a1: 74 04 je a7 <rcuref_put_slowpath+0x27> a3: 31 c0 xor %eax,%eax to: 9a: f0 0f b1 0b lock cmpxchg %ecx,(%rbx) 9e: 75 4c jne ec <rcuref_put_slowpath+0x6c> a0: b0 01 mov $0x1,%al No functional change intended. Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx> Link: https://lore.kernel.org/r/20230509150255.3691-1-ubizjak@xxxxxxxxx --- lib/rcuref.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/lib/rcuref.c b/lib/rcuref.c index 5ec00a4..97f300e 100644 --- a/lib/rcuref.c +++ b/lib/rcuref.c @@ -248,7 +248,7 @@ bool rcuref_put_slowpath(rcuref_t *ref) * require a retry. If this fails the caller is not * allowed to deconstruct the object. */ - if (atomic_cmpxchg_release(&ref->refcnt, RCUREF_NOREF, RCUREF_DEAD) != RCUREF_NOREF) + if (!atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD)) return false; /*
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