From 4fbf8b136ded943f8661cf48270482ad1f5ce7bd Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Tue, 9 May 2023 17:02:55 +0200 Subject: [PATCH] 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 a3: 31 c0 xor %eax,%eax to: 9a: f0 0f b1 0b lock cmpxchg %ecx,(%rbx) 9e: 75 4c jne ec a0: b0 01 mov $0x1,%al No functional change intended. Signed-off-by: Uros Bizjak Signed-off-by: Ingo Molnar Cc: Peter Zijlstra Cc: Paul E. McKenney Link: https://lore.kernel.org/r/20230509150255.3691-1-ubizjak@gmail.com --- lib/rcuref.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/lib/rcuref.c b/lib/rcuref.c index 5ec00a4a64d1..97f300eca927 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; /*