[tip: locking/core] locking/atomics: Use atomic_try_cmpxchg_release() to micro-optimize rcuref_put_slowpath()
From: tip-bot2 for Uros Bizjak
Date: Tue Oct 10 2023 - 04:27:21 EST
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;
/*