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Date:   Tue, 10 Oct 2023 08:27:04 -0000
From:   "tip-bot2 for Uros Bizjak" <tip-bot2@linutronix.de>
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Subject: [tip: locking/core] locking/atomics: Use atomic_try_cmpxchg_release()
 to micro-optimize rcuref_put_slowpath()
Cc:     Uros Bizjak <ubizjak@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
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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@gmail.com>
AuthorDate:    Tue, 09 May 2023 17:02:55 +02:00
Committer:     Ingo Molnar <mingo@kernel.org>
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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
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 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;
 
 		/*