linux-kernel - [PATCH] atomics: Use atomic_try_cmpxchg_release in rcuref_put

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Message-Id: <20230509150255.3691-1-ubizjak@gmail.com>
Date:   Tue,  9 May 2023 17:02:55 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] atomics: Use atomic_try_cmpxchg_release in rcuref_put_slowpath()

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in rcuref_put_slowpath(). 86 CMPXCHG instruction returns success in
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.

Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Uros Bizjak <ubizjak@...il.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;
 
 		/*
-- 
2.40.1