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Subject[PATCH] atomics: Use atomic_try_cmpxchg_release in rcuref_put_slowpath()
Date
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@linutronix.de>
Signed-off-by: Uros Bizjak <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;

/*
--
2.40.1
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