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locking/x86: Implement local_xchg() using CMPXCHG without the LOCK prefix

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Implement local_xchg() using the CMPXCHG instruction without the LOCK prefix.
XCHG is expensive due to the implied LOCK prefix.  The processor
cannot prefetch cachelines if XCHG is used.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20240124105816.612670-1-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2024-01-24 11:58:16 +01:00 committed by Ingo Molnar
parent f3e3620f1a
commit e807c2a370
1 changed files with 14 additions and 2 deletions
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16
arch/x86/include/asm/local.h
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@ -131,8 +131,20 @@ static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
(typeof(l->a.counter) *) old, new);
}
/* Always has a lock prefix */
#define local_xchg(l, n) (xchg(&((l)->a.counter), (n)))
/*
* Implement local_xchg using CMPXCHG instruction without the LOCK prefix.
* XCHG is expensive due to the implied LOCK prefix. The processor
* cannot prefetch cachelines if XCHG is used.
*/
static __always_inline long
local_xchg(local_t *l, long n)
{
long c = local_read(l);
do { } while (!local_try_cmpxchg(l, &c, n));
return c;
}
/**
* local_add_unless - add unless the number is already a given value