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x86/percpu: Use raw_cpu_try_cmpxchg() in preempt_count_set()

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Use raw_cpu_try_cmpxchg() instead of raw_cpu_cmpxchg(*ptr, old, new) == old.
x86 CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after CMPXCHG (and related MOV instruction in front of CMPXCHG).

Also, raw_cpu_try_cmpxchg() implicitly assigns old *ptr value to "old" when
cmpxchg fails. There is no need to re-read the value in the loop.

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>
Link: https://lore.kernel.org/r/20230830151623.3900-2-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2023-08-30 17:13:57 +02:00 committed by Ingo Molnar
parent 5f863897d9
commit b8e3dfa16e
1 changed files with 2 additions and 2 deletions
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4
arch/x86/include/asm/preempt.h
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@ -31,11 +31,11 @@ static __always_inline void preempt_count_set(int pc)
{
int old, new;
old = raw_cpu_read_4(pcpu_hot.preempt_count);
do {
old = raw_cpu_read_4(pcpu_hot.preempt_count);
new = (old & PREEMPT_NEED_RESCHED) |
(pc & ~PREEMPT_NEED_RESCHED);
} while (raw_cpu_cmpxchg_4(pcpu_hot.preempt_count, old, new) != old);
} while (!raw_cpu_try_cmpxchg_4(pcpu_hot.preempt_count, &old, new));
}
/*