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

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

diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
index 2d13f25..4527e14 100644 (file)
--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -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));
 }
 
 /*