linux-kernel - [tip: x86/asm] x86/percpu: Use raw_cpu_try_cmpxchg() in preempt_count

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Date:   Fri, 15 Sep 2023 11:25:01 -0000
From:   "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/asm] x86/percpu: Use raw_cpu_try_cmpxchg() in preempt_count_set()

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     b8e3dfa16ec55f310dd95831614af3d24abf5ed5
Gitweb:        https://git.kernel.org/tip/b8e3dfa16ec55f310dd95831614af3d24abf5ed5
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Wed, 30 Aug 2023 17:13:57 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 15 Sep 2023 13:19:22 +02:00

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@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Link: https://lore.kernel.org/r/20230830151623.3900-2-ubizjak@gmail.com
---
 arch/x86/include/asm/preempt.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

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