linux-kernel - [PATCH] kexec: Use atomic_try_cmpxchg in crash

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Message-ID: <20231114161228.108516-1-ubizjak@gmail.com>
Date:   Tue, 14 Nov 2023 17:12:01 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: [PATCH] kexec: Use atomic_try_cmpxchg in crash_kexec

Use atomic_try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
crash_kexec().  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg.

No functional change intended.

Cc: Eric Biederman <ebiederm@...ssion.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 kernel/kexec_core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index be5642a4ec49..bc4c096ab1f3 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1063,9 +1063,10 @@ __bpf_kfunc void crash_kexec(struct pt_regs *regs)
 	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
 	 * may stop each other.  To exclude them, we use panic_cpu here too.
 	 */
+	old_cpu = PANIC_CPU_INVALID;
 	this_cpu = raw_smp_processor_id();
-	old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, this_cpu);
-	if (old_cpu == PANIC_CPU_INVALID) {
+
+	if (atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu)) {
 		/* This is the 1st CPU which comes here, so go ahead. */
 		__crash_kexec(regs);
 
-- 
2.41.0