[tip: x86/percpu] x86/smp: Use atomic_try_cmpxchg in native_stop_other_cpus()

[tip-bot2 for Uros Bizjak]

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

Commit-ID:     9e9d673b2c84719937db5d6ab1d8cbcd7d45e974
Gitweb:        https://git.kernel.org/tip/9e9d673b2c84719937db5d6ab1d8cbcd7d45e974
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Thu, 23 Nov 2023 21:34:23 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 30 Nov 2023 20:25:09 +01:00

x86/smp: Use atomic_try_cmpxchg in native_stop_other_cpus()

Use atomic_try_cmpxchg() instead of atomic_cmpxchg(*ptr, old, new) == old.

X86 CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after the CMPXCHG.

Tested by building a native Fedora-38 kernel and rebooting
a 12-way SMP system using "shutdown -r" command some 100 times.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231123203605.3474745-2-ubizjak@xxxxxxxxx
---
 arch/x86/kernel/smp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 65dd44e..1bb7952 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -148,15 +148,16 @@ static int register_stop_handler(void)
 
 static void native_stop_other_cpus(int wait)
 {
-	unsigned int this_cpu;
+	unsigned int old_cpu, this_cpu;
 	unsigned long flags, timeout;
 
 	if (reboot_force)
 		return;
 
 	/* Only proceed if this is the first CPU to reach this code */
+	old_cpu = -1;
 	this_cpu = smp_processor_id();
-	if (atomic_cmpxchg(&stopping_cpu, -1, this_cpu) != -1)
+	if (!atomic_try_cmpxchg(&stopping_cpu, &old_cpu, this_cpu))
 		return;
 
 	/* For kexec, ensure that offline CPUs are out of MWAIT and in HLT */