[PATCH v2 2/2] x86/smp: Use atomic_try_cmpxchg in native_stop_other_cpus()

[Uros Bizjak]

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 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.

Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
v2: Split from v1 patch.  Simplify commit entry and state
    how the patch was tested.
---
 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 97f6f0cbb43a..2908e063d7d8 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 */
-- 
2.42.0