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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231123203605.3474745-2-ubizjak@gmail.com

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 65dd44e3fc1cfaf5cc8728624af6b3fbed47f8a1..1bb79526c21793a9bf78afa02b5ca62b7444f171 100644 (file)
--- 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 */