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 <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ubizjak

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 */