smpboot: use atomic_try_cmpxchg in cpu_wait_death and cpu_report_death

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in cpu_wait_death and cpu_report_death.  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg (and
related move instruction in front of cmpxchg).  Also, atomic_try_cmpxchg
implicitly assigns old *ptr value to "old" when cmpxchg fails, enabling
further code simplifications.

No functional change intended.

Link: https://lkml.kernel.org/r/20220825145603.5811-1-ubizjak@gmail.comSigned-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/smpboot.c

@@ -433,7 +433,7 @@ bool cpu_wait_death(unsigned int cpu, int seconds)
 
 	/* The outgoing CPU will normally get done quite quickly. */
 	if (atomic_read(&per_cpu(cpu_hotplug_state, cpu)) == CPU_DEAD)
-		goto update_state;
+		goto update_state_early;
 	udelay(5);
 
 	/* But if the outgoing CPU dawdles, wait increasingly long times. */
@@ -444,16 +444,17 @@ bool cpu_wait_death(unsigned int cpu, int seconds)
 			break;
 		sleep_jf = DIV_ROUND_UP(sleep_jf * 11, 10);
 	}
-update_state:
+update_state_early:
 	oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
+update_state:
 	if (oldstate == CPU_DEAD) {
 		/* Outgoing CPU died normally, update state. */
 		smp_mb(); /* atomic_read() before update. */
 		atomic_set(&per_cpu(cpu_hotplug_state, cpu), CPU_POST_DEAD);
 	} else {
 		/* Outgoing CPU still hasn't died, set state accordingly. */
-		if (atomic_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
-				   oldstate, CPU_BROKEN) != oldstate)
+		if (!atomic_try_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
+					&oldstate, CPU_BROKEN))
 			goto update_state;
 		ret = false;
 	}
@@ -475,14 +476,14 @@ bool cpu_report_death(void)
 	int newstate;
 	int cpu = smp_processor_id();
 
+	oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
 	do {
-		oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
 		if (oldstate != CPU_BROKEN)
 			newstate = CPU_DEAD;
 		else
 			newstate = CPU_DEAD_FROZEN;
-	} while (atomic_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
-				oldstate, newstate) != oldstate);
+	} while (!atomic_try_cmpxchg(&per_cpu(cpu_hotplug_state, cpu),
+				     &oldstate, newstate));
 	return newstate == CPU_DEAD;
 }