Commit 9a15193e authored by Uros Bizjak's avatar Uros Bizjak Committed by Andrew Morton
Browse files

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.com


Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 5fdfa161

kernel/smpboot.c

+8 −7
Original line number Diff line number Diff line
@@ -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();



	do {

	oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));

	do {

		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;

}