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: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index b9f5454..2c7396d 100644 (file)
--- a/kernel/smpboot.c
+++ b/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;
 }