Messages in this thread Patch in this message | | | From | Uros Bizjak <> | Subject | [PATCH v2] smpboot: Use atomic_try_cmpxchg in cpu_wait_death and cpu_report_death | Date | Thu, 25 Aug 2022 16:56:03 +0200 |
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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.
Reported-by: kernel test robot <lkp@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- Changes since v1: - Fix uninitialized variable in case cpu dies early. --- kernel/smpboot.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-)
diff --git a/kernel/smpboot.c b/kernel/smpboot.c index b9f54544e749..2c7396da470c 100644 --- 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; } -- 2.37.1
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