From: Uros Bizjak Date: Thu, 26 Jan 2023 15:08:35 +0000 (+0100) Subject: x86/pvclock: Improve atomic update of last_value in pvclock_clocksource_read() X-Git-Tag: v6.6.7~3526^2~9 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=5c9da9fe826d4a0d84bb322cca27cc8ad8d23b24;p=platform%2Fkernel%2Flinux-starfive.git x86/pvclock: Improve atomic update of last_value in pvclock_clocksource_read() Improve atomic update of last_value in pvclock_clocksource_read: - Atomic update can be skipped if the "last_value" is already equal to "ret". - The detection of atomic update failure is not correct. The value, returned by atomic64_cmpxchg should be compared to the old value from the location to be updated. If these two are the same, then atomic update succeeded and "last_value" location is updated to "ret" in an atomic way. Otherwise, the atomic update failed and it should be retried with the value from "last_value" - exactly what atomic64_try_cmpxchg does in a correct and more optimal way. Signed-off-by: Uros Bizjak Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Link: https://lkml.kernel.org/r/20230118202330.3740-1-ubizjak@gmail.com Link: https://lore.kernel.org/r/20230126151323.643408110@infradead.org --- diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c index eda37df..5a2a517 100644 --- a/arch/x86/kernel/pvclock.c +++ b/arch/x86/kernel/pvclock.c @@ -102,10 +102,9 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) */ last = atomic64_read(&last_value); do { - if (ret < last) + if (ret <= last) return last; - last = atomic64_cmpxchg(&last_value, last, ret); - } while (unlikely(last != ret)); + } while (!atomic64_try_cmpxchg(&last_value, &last, ret)); return ret; }