The following commit has been merged into the sched/core branch of tip: Commit-ID: 5c9da9fe826d4a0d84bb322cca27cc8ad8d23b24 Gitweb: https://git.kernel.org/tip/5c9da9fe826d4a0d84bb322cca27cc8ad8d23b24 Author: Uros Bizjak <ubizjak@xxxxxxxxx> AuthorDate: Thu, 26 Jan 2023 16:08:35 +01:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Tue, 31 Jan 2023 15:01:46 +01:00 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 <ubizjak@xxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Link: https://lkml.kernel.org/r/20230118202330.3740-1-ubizjak@xxxxxxxxx Link: https://lore.kernel.org/r/20230126151323.643408110@xxxxxxxxxxxxx --- arch/x86/kernel/pvclock.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) 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; }
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