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x86/pvclock: Improve atomic update of last_value in pvclock_clocksource_read()

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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@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20230118202330.3740-1-ubizjak@gmail.com
Link: https://lore.kernel.org/r/20230126151323.643408110@infradead.org
This commit is contained in:
Uros Bizjak 2023-01-26 16:08:35 +01:00 committed by Ingo Molnar
parent 7aab7aa4b4
commit 5c9da9fe82
1 changed files with 2 additions and 3 deletions
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5
arch/x86/kernel/pvclock.c
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@ -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;
}