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;
}
|