Commit eb1b4af0 authored by Rik van Riel's avatar Rik van Riel Committed by Ingo Molnar

sched, time: Atomically increment stime & utime

The functions task_cputime_adjusted and thread_group_cputime_adjusted()
can be called locklessly, as well as concurrently on many different CPUs.

This can occasionally lead to the utime and stime reported by times(), and
other syscalls like it, going backward. The cause for this appears to be
multiple threads racing in cputime_adjust(), both with values for utime or
stime that is larger than the original, but each with a different value.

Sometimes the larger value gets saved first, only to be immediately
overwritten with a smaller value by another thread.

Using atomic exchange prevents that problem, and ensures time
progresses monotonically.
Signed-off-by: default avatarRik van Riel <>
Signed-off-by: default avatarPeter Zijlstra (Intel) <>
Cc: Linus Torvalds <>
Link: default avatarIngo Molnar <>
parent e78c3496
......@@ -602,9 +602,12 @@ static void cputime_adjust(struct task_cputime *curr,
* If the tick based count grows faster than the scheduler one,
* the result of the scaling may go backward.
* Let's enforce monotonicity.
* Atomic exchange protects against concurrent cputime_adjust().
prev->stime = max(prev->stime, stime);
prev->utime = max(prev->utime, utime);
while (stime > (rtime = ACCESS_ONCE(prev->stime)))
cmpxchg(&prev->stime, rtime, stime);
while (utime > (rtime = ACCESS_ONCE(prev->utime)))
cmpxchg(&prev->utime, rtime, utime);
*ut = prev->utime;
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