Commit-ID: eb1b4af0a64ac7bb0ee36f579c1c7cefcbc3ac2c Gitweb: http://git.kernel.org/tip/eb1b4af0a64ac7bb0ee36f579c1c7cefcbc3ac2c Author: Rik van Riel <riel@xxxxxxxxxx> AuthorDate: Fri, 15 Aug 2014 16:05:38 -0400 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Mon, 8 Sep 2014 08:17:02 +0200 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: Rik van Riel <riel@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: umgwanakikbuti@xxxxxxxxx Cc: fweisbec@xxxxxxxxx Cc: akpm@xxxxxxxxxxxxxxxxxxxx Cc: srao@xxxxxxxxxx Cc: lwoodman@xxxxxxxxxx Cc: atheurer@xxxxxxxxxx Cc: oleg@xxxxxxxxxx Link: http://lkml.kernel.org/r/1408133138-22048-4-git-send-email-riel@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/sched/cputime.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index 49b7cfe..2b57031 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -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); out: *ut = prev->utime; -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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