Commit-ID: 057b87e3161d1194a095718f9918c01b2c389e74 Gitweb: http://git.kernel.org/tip/057b87e3161d1194a095718f9918c01b2c389e74 Author: John Stultz <john.stultz@xxxxxxxxxx> AuthorDate: Wed, 11 Mar 2015 21:16:34 -0700 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Fri, 13 Mar 2015 08:07:05 +0100 timekeeping: Try to catch clocksource delta underflows In the case where there is a broken clocksource where there are multiple actual clocks that aren't perfectly aligned, we may see small "negative" deltas when we subtract 'now' from 'cycle_last'. The values are actually negative with respect to the clocksource mask value, not necessarily negative if cast to a s64, but we can check by checking the delta to see if it is a small (relative to the mask) negative value (again negative relative to the mask). If so, we assume we jumped backwards somehow and instead use zero for our delta. Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx> Cc: Dave Jones <davej@xxxxxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Prarit Bhargava <prarit@xxxxxxxxxx> Cc: Richard Cochran <richardcochran@xxxxxxxxx> Cc: Stephen Boyd <sboyd@xxxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1426133800-29329-7-git-send-email-john.stultz@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/time/timekeeping.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 657414c..187149b 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -148,6 +148,13 @@ static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr) /* calculate the delta since the last update_wall_time */ delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + /* + * Try to catch underflows by checking if we are seeing small + * mask-relative negative values. + */ + if (unlikely((~delta & tkr->mask) < (tkr->mask >> 3))) + delta = 0; + /* Cap delta value to the max_cycles values to avoid mult overflows */ if (unlikely(delta > tkr->clock->max_cycles)) delta = tkr->clock->max_cycles; -- 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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