From: John Stultz Date: Thu, 12 Mar 2015 04:16:34 +0000 (-0700) Subject: timekeeping: Try to catch clocksource delta underflows X-Git-Tag: v4.1-rc1~144^2~7 X-Git-Url: http://git.cascardo.info/?p=cascardo%2Flinux.git;a=commitdiff_plain;h=057b87e3161d1194a095718f9918c01b2c389e74 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 Cc: Dave Jones Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Prarit Bhargava Cc: Richard Cochran Cc: Stephen Boyd Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1426133800-29329-7-git-send-email-john.stultz@linaro.org Signed-off-by: Ingo Molnar --- diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 657414cf2e46..187149be83ea 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;