@@ -224,6 +224,13 @@ static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk)
/* calculate the delta since the last update_wall_time: */
delta = clocksource_delta(cycle_now, tk->tkr.cycle_last, tk->tkr.mask);
+ /*
+ * Try to catch underflows by checking if we are seeing small
+ * mask-relative negative values.
+ */
+ if (((~delta+1) & tk->tkr.mask) < (tk->tkr.mask >> 3))
+ delta = 0;
+
/* Cap delta value to the max_cycles values to avoid mult overflows */
delta = min(delta, tk->tkr.clock->max_cycles);
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 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. Cc: Dave Jones <davej@codemonkey.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Stephen Boyd <sboyd@codeaurora.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: John Stultz <john.stultz@linaro.org> --- kernel/time/timekeeping.c | 7 +++++++ 1 file changed, 7 insertions(+)