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 <da...@codemonkey.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Richard Cochran <richardcoch...@gmail.com>
Cc: Prarit Bhargava <pra...@redhat.com>
Cc: Stephen Boyd <sb...@codeaurora.org>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: John Stultz <john.stu...@linaro.org>
---
kernel/time/timekeeping.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 9740fd8..4acfc7f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -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);
--
1.9.1
--
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