[PATCH 06/12] time: Try to catch clocksource delta underflows

From: John Stultz
Date: Thu Mar 12 2015 - 00:17:09 EST

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@xxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Richard Cochran <richardcochran@xxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
kernel/time/timekeeping.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index c06fe6e..2ba62fe 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -146,6 +146,13 @@ 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;

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