1 files changed, 22 insertions, 0 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e7acfb482a68..fa05e88aa76f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -518,6 +518,28 @@ void update_wall_time(void)
         /* correct the clock when NTP error is too big */
         clocksource_adjust(offset);
 
+        /*
+         * Since in the loop above, we accumulate any amount of time
+         * in xtime_nsec over a second into xtime.tv_sec, its possible for
+         * xtime_nsec to be fairly small after the loop. Further, if we're
+         * slightly speeding the clocksource up in clocksource_adjust(),
+         * its possible the required corrective factor to xtime_nsec could
+         * cause it to underflow.
+         *
+         * Now, we cannot simply roll the accumulated second back, since
+         * the NTP subsystem has been notified via second_overflow. So
+         * instead we push xtime_nsec forward by the amount we underflowed,
+         * and add that amount into the error.
+         *
+         * We'll correct this error next time through this function, when
+         * xtime_nsec is not as small.
+         */
+        if (unlikely((s64)clock->xtime_nsec < 0)) {
+                s64 neg = -(s64)clock->xtime_nsec;
+                clock->xtime_nsec = 0;
+                clock->error += neg << (NTP_SCALE_SHIFT - clock->shift);
+        }
+
         /* store full nanoseconds into xtime after rounding it up and
          * add the remainder to the error difference.
          */