Merge branch 'timers-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'timers-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  time: catch xtime_nsec underflows and fix them
  posix-cpu-timers: fix clock_gettime with CLOCK_PROCESS_CPUTIME_ID

2 files changed, 23 insertions, 1 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 895337b16a24..4e5288a831de 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -311,7 +311,7 @@ static int cpu_clock_sample_group(const clockid_t which_clock,
         struct task_cputime cputime;
 
         thread_group_cputime(p, &cputime);
-        switch (which_clock) {
+        switch (CPUCLOCK_WHICH(which_clock)) {
         default:
                 return -EINVAL;
         case CPUCLOCK_PROF:
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.
          */