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Commit 2a8c0883 authored by John Stultz's avatar John Stultz Committed by Thomas Gleixner
Browse files

time: Move xtime_nsec adjustment underflow handling timekeeping_adjust 



When we make adjustments speeding up the clock, its possible
for xtime_nsec to underflow. We already handle this properly,
but we do so from update_wall_time() instead of the more logical
timekeeping_adjust(), where the possible underflow actually
occurs.

Thus, move the correction logic to the timekeeping_adjust, which
is the function that causes the issue. Making update_wall_time()
more readable.

Signed-off-by: default avatarJohn Stultz <johnstul@us.ibm.com>
Reviewed-by: default avatarIngo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Link: http://lkml.kernel.org/r/1342156917-25092-8-git-send-email-john.stultz@linaro.org


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent f2a5a085

kernel/time/timekeeping.c

+21 −21
Original line number Diff line number Diff line
@@ -980,6 +980,27 @@ static void timekeeping_adjust(s64 offset) 
	timekeeper.xtime_nsec -= offset;

	timekeeper.ntp_error -= (interval - offset) <<

				timekeeper.ntp_error_shift;



	/*

	 * It may be possible that when we entered this function, xtime_nsec

	 * was very small.  Further, if we're slightly speeding the clocksource

	 * in the code above, its possible the required corrective factor to

	 * xtime_nsec could cause it to underflow.

	 *

	 * Now, since we already accumulated the second, 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)timekeeper.xtime_nsec < 0)) {

		s64 neg = -(s64)timekeeper.xtime_nsec;

		timekeeper.xtime_nsec = 0;

		timekeeper.ntp_error += neg << timekeeper.ntp_error_shift;

	}



}
@@ -1105,27 +1126,6 @@ static void update_wall_time(void) 
	/* correct the clock when NTP error is too big */

	timekeeping_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 timekeeping_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)timekeeper.xtime_nsec < 0)) {

		s64 neg = -(s64)timekeeper.xtime_nsec;

		timekeeper.xtime_nsec = 0;

		timekeeper.ntp_error += neg << timekeeper.ntp_error_shift;

	}



	/*

	* Store only full nanoseconds into xtime_nsec after rounding