• john stultz's avatar
    time: catch xtime_nsec underflows and fix them · 6c9bacb4
    john stultz authored
    Impact: fix time warp bug
    Alex Shi, along with Yanmin Zhang have been noticing occasional time
    inconsistencies recently. Through their great diagnosis, they found that
    the xtime_nsec value used in update_wall_time was occasionally going
    negative. After looking through the code for awhile, I realized we have
    the possibility for an underflow when three conditions are met in
      1) We have accumulated a second's worth of nanoseconds, so we
         incremented xtime.tv_sec and appropriately decrement xtime_nsec.
         (This doesn't cause xtime_nsec to go negative, but it can cause it
          to be small).
      2) The remaining offset value is large, but just slightly less then
      3) clocksource_adjust() is speeding up the clock, causing a
         corrective amount (compensating for the increase in the multiplier
         being multiplied against the unaccumulated offset value) to be
         subtracted from xtime_nsec.
    This can cause xtime_nsec to underflow.
    Unfortunately, since we notify the NTP subsystem via second_overflow()
    whenever we accumulate a full second, and this effects the error
    accumulation that has already occured, we cannot simply revert the
    accumulated second from xtime nor move the second accumulation to after
    the clocksource_adjust call without a change in behavior.
    This leaves us with (at least) two options:
    1) Simply return from clocksource_adjust() without making a change if we
       notice the adjustment would cause xtime_nsec to go negative.
    This would work, but I'm concerned that if a large adjustment was needed
    (due to the error being large), it may be possible to get stuck with an
    ever increasing error that becomes too large to correct (since it may
    always force xtime_nsec negative). This may just be paranoia on my part.
    2) Catch xtime_nsec if it is negative, then add back the amount its
       negative to both xtime_nsec and the error.
    This second method is consistent with how we've handled earlier rounding
    issues, and also has the benefit that the error being added is always in
    the oposite direction also always equal or smaller then the correction
    being applied. So the risk of a corner case where things get out of
    control is lessened.
    This patch fixes bug 11970, as tested by Yanmin Zhang
    Reported-by: alex.shi@intel.com
    Signed-off-by: default avatarJohn Stultz <johnstul@us.ibm.com>
    Acked-by: default avatar"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
    Tested-by: default avatar"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
timekeeping.c 15.2 KB