![]() 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 update_wall_time(): 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 cycle_interval. 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 http://bugzilla.kernel.org/show_bug.cgi?id=11970 Reported-by: alex.shi@intel.com Signed-off-by: John Stultz <johnstul@us.ibm.com> Acked-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com> Tested-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> |
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.. | ||
clockevents.c | ||
clocksource.c | ||
jiffies.c | ||
Kconfig | ||
Makefile | ||
ntp.c | ||
tick-broadcast.c | ||
tick-common.c | ||
tick-internal.h | ||
tick-oneshot.c | ||
tick-sched.c | ||
timekeeping.c | ||
timer_list.c | ||
timer_stats.c |