Re: [RFC][PATCH] Catch xtime_nsec underflows and fix them

From: Roman Zippel
Date: Tue Dec 02 2008 - 22:49:34 EST


On Wed, 3 Dec 2008, Zhang, Yanmin wrote:

> > This doesn't explain the problem entirely, I considered a negative
> > xtime_nsec before, but xtime_nsec+offset should still be positive
> ïxtime_nsec underflows after ïclocksource_adjust. Before ïclocksource_adjust,
> ïxtime_nsec is a small positive.
> When ïïxtime_nsec underflows at the first time, xtime.tv_nsec becomes -1.
> Later on when the second tick arrives, below statement in the while loop
> clock->xtime_nsec += clock->xtime_interval;
> will cause ïclock->xtime_nsec becomes positive again. So the second tick
> appears a going-backward time.

Yes, but only by 1nsec, so normally it wouldn't be noticable.

> > and
> > produce the correct result, at least I can't find anything in
> > getnstimeofday().
> The testing uses vsyscall to get call gettimeofday. vsyscall_gtod_data.wall_time_nsec
> is a u32 while timespec->tv_nsec is a signed long.

Ok, I was missing this part, I looked at the 32bit version of
getnstimeofday() and there xtime.tv_nsec was correctly sign extended.
To be safe for the future wall_time_nsec should also be a s32.

bye, Roman