lkml.org 
[lkml]   [2008]   [Dec]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC][PATCH] Catch xtime_nsec underflows and fix them
Hi,

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
> xtime_nsec underflows after clocksource_adjust. Before clocksource_adjust,
> xtime_nsec is a small positive.
>
> When xtime_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 clock->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
\
 
 \ /
  Last update: 2008-12-03 04:51    [W:0.321 / U:0.052 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site