Re: do_posix_clock_monotonic_gettime() returns negative nsec

[George Anzinger]

Herbert Poetzl wrote:
> On Thu, Dec 02, 2004 at 07:08:23PM -0800, Andrew Morton wrote:
>
> > Herbert Poetzl <herbert@xxxxxxxxxxxx> wrote:
> >
> > > Hi Folks!
> > >
> > > recent kernels (tested 2.6.10-rc2 and 2.6.10-rc2-bk15)
> > > produce funny output in /proc/uptime like this:
> > >
> > > 	# cat /proc/uptime
> > > 	  12.4294967218 9.05
> > > 	# cat /proc/uptime
> > > 	  13.4294967251 10.33
> > > 	# cat /proc/uptime
> > > 	  14.4294967295 11.73
> > >
> > > a short investigation of the issue, ended at
> > > do_posix_clock_monotonic_gettime() which can (and 
> > > often does) return negative nsec values (within
> > > one second), so while the actual 'time' returned
> > > is correct, some parts of the kernel assume that
> > > those part is within the range (0 - NSEC_PER_SEC)
> > >
> > >        len = sprintf(page,"%lu.%02lu %lu.%02lu\n",
> > >                        (unsigned long) uptime.tv_sec,
> > >                        (uptime.tv_nsec / (NSEC_PER_SEC / 100)),
> > >
> > > as the function itself corrects overflows, it would
> > > make sense to me to correct underflows too, for 
> > > example with the following patch:
> > >
> > > --- ./kernel/posix-timers.c.orig	2004-11-19 21:11:05.000000000 +0100
> > > +++ ./kernel/posix-timers.c	2004-12-03 02:23:56.000000000 +0100
> > > @@ -1208,7 +1208,10 @@ int do_posix_clock_monotonic_gettime(str
> > > 	tp->tv_sec += wall_to_mono.tv_sec;
> > > 	tp->tv_nsec += wall_to_mono.tv_nsec;
> > >
> > > -	if ((tp->tv_nsec - NSEC_PER_SEC) > 0) {
> > > +	if (tp->tv_nsec < 0) {
> > > +		tp->tv_nsec += NSEC_PER_SEC;
> > > +		tp->tv_sec--;
> > > +	} else if ((tp->tv_nsec - NSEC_PER_SEC) > 0) {
> > > 		tp->tv_nsec -= NSEC_PER_SEC;
> > > 		tp->tv_sec++;
> > > 	}
> >
> > Doesn't this imply that do_posix_clock_monotonic_gettime_parts() is
> > returning a negative tv_nsec?
>
>
> nope, not necessarily, because after that ...
>
>         tp->tv_sec += wall_to_mono.tv_sec;
>         tp->tv_nsec += wall_to_mono.tv_nsec;
>
> might add a negative value, which explains the
> underflow ...
>
> and if you look closer:
>
> 	xtime.tv_sec = get_cmos_time();
>         wall_to_monotonic.tv_sec = -xtime.tv_sec;
>         xtime.tv_nsec = (INITIAL_JIFFIES % HZ) * (NSEC_PER_SEC / HZ);
>         wall_to_monotonic.tv_nsec = -xtime.tv_nsec;

Yep, that IS the problem.  It should be normalized here,  I.e.
	set_normalized_timespec(wall_to_monotonic,
		wall_to_monotonic.tv_sec - xtime.tv_sec,
		wall_to_monotonic.tv_nsec - xtime.tv_nsec);
with the obvious delets :)

Still, this should be corrected by the first settimeofday, which most systems do 
on the way up, or is that just those who use NTP?


--
George Anzinger   george@xxxxxxxxxx
High-res-timers:  http://sourceforge.net/projects/high-res-timers/

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