Re: do_posix_clock_monotonic_gettime() returns negative nsec

[George Anzinger]

john stultz wrote:
> On Thu, 2004-12-02 at 18:03, Herbert Poetzl wrote:
>
> > 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++;
> > 	}
>
>
> Sounds like its a good fix to me. 
>
> George: You have any comment?

Two, in fact.  First, the result here is the sum of wall_to_monotonic and 
getnstimeofday().  If nsec < 0, one or more of these must be also.  Both of 
these values are SUPPOSED to be normalized.

Second, I would rather see:
	set_normalized_timespec(tp, tp->tv_sec + wall_to_mono.tv_sec, 		
				tp->tv_nsec + wall_to_mono.tv_nsec);

Still, doing this paves over the first issue....

> thanks
> -john

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

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