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

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Date:	Wed, 3 Dec 2008 04:48:16 +0100 (CET)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
cc:	john stultz <johnstul@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, alex.shi@...el.com,
	Thomas Gleixner <tglx@...x.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [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