linux-kernel - Re: nanosleep() uses CLOCK_MONOTONIC, should be CLOCK

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Date:	Wed, 25 Jun 2008 08:13:57 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Michael Kerrisk <mtk.manpages@...glemail.com>
cc:	Roman Zippel <zippel@...ux-m68k.org>,
	Bart Van Assche <bart.vanassche@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: nanosleep() uses CLOCK_MONOTONIC, should be CLOCK_REALTIME?

On Tue, 24 Jun 2008, Michael Kerrisk wrote:
> > If you check the man page for clock_settime, it specifically
> > mentions that pending relative timer (including nanosleep) aren't affected
> > by the changed time, thus if CLOCK_MONOTONIC and CLOCK_REALTIME advance
> > equally, it doesn't matter which you use for relative timer.
> 
> Well, I was going to say that that's just a man page, and man page
> authors are fallible ;-).  But then I went and had a look at the POSIX
> spec for clock_settime().  It includes the following paragraph:
> 
>        Setting the value of the CLOCK_REALTIME  clock  via  clock_set-
>        time() shall have no effect on threads that are blocked waiting
>        for a relative time service based upon  this  clock,  including
>        the  nanosleep()  function;  nor  on the expiration of relative
>        timers  based  upon  this  clock.   Consequently,  these   time
>        services  shall  expire  when  the  requested relative interval
>        elapses, independently of the new or old value of the clock.
> 
> So that rather flatly contradicts the alternative semantics that I
> suggested were possible in my reply to Bart a few minutes ago.
> 
> So in my reading of things now, it looks as though the Linux
> implementation is probably fine, since the fact that relative
> timers/sleeps are explicitly unaffected by jumps in CLOCK_REALTIME
> means that the semantics are effectively the same as if the relative
> interval was measured against CLOCK_MONOTONIC (unless the two clocks
> counted time at different rates; not sure if that  would be possible
> in theory, but certainly seems very unlikely in practice).

We use CLOCK_MONOTONIC for the relative timeouts simply to avoid
trickery vs. clock_settime(CLOCK_REALTIME). That's an kernel internal
implementation detail which does not have any visible effect to the
user space interface.

CLOCK_MONOTONIC and CLOCK_REALTIME are using the same timebase
internally and therefor we can safely use CLOCK_MONOTONIC for the
relative timeouts.

Thanks,

	tglx
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