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

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Message-ID: <cfd18e0f0806242316g11fe654gf23bb0c60f29ac3f@mail.gmail.com>
Date:	Wed, 25 Jun 2008 08:16:56 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
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 Wed, Jun 25, 2008 at 8:13 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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 Thomas -- that's what I was beginning to understand, butit's
nice to have confirmation.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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