linux-kernel - Re: [tip:timers/core] timerfd: Allow timers to be cancelled when clock was set

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Date:	Wed, 11 May 2011 15:52:14 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	johnstul@...ibm.com, davidel@...ilserver.org, virtuoso@...nd.org,
	tglx@...utronix.de, chris.friesen@...band.com,
	kirill@...temov.name, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/core] timerfd: Allow timers to be cancelled when
 clock was set

On Wed, May 11, 2011 at 15:45, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2011-05-11 at 13:58 +0200, Kay Sievers wrote:
>> On Wed, May 11, 2011 at 12:23, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Mon, 2011-05-02 at 19:44 +0000, tip-bot for Thomas Gleixner wrote:
>> >> Commit-ID:  99ee5315dac6211e972fa3f23bcc9a0343ff58c4
>> >> Gitweb:     http://git.kernel.org/tip/99ee5315dac6211e972fa3f23bcc9a0343ff58c4
>> >> Author:     Thomas Gleixner <tglx@...utronix.de>
>> >> AuthorDate: Wed, 27 Apr 2011 14:16:42 +0200
>> >> Committer:  Thomas Gleixner <tglx@...utronix.de>
>> >> CommitDate: Mon, 2 May 2011 21:39:15 +0200
>> >>
>> >> timerfd: Allow timers to be cancelled when clock was set
>> >>
>> >> Some applications must be aware of clock realtime being set
>> >> backward. A simple example is a clock applet which arms a timer for
>> >> the next minute display. If clock realtime is set backward then the
>> >> applet displays a stale time for the amount of time which the clock
>> >> was set backwards. Due to that applications poll the time because we
>> >> don't have an interface.
>> >
>> > Shouldn't that clock applet use CLOCK_MONOTONIC for the timer anyway?
>>
>> Like: Hey let's meet at the bar 5 hours after bootup? :)
>
> Uhm, the example was a timer to update the displayed time, so a timer
> like: wake me next minute, suffices.

No, fixed time spans have never been a problem, and are not the
example here. It's about the normal wall clock, that wakes up every
minute and updates the numbers on the screen.

Kay
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