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 15:45:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kay Sievers <kay.sievers@...y.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, 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.

The whole what to do with appointments when switching timezones wasn't
mentioned, but that too should be able to be sorted with a clkadjust
notifier.

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