linux-kernel - Re: [PATCH 1/7] notify userspace about time changes

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Date:	Fri, 17 Sep 2010 12:22:36 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Alexander Shishkin <virtuoso@...nd.org>,
	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Greg KH <gregkh@...e.de>,
	Chris Friesen <chris.friesen@...band.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jon Hunter <jon-hunter@...com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Avi Kivity <avi@...hat.com>, John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH 1/7] notify userspace about time changes

On Fri, Sep 17, 2010 at 12:33, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> Certain userspace applications (like "clock" desktop applets or cron) might
>> want to be notified when some other application changes the system time. It
>> might also be important for an application to be able to distinguish between
>> its own and somebody else's time changes.
>
> A program that cannot work out if it or someone else changed the time is
> very very broken indeed !

Yeah, that seems a bit weird to me too.

> Clocks apps don't care because they check the actual time so notice it
> shfited. Cron and anacron appear to contain the needed internal handling.

Cron wakes up every minute to check if the time has changed. We don't
want such silly behavior, but there are no other options at the moment
for scheduling re-occurring events.

> Anything sleeping until a time occurs maybe ? In which case its a lot
> simpler and cleaner than events to provide a new itimer which wakes the
> process when the wall time hits the time specified in the timer.

That seems not sufficient. The details should be in the changelog of the patch.

Kay
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