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

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Message-ID: <20100917110803.GC10463@shisha.kicks-ass.net>
Date:	Fri, 17 Sep 2010 14:08:03 +0300
From:	Alexander Shishkin <virtuoso@...nd.org>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, 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>,
	Alexander Shishkin <virtuoso@...nd.org>
Subject: Re: [PATCH 1/7] notify userspace about time changes

On Fri, Sep 17, 2010 at 12:22:36 +0200, Kay Sievers wrote:
> 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.

Ok, I get it. Filtering has to go.

> > 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.

Yes, I did include your description of the problem in the 0/7, but
that doesn't seem sufficient. I really need to compile all the
usecases from previous threads and include them in 1/7 as well.

Regards,
--
Alex
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