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

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Date:	Fri, 17 Sep 2010 11:33:25 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Alexander Shishkin <virtuoso@...nd.org>
Cc:	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Kay Sievers <kay.sievers@...y.org>, 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

> 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 !

> This patch implements a notification interface via eventfd mechanism. Proccess
> wishing to be notified about time changes should create an eventfd and pass it
> to time_change_notify() syscall along with notification options.

This seems complete overkill and it doesn't really help applications much
that I can see because of suspend/resume.

What are your actual use cases ?

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.

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.

Alan
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