linux-kernel - Re: [PATCH] [RFC] notify userspace about time changes

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Message-ID: <20100805100535.GA1754@shutemov.name>
Date:	Thu, 5 Aug 2010 13:05:35 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	lkml@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>, "H. Peter Anvin" <hpa@...or.com>,
	John Kacur <jkacur@...hat.com>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH] [RFC] notify userspace about time changes

On Wed, Aug 04, 2010 at 06:46:38PM +0300, Alexander Shishkin wrote:
> On Wed, Aug 04, 2010 at 06:32:21 +0300, Kirill A. Shutemov wrote:
> > On Wed, Aug 04, 2010 at 03:48:28PM +0300, Alexander Shishkin wrote:
> > > Certain userspace applications (like "clock" desktop applets or ntpd) 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.
> > > 
> > > This patch implements a notification interface via eventfd mechanism. Proccess
> > > wishing to be notified about time changes should create an eventfd and echo
> > > its file descriptor to /sys/kernel/time_notify. After that, any calls to
> > > settimeofday()/stime()/adjtimex() made by other processes will be signalled
> > > to this eventfd. Credits for suggesting the eventfd mechanism for this
> > > purpose go te Kirill Shutemov.
> > > 
> > > So far, this implementation can only filter out notifications caused by
> > > time change calls made by the process that wrote the eventfd descriptor to
> > > sysfs, but not its children which (might) have inherited the eventfd. It
> > > is so far not clear to me whether this is bad and more confusing than
> > > excluding such children as well.
> > 
> > I think it's a bad idea to filter notifications. Let's leave it for
> 
> Why?

Someone might want to recieve notifications about its own activity.
It's a policy. Kernel should provide mechanism, not policy.

-- 
 Kirill A. Shutemov
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