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

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Date:	Fri, 6 Aug 2010 16:44:10 -0700
From:	Greg KH <gregkh@...e.de>
To:	Kay Sievers <kay.sievers@...y.org>,
	john stultz <johnstul@...ibm.com>, lkml@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Thomas Gleixner <tglx@...utronix.de>,
	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
Subject: Re: [PATCH] [RFC] notify userspace about time changes

On Fri, Aug 06, 2010 at 10:21:45AM +0300, Alexander Shishkin wrote:
> On Thu, Aug 05, 2010 at 03:38:54 -0700, Greg KH wrote:
> > On Fri, Aug 06, 2010 at 12:22:27AM +0200, Kay Sievers wrote:
> > > On Thu, Aug 5, 2010 at 23:38, Greg KH <gregkh@...e.de> wrote:
> > > > On Thu, Aug 05, 2010 at 02:11:05PM -0700, john stultz wrote:
> > > >> On Thu, 2010-08-05 at 15:33 +0300, Alexander Shishkin wrote:
> > > >> > On 4 August 2010 18:58, john stultz <johnstul@...ibm.com> wrote:
> > > >> > > Is there a actual use case that you need this for?  I don't really have
> > > >> > > an issue with the code I just really want to make sure the feature would
> > > >> > > be useful enough to justify the API and code maintenance going forward.
> > > >> >
> > > >> > Yes. What we have here is an application which takes care of different means
> > > >> > of time synchronization (trusted time servers, different GSM operators, etc)
> > > >> > and also different kinds of time-based events/notifications (like "dentist
> > > >> > appointment next thursday"). When it encounters a time change that is
> > > >> > made by some other application, it basically wants to disable automatic
> > > >> > time adjustment and trigger the events/notifications which are due at this
> > > >> > (new) time.
> > > >>
> > > >> Ok. Something specific is always more helpful then theoretical uses.
> > > >>
> > > >> I think the filtering is still a bit controversial, so you might want to
> > > >> respin it without that. But otherwise I'm ok with it as long as no one
> > > >> else objects to any of the minor details of the interface
> > > >>
> > > >> GregKH: Does /sys/kernel/time_notify seem ok by you?
> > > >
> > > > Um, it depends, what is that file going to do?  I don't see a
> > > > Documentation/ABI/ entry here that describes it fully :)
> > > 
> > > I think that's really awkward interface, to pass file descriptor
> > > numbers around and write them to magic sysfs files.
> > 
> > Ick, really?  That's not ok for sysfs.
> 
> This is how cgroups notifications work [1], for example.

Yes, but that's not the way that sysfs works.  sysfs is
one-value-per-file simple text files.  With a few binary files that are
"pass-through" to the hardware.

It doesn't have these types of "magic" files, please don't try to add
them without a very large discussion, and boatloads of documentation :)

thanks,

greg k-h
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