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

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Message-ID: <AANLkTimYmtXZe1Ue-z2D_5512RXz3xUSRWPiUUUY6rrz@mail.gmail.com>
Date:	Fri, 6 Aug 2010 00:22:27 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Greg KH <gregkh@...e.de>
Cc:	john stultz <johnstul@...ibm.com>,
	Alexander Shishkin <virtuoso@...nd.org>, 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 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.

I would very much prefer a file that contains the current time, and
wakes up possible users with a POLL_ERR on changes caused by some
other process. That works very well for things like /proc/mounts, is
easy to get, and does not need a full page of weird instructions to
get stuff done. :)

Kay
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