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

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Message-ID: <AANLkTinOxPoyPx3ERFKGFA+iLkvv==1=AYdtMsssvfRs@mail.gmail.com>
Date:	Fri, 6 Aug 2010 00:39:54 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Greg KH <gregkh@...e.de>, 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>, John Kacur <jkacur@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] notify userspace about time changes

On Fri, Aug 6, 2010 at 00:34, H. Peter Anvin <hpa@...or.com> wrote:
> On 08/05/2010 03:22 PM, Kay Sievers wrote:
>>
>> 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. :)
>>
>
> Okay, what's wrong with having a file descriptor that gets *written to*
> on a notification?

Because it needs documentation, and is just not needed for such a
simple thing, I think. Why would you want to write a fd number to a
magic file, which can be your fd right away, even passing you the data
on read().

> Why POLL_ERR?

Because normal files can not be poll()ed, and it's not that new data
has arrived, it just tells you to rewind and read it again. It's
commonly used:
  http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5addc5dd8836aa061f6efc4a0d9ba6323726297a

Kay
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