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

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Date:	Thu, 05 Aug 2010 15:43:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kay Sievers <kay.sievers@...y.org>
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 08/05/2010 03:39 PM, Kay Sievers wrote:
>>
>> 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().
> 

I didn't mean that, I meant a note that you open and get a pipe/socket/*.

>> 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

It makes sense there, I guess, as some kind of sideband notification is
highly useful.  Too bad we don't have a generic mechanism on files other
than inotify... a lot of things could use it (including tail, which I
think uses inotify now.)

	-hpa

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