linux-kernel - Re: [patch 3/3] timerfd: Implement write method

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Message-ID: <CAHO5Pa3NFndwZUKGdEt8AYnYLXvd4aJB1RdWOQ-Kcht+Y0WEyA@mail.gmail.com>
Date:	Thu, 22 May 2014 08:32:45 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	shawn@...rchofgit.com, Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Vagin <avagin@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>, vdavydov@...allels.com,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Subject: Re: [patch 3/3] timerfd: Implement write method

[Thomas, thanks for pinging me on this.]

Hi Cyril

Please CC linux-api on changes that affect kernel-user-space ABI/API.

On Thu, May 22, 2014 at 12:35 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Thu, May 22, 2014 at 07:12:30AM +0900, Thomas Gleixner wrote:
>>
>> There is a world outside of checkpoint/restore, really.
>
> Yes, I simply don't know who else might use this write()
> functionality for other purpose, I mean i don't see a
> point to use it for anything else.

Nevertheless, when making API changes like this, we should be thinking
as generally as possible, rather than looking from the perspective of
a single use case.

>> So what's the semantics of that write function? We really want to have
>> that agreed on and documented in the man page.
>
> The idea was to provide a way to setup @ticks into (nonzero) value
> which we get from show_fdinfo output. Then when we restore it
> we setup the timer and set @ticks to the value it had at dump
> moment.
>
>> Right now the write will just update the ticks and nothing else. So
>> what if there is a waiter already? What if there is a timer armed?
>>
>> Can you please describe how checkpoint/restore is going to use all of
>> this. How is the timer restored and how/when is the reader which was
>> waiting in read/poll at the time of suspend reattached to it.
>
> Thomas, I see what you mean. Need to think (I must admit I forgot about
> polling of timerfds :( I were to restore timerfds like this
>
>  - fetch data from fdinfo
>  - use timer_create/settime to arm it
>  - write @ticks then
>
> but i didn't try restore polling waiters, my bad. Letme rework this
> trying addressing your comments.

Great. Please CC me and linux-api@ on the next round.

Cheers,

Michael

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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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