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

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Message-ID: <20140521223511.GI12819@moon>
Date:	Thu, 22 May 2014 02:35:11 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, shawn@...rchofgit.com,
	akpm@...ux-foundation.org, avagin@...nvz.org, xemul@...allels.com,
	vdavydov@...allels.com
Subject: Re: [patch 3/3] timerfd: Implement write method

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.

> 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.
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