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

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Message-ID: <alpine.DEB.2.02.1405220826460.9695@ionos.tec.linutronix.de>
Date:	Thu, 22 May 2014 08:30:04 +0900 (JST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Cyrill Gorcunov <gorcunov@...il.com>
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, 22 May 2014, Cyrill Gorcunov 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.
> 
> > 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.

That's not describing the semantics. It's describing what you use it
for.
 
> > 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

That's clear to me.

So again you have to answer the questions:

   Do we just allow the write unconditionally?
   Do we care about waking readers/pollers?
 
Whatever the answer is, it needs to be documented coherently in the
changelog, in the code and in the man page.

Thanks,

	tglx
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