linux-kernel - Re: [PATCH] Revised timerfd() interface

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Message-ID: <20070905225033.236700@gmx.net>
Date:	Thu, 06 Sep 2007 00:50:33 +0200
From:	"Michael Kerrisk" <mtk-manpages@....net>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	rdunlap@...otime.net, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	drepper@...hat.com, stable@...nel.org, hch@....de,
	jengelh@...putergmbh.de, corbet@....net, akpm@...ux-foundation.org
Subject: Re: [PATCH] Revised timerfd() interface

Hi Davide,

> > > > > > As I think about this more, I see more problems with
> > > > > > your argument.  timerfd needs the ability to get and 
> > > > > > get-while-setting just as much as the earlier APIs.
> > > > > > Consider a library that creates a timerfd file descriptor that
> > > > > > is handed off to an application: that library may want
> > > > > > to modify the timer settings without having to create a
> > > > > > new file descriptor (the app mey not be able to be told about
> > > > > > the new fd).  Your argument just doesn't hold, AFAICS.
> > > > > 
> > > > > Such hypotethical library, in case it really wanted to offer such 
> > > > > functionality, could simply return an handle instead of the raw
> > > > > fd, and take care of all that stuff in userspace.
> > > > 
> > > > Did I miss something?  Is it not the case that as soon as the
> > > > library returns a handle, rather than an fd, then the whole
> > > > advantage of timerfd() (being able to select/poll/epoll on 
> > > > the timer as well as other fds) is lost?  
> > > 
> > > Why? The handle would simply be a little struct where the timerfd fd
> > > is 
> > > stored, and a XXX_getfd() would return it.
> > > So my point is, I doubt such functionalities are really needed, and I 
> > > also argue that the kernel is the best place for such wrapper code
> > > to go.
> > 
> > So what happens if one thread (via the library) wants modify
> > a timer's settings at the same timer as another thread is 
> > select()ing on it?  The first thread can't do this by creating
> > a new timerfd timer, since it wants to affect the select()
> > in the other thread?
> 
> It can be done w/out any problems. The select thread will be notified 
> whenever the new timer setting expires.

We are going in circles here.  I think you are missing my point.
Consider the following

[[
Thread A: calls library function which creates a timerfd file
descriptor.

Thread B: calls select() on the timerfd file descriptor.

Thread A: calls library function which wants to:
   a) modify timer settings, and retrieve copy of current timer
      settings, and later
   b) restore old timer settings.
]]

This seems a quite reasonable use-case to me, and the existing
interface simply can't support it.

Cheers,

Michael
-- 
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7 

Want to help with man page maintenance?  
Grab the latest tarball at
http://www.kernel.org/pub/linux/docs/manpages , 
read the HOWTOHELP file and grep the source 
files for 'FIXME'.

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