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

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Message-ID: <Pine.LNX.4.64.0709051251290.11804@alien.or.mcafeemobile.com>
Date:	Wed, 5 Sep 2007 12:57:24 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Michael Kerrisk <mtk-manpages@....net>
cc:	Andrew Morton <akpm@...ux-foundation.org>, corbet@....net,
	jengelh@...putergmbh.de, hch@....de, stable@...nel.org,
	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	tglx@...utronix.de, rdunlap@...otime.net
Subject: Re: [PATCH] Revised timerfd() interface

On Wed, 5 Sep 2007, Michael Kerrisk wrote:

> 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.


- Davide


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