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

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Message-ID: <20070905162352.236680@gmx.net>
Date:	Wed, 05 Sep 2007 18:23:52 +0200
From:	"Michael Kerrisk" <mtk-manpages@....net>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	akpm@...ux-foundation.org, corbet@....net, jengelh@...putergmbh.de,
	hch@....de, stable@...nel.org, drepper@...hat.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, rdunlap@...otime.net
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?

Cheers,

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

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