linux-kernel - Re: RFC: A revised timerfd API

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Message-ID: <20070918110841.235150@gmx.net>
Date:	Tue, 18 Sep 2007 13:08:41 +0200
From:	"Michael Kerrisk" <mtk-manpages@....net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	david@...deman.nu, davidel@...ilserver.org, drepper@...hat.com,
	geoff@...are.org.uk, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, hch@....de, corbet@....net,
	rdunlap@...otime.net, vda.linux@...glemail.com,
	torvalds@...ux-foundation.org, Lee.Schermerhorn@...com
Subject: Re: RFC: A revised timerfd API

Hi Thomas,

> Von: Thomas Gleixner <tglx@...utronix.de>
> On Tue, 2007-09-18 at 11:30 +0200, Michael Kerrisk wrote:
> > > This way we have it nicely integrated into the posix timer code and keep
> > > the existing semantics of posix timers intact.
> > > 
> > > We need to think about the open file descriptor in the timer_delete()
> > > case as well, but this should be not too hard to sort out.
> > 
> > This seems like a workable idea also.  But note David Härdeman's
> > critique of options c & d: the existence of a coupled timerfd 
> > and a timerid means that the application must maintain a mapping
> > between the two, so that after an epoll call (for example) that 
> > says the timerfd is ready, the timer can be manipulated using
> > the corresponding timerfd.  This isn't IMO a fatal flaw, but
                      ^^^^^^^
hmmm, of course I meant "timerid" in that last line.

> > it does make the API a little more clumsy.
> 
> Hmm, we might do something like:
> 
> 	timer_gettime(fd | POSIX_TIMER_FD, .....);
> 
> So the kernel looks up the fd in order to figure out the timer_id, which
> needs to be referenced in filep->private_data anyway.

And you'd need similar for timer_settime() and, perhaps,
timer_getoverrun().  But it seems slightly ugly, in the same way that
my idea in option (d) of returning a file descriptor from
timer_create() seems a slightly ugly.  (And can we guarantee that
the [timerid] space is distinct from the [fd|POSIX_TIMER_FD] space?)

Cheers,

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

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