Subject: Re: RFC: A revised timerfd API
From: Thomas Gleixner <tglx@linutronix.de>
To: Michael Kerrisk <mtk-manpages@gmx.net>
Cc: "\"David" =?ISO-8859-1?Q?H=E4rdeman=22?= <david@hardeman.nu>,
       lee.schermerhorn@hp.com, torvalds@linux-foundation.org,
       vda.linux@googlemail.com, rdunlap@xenotime.net, corbet@lwn.net,
       hch@lst.de, akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       geoff@gclare.org.uk, drepper@redhat.com, davidel@xmailserver.org
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Date: Tue, 18 Sep 2007 11:27:15 +0200
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On Tue, 2007-09-18 at 11:01 +0200, Michael Kerrisk wrote:
> > With solution c) you have to keep two
> > references to the same timer around and use one of them depending on what
> > you want to do with the timer.
> 
> Yes.  (And the same for option (d).)
> 
> > Also, if the timerfd is close():d, does that remove the underlying timer
> > (invalidate the timerid) as well?
> 
> My gut feeling would be to say that closing the timerfd would not
> remove the underlying timer (so the timerid would remain valid).
> One could even do things like recreating a file descriptor
> for the timer using another timerfd() call.  
> 
> But now that raises the question: what are the semantics if
> timerfd() is called  more than once on the same timerid? 
> Perhaps a read() from any one of them (destructively)
> reads the expiration count, as though one had read from a 
> dup()ed the file descriptor.  All in all, solution (c) 
> starts to look overly complex, and maybe suffers from 
> various dirty corners in the API.  (Solution (d) feels 
> slightly better, because the creation of the file descriptor
> and the timerid are integrated into a single call, and the
> fact that it integrates with an existing API, but
> it still has the limitation you describe above.)

I don't think it is a big problem to have several open file descriptors
on a single posix timer without having destructive reads, we just need
to store the event count per file descriptor in file->private_data. We
solved this in the UIO code already and it works perfectly fine.

	tglx


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