Re: RFC: A revised timerfd API

[Thomas Gleixner <tglx@linutronix.de>]

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