Re: [Xen-devel] [PATCH 02/20] libxl: support multiple libxl__ev_fds for the same fd

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-04-16 at 11:26 +0100, Ian Jackson wrote:
> Ian Campbell writes ("Re: [Xen-devel] [PATCH 02/20] libxl: support multiple 
> libxl__ev_fds for the same fd"):
> > On Fri, 2012-04-13 at 19:39 +0100, Ian Jackson wrote:
> > > We need a slightly more sophisticated data structure to allow this,
> > > where we record the slot not just for each fd but also for each
> > > (fd,eventbit) where eventbit is POLLIN, POLLPRI, POLLOUT.
> > 
> > Just to be sure I'm following: By multiple you mean you can have one
> > libxl__ev_fds listening for e.g. POLLIN and another for POLLOUT but you
> > specifically exclude the case where two libxl__ev_fds both want to
> > listen for POLLIN? Similarly one listening for POLLIN|POLLPRI and the
> > other for POLLOUT|POLLPRI (overlapping) is forbidden.
> 
> Yes, exactly.  As I say in the new doc comment:
> +   *
> +   * It is not permitted to listen for the same or overlapping events
> +   * on the same fd using multiple different libxl__ev_fd's.
> 
> This restriction is actually stronger than that required by the code.
> The code merely requires that for every distinct libxl__ev_fd
> listening on the same fd, it has at least one of the three flags all
> to itself.  So your second scenario would actually work.
> 
> Would it be worth documenting this precise restriction ?

I guess it would just confuse the language but only really add anything
for what is a pretty small corner case?

Since it's a libxl internal thing I'm not too concerned about people
coming to rely on the actual broader behaviour.

> 
> > > +     * fd_rindices, and each elemebnt in the rindices is three indices
> > 
> >                                element
> 
> Fixed.
> 
> > So, other than the typoe:
> > 
> > Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 
> Thanks.
> 
> Ian.



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