Re: [Xen-devel] [PATCH 5/6] libxl: events: Deregister evtchn fd when not needed

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-11-28 at 14:47 +0000, Ian Jackson wrote:
> Ian Campbell writes ("Re: [PATCH 5/6] libxl: events: Deregister evtchn fd 
> when not needed"):
> > On Thu, 2014-11-27 at 18:27 +0000, Ian Jackson wrote:
> > > We want to have no fd events registered when we are idle.
> > > In this patch, deal with the evtchn fd:
> > > 
> > >  * Defer setup of the evtchn handle to the first use.
> > >  * Defer registration of the evtchn fd; register as needed on use.
> > >  * When cancelling an evtchn wait, or when wait setup fails, check
> > >    whether there are now no evtchn waits and if so deregister the fd.
> > >  * On libxl teardown, the evtchn fd should therefore be unregistered.
> > >    assert that this is the case.
> > 
> > Is there no locking required when registering/deregistering? Since there
> > are list manipulations in most of these places I presume it already
> > exists, but I thought I should check.
> 
> libxl__ev_evtchn_* is always called with the ctx lock held.

For the most part this is implicit due to the caller being in an ao
callback, right?

> However, that they don't take the lock is contrary to the rules stated
> for libxl__ev_* in the doc comment.  That should be fixed for 4.6.

OK.

> libxl__ev_fd_* already take and release the lock to protect their own
> data structures etc.
> 
> Ian.



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