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

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

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.

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.

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

Ian.

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