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Re: [libvirt] PATCH: 0/4: Fix the event loop (again) no really, this time its fixed.



On Mon, May 11, 2009 at 12:47:38PM +0100, Mark McLoughlin wrote:
> On Mon, 2009-05-11 at 12:11 +0100, Daniel P. Berrange wrote:
> > Back in March I attempted to fix the event loop handling of deleted file
> > handles
> > 
> > http://www.redhat.com/archives/libvir-list/2009-March/msg00246.html
> > 
> > but have merely succeeded in screwing it up in a different fun & exciting
> > way. That patch fixed a case where there was a deleted file handle in the
> > list at the time virEventRunOnce() starts. Which was nice. Not nice, though
> > was that it then breaks the case where a file handle is deleted during the
> > course of dispatching events afer poll(). Investigating this also identified
> > that when we were marking a file handle as deleted, we were forgetting to
> > run virEventInterrupt(), which meant that in some cases the main thread
> > would not be woken up until the next event triggers. Not very critical,
> > but at the same time, not good  because it delays cleanup & release of
> > resources.
> > 
> > This series of patches fixes the problems, and more importantly, adds a
> > unit test which cover these nasty edge cases.
> 
> It's not obvious to me from your description, so for others, one of the
> problems these patches fix is where destroying one running guest can
> cause another to also be destroyed:
> 
>   https://bugzilla.redhat.com/499698

NB That's just one symptom. The core problem is that you end up with an
off-by-1 error when dispatching the callbacks. ie, instead of dispatching
the nth callback, we dispatch the nth+1 callbck.  There are lots of 
different things registering callbacks so effects are pretty unpredictable.
Normally the callbacks for running guests don't align, but if you restart
libvirtd while a set of guests are running, you end up with all the guest
callbacks registered in series, so are quite likely to see this double
destroy.

Daniel
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