[libvirt] [PATCH v5 3/3] libvirtd: fix crash on termination

Erik Skultety eskultet at redhat.com
Thu Dec 21 12:14:09 UTC 2017

On Thu, Dec 21, 2017 at 12:48:44PM +0300, Nikolay Shirokovskiy wrote:
> On 21.12.2017 11:49, Erik Skultety wrote:
> > On Thu, Dec 21, 2017 at 09:39:16AM +0100, Cedric Bosdonnat wrote:
> >> Hi John,
> >>
> >> On Wed, 2017-12-20 at 16:52 -0500, John Ferlan wrote:
> >>> So I moved the Unref(dmn) after virStateCleanup... That of course led to
> >>> the virt-manager issue.  So I figured I'd give adding this patch in and
> >>> that then fixed the virt-manager crash (obviously as we'd be Unref'ing
> >>> all those servers we added).
> >>
> >> Seems like I forgot to git-send my patch yesterday evening: here it is:
> >>
> >> https://www.redhat.com/archives/libvir-list/2017-December/msg00739.html
> >>
> >> That fixes the virt-manager crash at least.
> >
> > Hi, so the patch looks good, at first glance it makes complete sense to close
> > the client connections when closing the server and unreffing the objects when
> > disposing of the server object. However, since this and the other related
> > issues have been bugging us for quite some time I'm not going to say ACK just
> > yet, I'd like to think about it for a bit - but if someone else already has and
> > they feel brave enough :P, then I'm not going to block this to go in.
> >
> > Erik
> >
> Patch looks good to me too. But still original "libvirtd: fix crash on termination"
> fixes another issue and if applied fixes "virt-manager issue" as well as John
> figured out.

Finally I'm back on track with this (sorry for it taking so long), although
you're right about this, it's not the correct fix, it's just a byproduct of
your patch, however, the whole thing about closing connections and releasing
memory is a bit of a mess. For example, right now, we only dispose of service
objs (but we don't close them, we do that in virNetServerClose), but we both
close and dispose of the client objs. Another thing, we toggle the service to
stop accepting any new connections, but that's irrelevant at that point because
we've closed them already by that time as a product of calling
virNetDaemonClose->virNetServerClose->virNetServerServiceClose - so that
should be removed...I drifted a bit here, anyway, we need to make a clear
distinction when we're releasing memory and when we're shutting down some kind
of service - sometimes both can be done at the same time (see below), however
it's very clear we can't do it here. Your issue is that Close currently only
closes the sockets (this reflects my point of "shutting down a service") but
does nothing with the threadpool connected to the server, thus leaving the
worker threads to continue running and executing APIs the results of which they
shouldn't even be able to return back to the client, since we're shutting down.
Now the thing with threadpool is that both memory release and teardown are
merged into one "object disposal" operation and therefore done as part of
virNetServerDispose. Since I understand a removal from the hash table as a
memory release operation, we should not be calling virHashRemoveAll from
virNetDaemonClose. Now, I see 2 options:

1) split virThreadPoolFree into 2 functions, one which only broadcasts the
"die" message and joins the threads (or waits for them in this case...) and the
other releasing the resources - can't say I'm a fan of this one

2) call virThreadPoolFree from virNetServerClose instead...

None of those approaches is ideal, but I can't seem to think off anything
better at the moment.

I'm open to discuss any other suggestions.

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