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Re: [libvirt] [PATCH] Release VM lock before acquiring virDomainObjListPtr lock



On Mon, Apr 08, 2013 at 09:15:28PM -0600, Eric Blake wrote:
> On 02/11/2013 09:46 AM, Daniel P. Berrange wrote:
> > From: "Daniel P. Berrange" <berrange redhat com>
> > 
> > When removing a VM from the virDomainObjListPtr, we must not
> > be holding the VM lock while acquiring the list lock. Re-order
> > code to ensure that we can release the VM lock early.
> > ---
> >  src/conf/domain_conf.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> > index 5e16ddf..d92e54a 100644
> > --- a/src/conf/domain_conf.c
> > +++ b/src/conf/domain_conf.c
> > @@ -2115,11 +2115,10 @@ void virDomainObjListRemove(virDomainObjListPtr doms,
> >  {
> >      char uuidstr[VIR_UUID_STRING_BUFLEN];
> >  
> > -    virObjectLock(doms);
> >      virUUIDFormat(dom->def->uuid, uuidstr);
> > -
> >      virObjectUnlock(dom);
> >  
> > +    virObjectLock(doms);
> 
> This patch seems to be implicated in Peter's latest proof of a
> use-after-free data race:
> https://www.redhat.com/archives/libvir-list/2013-April/msg00674.html

The caller of this method should own a reference on virDomainObjPtr.
This method will result in that reference being released, as the
dom is removed from the list.

If any other thread experiances a use-after-free, then that thread
must have been missing a reference.

> I'm trying to understand what the behavior was before this patch went in.

Well this was just fixing a deadlock introduced in a previous patch.
You need to look further back than just this patch. Originally the
global QEMU driver lock would be held preventing any kind of concurrent
execution.

Regards,
Daniel
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