Re: [libvirt] [PATCH] Fix a deadlock in bi-directional p2p concurrent migration.

On Fri, Jul 16, 2010 at 09:38:10AM -0400, Chris Lalancette wrote: > If you try to execute two concurrent migrations p2p > from A->B and B->A, the two libvirtd's will deadlock > trying to perform the migrations. The reason for this is > that in p2p migration, the libvirtd's are responsible for > making the RPC Prepare, Migrate, and Finish calls. However, > they are currently holding the driver lock while doing so, > which basically guarantees deadlock in this scenario. > > This patch fixes the situation by adding > qemuDomainObjEnterRemoteWithDriver and > qemuDomainObjExitRemoteWithDriver helper methods. The Enter > take an additional object reference, then drops both the > domain object lock and the driver lock. The Exit takes > both the driver and domain object lock, then drops the > reference. Adding calls to these Enter and Exit helpers > around remote calls in the various migration methods > seems to fix the problem for me in testing. > > This should make the situation safe. The additional domain > object reference ensures that the domain object won't disappear > while this operation is happening. The BeginJob that is called > inside of qemudDomainMigratePerform ensures that we can't execute a > second migrate (or shutdown, or save, etc) job while the > migration is active. Finally, the additional check on the state > of the vm after we reacquire the locks ensures that we can't > be surprised by an external event (domain crash, etc). > > Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt; ACK Daniel