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

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).