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

[Chris Lalancette]

On 07/16/10 - 09:38:10AM, 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).

Ping, any reviews here?

--
Chris Lalancette