In connectOplogNotifier, the notifier thread grabs bgsync's _mutex and then calls syncSourceFeedback.connect() which grabs SyncSourceFeedback's _mtx and _connmtx. Meanwhile SyncSourceFeedback's run() loop grabs its _mtx and _connmtx and then calls bgsync's getSyncTarget() which grabs bgsync's _mutex. If each of these threads grabs their first lock, we end up deadlocked.