The ReplicaSetMonitorManager's mutex has a hierarchical locking level of 6. This means that it must be acquired before any lower-numbered mutex (on the same thread). This mutex is locked every time that a ReplicaSetMonitor is destructed. Enough uses of mutexes at levels 6 and below exist that it should be re-evaluated whether it's safe to lock a level 6 mutex in such a common code path.