There is specifically a cycle of STEP::_mutex --> ShardREgistry::_mutex --> RSMM::_mutex --> STEP::_mutex).

An example manifestation would be:
STEP mutex is taken while holding RSMM mutex via RSMM::removeMonitor --> StreamableRSM::drop --> STEP::REplicaSetChangeListener::onDroppedSet

RSM mutex taken while holding ShardREgistry mutex via ShardingReplicaSetChangedLitener::onConfirmedSet --> ShardRegistry::updateReplSetHosts --> ShardFactory::createShard --> RSMM::getOrCreateMonitor

SR mutex taken while holding STEP mutex via ConnectionPool::get --> STEP::addHost --> ShardRegistry::isConfigServer --> ShardRegistry::getConfigShard