This was based from a user report at google groups:

https://groups.google.com/forum/?hl=en-US&fromgroups#!topic/mongodb-dev/g1BULoCq1ck

Condition to actually trigger the deadlock (not the just the MutexDebugger warning) is to call connPoolSync when the ReplicaSetMonitor's master node was flagged as not ok.

This is not an issue in the master branch because SERVER-6972 removed the ScopedDbConnection inside ReplicaSetMonitor::_checkStatus.