I downloaded master of mongodb to get a feel of how its put together.

I casually noticed that in syncclusterconnection.cpp there is the line:

SyncClusterConnection::SyncClusterConnection( const list<HostAndPort> & L) : _mutex("SynClusterConnection") {

While the other overloads have a different mutex name (SyncCluster rather than SynCluster).

I don't know enough about the code yet to know whether this is a possible problem (ie two mutexes may exist instead of one for this class if a separate overload is used).