There appears to be a re-entrant mutex condition in the replica set connection pooling. Stack trace:

https://gist.githubusercontent.com/cheald/5ed01172c5b2c9943c87/raw/63075158dac4c78c1775cac8bf84ba3b6537bc1e/gistfile1.txt

The original lock occurs here: https://github.com/mongodb/mongo-ruby-driver/blob/1.x-stable/lib/mongo/connection/pool_manager.rb#L60

and then the process of reconnecting ends up attempting to resynchronize the same lock here: https://github.com/mongodb/mongo-ruby-driver/blob/1.x-stable/lib/mongo/connection/pool_manager.rb#L150