There are presently code paths in which upsert takes exclusive database locks (e.g., SERVER-45955). It looks like there is also an update helper that takes an exclusive database lock for upsert. This update helper is currently used in various places, e.g., during oplog application, as well as in the dbhelpers upsert and putSingleton methods.

Any usages of this old-style update() method should be replaced with an equivalent write_ops_exec based implementation, to avoid the costly exclusive database lock and to guarantee consistency of implementation.