In the current FCV upgrade logic, the primary node in a replica set executes a command and has the opportunity to acquire strong locks for consistency. However, the secondaries only get to observe writes to the FCV document which are executed while already holding a hierarchy of locks and as a result, they are not allowed to take any further locks on their own out of risk of introducing deadlocks.

During 4.0 development we uncovered cases (SERVER-34483) where upgrade requires the acquisition of strong locks on secondaries and because of this, it would be much more convenient if the FCV upgrade logic wrote a command oplog entry, which on the secondary nodes executes serially.