To try to prevent corruption from two clients writing to the same RBD at the same time, exclusive locking allows the client to acquire a lock to disallow any other client from writing to the RBD. It's important to note that clients can always request the lock to be transferred to themselves and so the lock is only to protect the RBD device itself; a non-clustered filesystem will still likely be corrupted if two clients try to mount it, regardless of the exclusive locking.