As explained in another thread I really think we need to get DAX to stop pretending to be direct I/O, which should also take care of the locking. The same issue also exists for ext2 and XFS so it needs to be solved at a higher level.
- [RFC PATCH] ext4: Don't release mutex for DAX write Waiman Long
- Re: [RFC PATCH] ext4: Don't release mutex for DAX w... Christoph Hellwig