On Wed, Nov 25, 2020 at 03:20:23PM -0500, Tejun Heo wrote:

Agreed. It'd be nice to replace the stale comment.

I've updated the comment.

quoted

Jan added lookup_sem in commit 56c0908c855afbb to prevent a three way
race between del_gendisk and blkdev_open due to the weird bdev vs
gendisk lifetime rules.  None of that can happen with the new lookup
scheme.

Understood. I think it'd be worthwhile to note that in the commit log.

So after the idea of just holding a disk reference from each bdev did
not work out very well I've reworked this a bit, and lookup_sem stays,
but becomes global to protect the disk lookup (similar to the previous
global bdev_map_lock).