> BTW, this part of commit message in 2c18a63b760a is rather confused:
> Recent rework moved block device closing out of sb->put_super() and into
> sb->kill_sb() to avoid deadlocks as s_umount is held in put_super() and
> blkdev_put() can end up taking s_umount again.
>
> That was *NOT* what a recent rework had done. Block device closing had never
> been inside ->put_super() - at no point since that (closing, that is) had been
> introduced back in 0.97 ;-) ->put_super() predates it (0.95c+).
I think the commit message probably just isn't clear enough. The main
block device of a superblock isn't closed in sb->put_super(). That's
always been closed in kill_block_super() after generic_shutdown_super().
But afaict filesystem like ext4 and xfs may have additional block
devices open exclusively and closed them in sb->put_super():
xfs_fs_put_super()
-> xfs_close_devices()
-> xfs_blkdev_put()
-> blkdev_put()
ext4_put_super()
-> ext4_blkdev_remove()
-> blkdev_put()
