Re: [PATCH] fs: Avoid grabbing sb->s_umount under bdev->bd_holder_lock

[Christian Brauner <brauner@kernel.org>]

On Wed, Oct 18, 2023 at 05:29:24PM +0200, Jan Kara wrote:
> The implementation of bdev holder operations such as fs_bdev_mark_dead()
> and fs_bdev_sync() grab sb->s_umount semaphore under
> bdev->bd_holder_lock. This is problematic because it leads to
> disk->open_mutex -> sb->s_umount lock ordering which is counterintuitive
> (usually we grab higher level (e.g. filesystem) locks first and lower
> level (e.g. block layer) locks later) and indeed makes lockdep complain
> about possible locking cycles whenever we open a block device while
> holding sb->s_umount semaphore. Implement a function

This patches together with my series that Christoph sent out for me
Link: https://lore.kernel.org/r/20231017184823.1383356-1-hch@lst.de
two days ago (tyvm!) the lockdep false positives are all gone and we
also eliminated the counterintuitive ordering requirement that forces us
to give up s_umount before opening block devices.

I've verified that yesterday and did a bunch of testing via sudden
device removal.

Christoph had thankfully added generic/730 and generic/731 to emulate
some device removal. I also messed around with the loop code and
specifically used LOOP_CHANGE_FD to force a disk_force_media_change() on
a filesystem.