Re: [PATCH 4/5] btrfs: open block devices after superblock creation

[David Sterba <dsterba@suse.cz>]

On Wed, Feb 14, 2024 at 10:58:09AM -0800, Boris Burkov wrote:
> On Wed, Feb 14, 2024 at 08:42:15AM -0800, Johannes Thumshirn wrote:
> > From: Christoph Hellwig <hch@lst.de>
> > 
> > Currently btrfs_mount_root opens the block devices before committing to
> > allocating a super block. That creates problems for restricting the
> > number of writers to a device, and also leads to a unusual and not very
> > helpful holder (the fs_type).
> > 
> > Reorganize the code to first check whether the superblock for a
> > particular fsid does already exist and open the block devices only if it
> > doesn't, mirroring the recent changes to the VFS mount helpers.  To do
> > this the increment of the in_use counter moves out of btrfs_open_devices
> > and into the only caller in btrfs_mount_root so that it happens before
> > dropping uuid_mutex around the call to sget.
> 
> I believe this commit message is now out of date as of
> 'btrfs: remove old mount API code'
> which got rid of btrfs_mount_root.

It's not just that, this patchset was sent before the conversion to new
mount API that changed how devices are scanned (and potentially race
with mount). The changelog should be updated at minimum.

I haven't found any problems so far, the locking around device opening
should serialize any races so the one thread winning will open the super
block and the other will inherit the fs_devices.