Re: [PATCH 2/2] bcache: Fix bcache device claiming

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jun 21, 2023 at 06:23:27PM +0200, Jan Kara wrote:
> Commit 2736e8eeb0cc ("block: use the holder as indication for exclusive
> opens") introduced a change that blkdev_put() has to get exclusive
> holder of the bdev as an argument. However it overlooked that
> register_bdev() and register_cache() overwrite the bdev->bd_holder field
> in the block device to point to the real owning object which was not
> available at the time we called blkdev_get_by_path(). Messing with bdev
> internals like this is a layering violation and it also causes
> blkdev_put() to issue warning about mismatching holders.

Ugg, yes.

> Fix bcache to reopen the block device with appropriate holder once it is
> available which also restores the behavior that multiple bcache caches
> cannot claim the same device which was also broken by commit
> 2736e8eeb0cc.

That was actually an intentional and documented change in commit
29499ab060fe ("bcache: don't pass a stack address to
blkdev_get_by_path") because the old behavior was broken already in
addition to the changing of the block stuff underneath.  Not that I'm
arguing against your changes here, but the commit log probably needs a
bit of tweaking.