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Date:   Fri, 28 Jan 2022 08:08:03 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Jan Kara <jack@suse.cz>, linux-block@vger.kernel.org,
        Ming Lei <ming.lei@redhat.com>
Subject: Re: yet another approach to fix loop autoclear for xfstets xfs/049
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References: <20220126155040.1190842-1-hch@lst.de> <7bebf860-2415-7eb6-55a1-47dc4439d9e9@I-love.SAKURA.ne.jp>
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[nit: please properly trim the lines in your mails, this needed a
 reformat to be readable at all]

On Thu, Jan 27, 2022 at 10:05:45AM +0900, Tetsuo Handa wrote:
> I want to remove disk->open_mutex => lo->lo_mutex => I/O completion chain
> itself from /proc/lockdep . Even if real deadlock does not happen due to
> lo->lo_refcnt exclusion, I consider that disk->open_mutex => lo->lo_mutex
> dependency being recorded is a bad sign.
> It makes difficult to find real possibility of deadlock when things change
> in future. I consider that lo_release() is doing too much things under
> disk->open_mutex.
> 
> I tried to defer lo->lo_mutex in lo_release() as much as possible.
> But this chain still remains.

Yes.  To completely remove it we'd need something like:

 - remove lo_refcnt and rely on bd_openers on the whole device bdev
 - add a block layer flag to temporarily disable a gendisk and fail
   all opens for it.

For now I'd really like to just fix the regression, though.