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Date:   Tue, 29 Mar 2022 08:52:34 +0200
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>,
        Josef Bacik <josef@toxicpanda.com>,
        Minchan Kim <minchan@kernel.org>,
        Nitin Gupta <ngupta@vflare.org>, Jan Kara <jack@suse.cz>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Ming Lei <ming.lei@redhat.com>,
        Matteo Croce <mcroce@microsoft.com>,
        linux-block@vger.kernel.org, nbd@other.debian.org
Subject: Re: [PATCH 13/14] loop: remove lo_refcount and avoid lo_mutex in
 ->open / ->release
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References: <20220325063929.1773899-1-hch@lst.de> <20220325063929.1773899-14-hch@lst.de> <03628e13-ca56-4ed0-da5a-ee698c83f48d@I-love.SAKURA.ne.jp>
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Hi Tetsuo,

I'm a bit confused. Partially due to the two patches in one mail, but
also because I can't actually find a try that they cleanly apply to.

But let me add my thoughts here:

 - I think the change 

On Sat, Mar 26, 2022 at 11:52:36AM +0900, Tetsuo Handa wrote:
> Since __loop_clr_fd() is currently holding loop_validate_mutex and lo->lo_mutex,
> "avoid lo_mutex in ->release" part is incomplete.
> 
> The intent of holding loop_validate_mutex (which causes disk->open_mutex =>
> loop_validate_mutex => lo->lo_mutex => blk_mq_freeze_queue()/blk_mq_unfreeze_queue()
> chain) is to make loop_validate_file() traversal safe.
> 
> Since ->release is called with disk->open_mutex held, and __loop_clr_fd() from
> lo_release() is called via ->release when disk_openers() == 0, we are guaranteed
> that "struct file" which will be passed to loop_validate_file() via fget() cannot
> be the loop device __loop_clr_fd(lo, true) will clear. Thus, there is no need to
> hold loop_validate_mutex and lo->lo_mutex from __loop_clr_fd() if release == true.

This part looks reasonable.  Can you give a signoff and proper commit
log and I'll slot it in before the lo_refcount removal.