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Date:   Mon, 29 Nov 2021 19:36:27 +0900
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Subject: Re: [syzbot] possible deadlock in blkdev_put (2)
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To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jan Kara <jack@suse.cz>, Dave Chinner <dchinner@redhat.com>,
        linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
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From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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On 2021/11/29 19:21, Christoph Hellwig wrote:
> On Sun, Nov 28, 2021 at 04:42:35PM +0900, Tetsuo Handa wrote:
>> Is dropping disk->open_mutex inside lo_release()
>> ( https://lkml.kernel.org/r/e4bdc6b1-701d-6cc1-5d42-65564d2aa089@I-love.SAKURA.ne.jp ) possible?
> 
> I don't think we can drop open_mutex inside ->release. What is the
> problem with offloading the clearing to a different context than the
> one that calls ->release?
> 

Offloading to a WQ context?

If the caller just want to call ioctl(LOOP_CTL_GET_FREE) followed by
ioctl(LOOP_CONFIGURE), deferring __loop_clr_fd() would be fine.

But the caller might want to unmount as soon as fput(filp) from __loop_clr_fd() completes.
I think we need to wait for __loop_clr_fd() from lo_release() to complete.