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Date:   Fri, 20 Aug 2021 12:02:02 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Hillf Danton <hdanton@sina.com>
Cc:     Sven Schnelle <svens@linux.ibm.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, nbd@other.debian.org,
        linux-kernel@vger.kernel.org
Subject: Re: WARNING: possible circular locking dependency detected in nbd
Message-ID: <20210820100202.GB9872@lst.de>
References: <yt9dsfz7xm6e.fsf@linux.ibm.com> <20210819072938.3839-1-hdanton@sina.com>
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On Thu, Aug 19, 2021 at 03:29:38PM +0800, Hillf Danton wrote:
> On Wed, 18 Aug 2021 09:10:49 +0200 Sven Schnelle wrote:
> > Hi,
> > 
> > i'm seeing the lockdep splat below in CI. I think this is because
> 
> Thanks for reporting it.
> 
> > nbd_open is called with disk->open_mutex held, and acquires
> > nbd_index_mutex. However, nbd_put() first takes the nbd_index_lock,
> > and calls del_gendisk, which locks disk->open_mutex, so the order is
> > reversed.
> 
> Right. See diff attached.

This is already fixed in linux-next.