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Date:   Mon, 20 Sep 2021 14:45:57 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>, Dave Chinner <david@fromorbit.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        syzbot <syzbot+d6c75f383e01426a40b4@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com, Waiman Long <llong@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [syzbot] WARNING in __init_work
Message-ID: <20210920124557.GA18317@lst.de>
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On Mon, Sep 20, 2021 at 05:38:59AM -0700, Paul E. McKenney wrote:
> > Well, the block code already does a bdi_unregister in del_gendisk.
> > So if we end up freeing the whole device bdev with a registered bdi
> > something is badly going wrong.  Unfortunately the log in this report
> > isn't much help on how we got there.  IIRC syzbot will eventually spew
> > out a reproducer, so it might be worth to wait for that.
> 
> If it does turn out that you need to block in an RCU callback,
> queue_rcu_work() can be helpful.  This schedules a workqueue from the RCU
> callback, allowing the function passed to the preceding INIT_RCU_WORK()
> to block.

In this case we really should not block here.  The problem is that
we are hitting the strange bdi auto-unregister misfeature due to a bug
elsewhere.  Which reminds that I have a patch series to remove this
auto unregistration which I need to bring bag once this is fixed.

That being said queue_rcu_work would have been really useful in a few
places I touched in that past.