linux-kernel - Re: [syzbot] WARNING in __init

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Message-ID: <20210920124557.GA18317@lst.de>
Date:   Mon, 20 Sep 2021 14:45:57 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Dave Chinner <david@...morbit.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+d6c75f383e01426a40b4@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com, Waiman Long <llong@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [syzbot] WARNING in __init_work

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.