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Date:   Mon, 11 Jun 2018 10:20:53 -0700
From:   Tejun Heo <tj@kernel.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Dmitry Vyukov <dvyukov@google.com>,
        Jens Axboe <axboe@kernel.dk>,
        syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <david@fromorbit.com>,
        linux-block@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn()
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Hello,

On Mon, Jun 11, 2018 at 06:29:20PM +0200, Jan Kara wrote:
> > Would something like the following work or am I missing the point
> > entirely?
> 
> I was pondering the same solution for a while but I think it won't work.
> The problem is that e.g. wb_memcg_offline() could have already removed
> wb from the radix tree but it is still pending in bdi->wb_list
> (wb_shutdown() has not run yet) and so we'd drop reference we didn't get.

Yeah, right, so the root cause is that we're walking the wb_list while
holding lock and expecting the object to stay there even after lock is
released.  Hmm... we can use a mutex to synchronize the two
destruction paths.  It's not like they're hot paths anyway.

Thanks.

-- 
tejun