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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 01 May 2018 16:06:48 +0000
Message-ID: <CA+55aFxaa_+uZ=bOVdevcUwG7ncue7O+i06q4Kb=bWACGwCBjQ@mail.gmail.com>
Subject: Re: INFO: task hung in wb_shutdown (2)
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>,
        syzbot+c0cf869505e03bdf1a24@syzkaller.appspotmail.com,
        christophe.jaillet@wanadoo.fr,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, syzkaller-bugs@googlegroups.com,
        zhangweiping@didichuxing.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        linux-block <linux-block@vger.kernel.org>
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On Tue, May 1, 2018 at 3:27 AM Tetsuo Handa <
penguin-kernel@i-love.sakura.ne.jp> wrote:

> Can you review this patch? syzbot has hit this bug for nearly 4000 times
but
> is still unable to find a reproducer. Therefore, the only way to test
would be
> to apply this patch upstream and test whether the problem is solved.

Looks ok to me, except:

> >       smp_wmb();
> >       clear_bit(WB_shutting_down, &wb->state);
> > +     smp_mb(); /* advised by wake_up_bit() */
> > +     wake_up_bit(&wb->state, WB_shutting_down);

This whole sequence really should just be a pattern with a helper function.

And honestly, the pattern probably *should* be

     clear_bit_unlock(bit, &mem);
     smp_mb__after_atomic()
     wake_up_bit(&mem, bit);

which looks like it is a bit cleaner wrt memory ordering rules.

             Linus