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Subject: Re: INFO: task hung in wb_shutdown (2)
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From: Jens Axboe <axboe@kernel.dk>
Message-ID: <bd3e8460-9794-6b57-e7d6-7e18ea34ac0c@kernel.dk>
Date: Tue, 1 May 2018 15:30:59 -0600
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To: Linus Torvalds <torvalds@linux-foundation.org>, Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: 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>

On 5/1/18 10:06 AM, Linus Torvalds wrote:
> 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.

Agree, that construct looks saner than introducing a "random"
smp_mb(). As a pattern helper, should probably be introduced
after the fact.

-- 
Jens Axboe