linux-kernel - Re: possible deadlock in blkdev_reread

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Date:   Thu, 13 Sep 2018 22:43:34 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>, Jens Axboe <axboe@...nel.dk>
Cc:     Eric Biggers <ebiggers3@...il.com>,
        syzbot+4684a000d5abdade83fac55b1e7d1f935ef1936e@...kaller.appspotmail.com,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: possible deadlock in blkdev_reread_part

On 2018/09/13 21:58, Dmitry Vyukov wrote:
> On Wed, May 2, 2018 at 1:23 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> On Wed, May 2, 2018 at 12:30 PM, Tetsuo Handa
>> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>> Dmitry Vyukov wrote:
>>>>> syzbot is reporting various bugs which involve /dev/loopX.
>>>>> Two of them
>>>>>
>>>>>   INFO: rcu detected stall in lo_ioctl
>>>>>   https://syzkaller.appspot.com/bug?id=7b49fb610af9cca78c24e9f796f2e8b0d5573997
>>>>>
>>>>>   general protection fault in lo_ioctl (2)
>>>>>   https://syzkaller.appspot.com/bug?id=f3cfe26e785d85f9ee259f385515291d21bd80a3
>>>>
>>>> /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>>>>
>>>> Now there is a repro for this one. I've pushed it to kernel mailing lists:
>>>>
>>>> https://groups.google.com/d/msg/syzkaller-bugs/c8KUcTAzTvA/3o_7g6-tAwAJ
>>>
>>> OK, thanks. But among loop related reports, this will be a dup of
>>> "INFO: rcu detected stall in blkdev_ioctl" which already has C reproducer.
>>> Should we merge them?
>>
>> Yes, sure, I will take care of it.
> 
> 1. I forgot to take care of it.
> 
> 2. "INFO: rcu detected stall in blkdev_ioctl" was fixed 3 months ago:
> https://syzkaller.appspot.com/bug?id=1f7b710f4110f225aed1f4263ec2b98b8dbd472e
> 
> but this bug still happens up until now:
> https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d15889
> 
> So this is a different bug.

I'm not sure what you are talking about.
RCU stall and lockdep warning are obviously different bugs.

Regarding lockdep warning on loop module, we are still waiting for Jens to
come up a better alternative than
http://lkml.kernel.org/r/1527297408-4428-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp .
Since no alternative was proposed, I think we should start testing my patch.