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Subject: Re: [syzbot] possible deadlock in loop_add
To:     Miquel Raynal <miquel.raynal@bootlin.com>,
        Hillf Danton <hdanton@sina.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        syzbot <syzbot+118992efda475c16dfb0@syzkaller.appspotmail.com>,
        axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
References: <000000000000ec01e405c6c2cee3@google.com>
 <20210710131638.605-1-hdanton@sina.com> <20210712052740.GA8599@lst.de>
 <c3d4ebd5-5679-cd81-d1de-4f5f2cbe13db@gmail.com>
 <20210716010028.4218b0de@xps13>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <77da708c-b63d-dac0-c7e6-43ced0d49982@gmail.com>
Date:   Sat, 17 Jul 2021 17:41:03 +0800
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On 16/7/21 7:00 am, Miquel Raynal wrote:
> Hello,
> 
> Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> wrote on Mon, 12 Jul
> 2021 16:29:16 +0800:
> 
>> On 12/7/21 1:27 pm, Christoph Hellwig wrote:
>>> On Sat, Jul 10, 2021 at 09:16:38PM +0800, Hillf Danton wrote:
>>>> To break the lock chain, un/register blkdev without mtd_table_mutex held.
>>>
>>> Yes, Desmond Cheong Zhi Xi sent pretty much the same patch on June 18th
>>> (mtd: break circular locks in register_mtd_blktrans), but it did not get
>>> picked up.
>>>    
>>
>> I believe Miquèl was waiting for -rc1 to apply it.
> 
> Indeed, I already applied it but did not advertise yet.
> 

Thanks Miquèl!

>>
>> But taking a closer look, although the fix for the register path is the same, Hillf Danton's proposed patch additionally avoids inverting the lock hierarchy on the unregister path. So I believe this new patch should be more robust.
> 
> We can definitely do this in two steps if you want.
> 

Sounds good, I'll prepare a patch with Hillf's suggestion for the 
unregister path.

> Thanks,
> Miquèl
>