linux-kernel - Re: possible deadlock in __io_queue

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Message-ID: <504b4b08-30c1-4ca8-ab3b-c9f0b58f0cfa@kernel.dk>
Date:   Tue, 11 Aug 2020 08:21:12 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     syzbot <syzbot+996f91b6ec3812c48042@...kaller.appspotmail.com>,
        io-uring@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk
Subject: Re: possible deadlock in __io_queue_deferred

On 8/11/20 8:00 AM, Stefano Garzarella wrote:
> On Mon, Aug 10, 2020 at 09:55:17AM -0600, Jens Axboe wrote:
>> On 8/10/20 9:36 AM, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
>>> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+996f91b6ec3812c48042@...kaller.appspotmail.com
>>
>> Thanks, the below should fix this one.
> 
> Yeah, it seems right to me, since only __io_queue_deferred() (invoked by
> io_commit_cqring()) can be called with 'completion_lock' held.

Right

> Just out of curiosity, while exploring the code I noticed that we call
> io_commit_cqring() always with the 'completion_lock' held, except in the
> io_poll_* functions.
> 
> That's because then there can't be any concurrency?

Do you mean the iopoll functions? Because we're definitely holding it
for the io_poll_* functions.

-- 
Jens Axboe