Subject: Re: [syzbot] general protection fault in set_task_ioprio
To:     Eric Dumazet <edumazet@google.com>
Cc:     syzbot <syzbot+8836466a79f4175961b0@syzkaller.appspotmail.com>,
        Christoph Hellwig <hch@lst.de>, changbin.du@intel.com,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        David Miller <davem@davemloft.net>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Jakub Kicinski <kuba@kernel.org>, linux-block@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Yajun Deng <yajun.deng@linux.dev>
References: <000000000000c70eef05d39f42a5@google.com>
 <00000000000066073805d3a4f598@google.com>
 <CANn89i++5O_4_j3KO0wAiJHkEj=1zAeAHv=s9Lub_B6=cguwXQ@mail.gmail.com>
 <e3a30c8c-3f1a-47b5-57e7-1b456bbc8719@kernel.dk>
 <CANn89iJfEgkJCBqO9d7t9BHHMEh-6DQ1BJkqkiOQ59dxSHB2EQ@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <4019677f-7225-c359-a411-e4290cc717b0@kernel.dk>
Date:   Tue, 21 Dec 2021 09:30:06 -0700
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On 12/21/21 9:03 AM, Eric Dumazet wrote:
> On Tue, Dec 21, 2021 at 7:25 AM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 12/21/21 3:44 AM, Eric Dumazet wrote:
>>> On Tue, Dec 21, 2021 at 1:52 AM syzbot
>>> <syzbot+8836466a79f4175961b0@syzkaller.appspotmail.com> wrote:
>>>>
>>>> syzbot has bisected this issue to:
>>>>
>>>> commit e4b8954074f6d0db01c8c97d338a67f9389c042f
>>>> Author: Eric Dumazet <edumazet@google.com>
>>>> Date:   Tue Dec 7 01:30:37 2021 +0000
>>>>
>>>>     netlink: add net device refcount tracker to struct ethnl_req_info
>>>>
>>>
>>> Unfortunately this commit will be in the way of many bisections.
>>>
>>> Real bug was added in
>>>
>>> commit 5fc11eebb4a98df5324a4de369bb5ab7f0007ff7
>>> Author: Christoph Hellwig <hch@lst.de>
>>> Date:   Thu Dec 9 07:31:29 2021 +0100
>>>
>>>     block: open code create_task_io_context in set_task_ioprio
>>>
>>>     The flow in set_task_ioprio can be simplified by simply open coding
>>>     create_task_io_context, which removes a refcount roundtrip on the I/O
>>>     context.
>>>
>>>     Signed-off-by: Christoph Hellwig <hch@lst.de>
>>>     Reviewed-by: Jan Kara <jack@suse.cz>
>>>     Link: https://lore.kernel.org/r/20211209063131.18537-10-hch@lst.de
>>>     Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> There are only really 5 patches in between the broken commit and the one
>> that fixes it, and it only affects things trying to set the ioprio with
>> a dead task. Is this a huge issue? I don't see why this would cause a
>> lot of bisection headaches.
>>
> 
> I was saying that my commit was polluting syzbot bisection, this is a
> distraction in this report.
> (Or if you prefer, please ignore syzbot bisection)

Ah got it, yes makes sense.

> linux-next has still this bug in set_task_ioprio()

linux-next often trails by a few days, once it catches up hopefully
this will be behind us.

-- 
Jens Axboe