linux-kernel - Re: [syzbot] [io-uring?] [usb?] WARNING in io_get_cqe

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Message-ID: <768a51d0-8c63-4d9f-a636-c9cb52a0c4f4@gmail.com>
Date: Tue, 5 Nov 2024 01:20:15 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Jens Axboe <axboe@...nel.dk>,
 syzbot <syzbot+e333341d3d985e5173b2@...kaller.appspotmail.com>,
 io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [io-uring?] [usb?] WARNING in io_get_cqe_overflow (2)

On 11/4/24 17:05, Jens Axboe wrote:
> On 11/4/24 10:03 AM, Jens Axboe wrote:
>> On 11/4/24 9:54 AM, Pavel Begunkov wrote:
>>> On 11/4/24 15:43, Jens Axboe wrote:
>>>> On 11/4/24 8:34 AM, Pavel Begunkov wrote:
>>>>> On 11/4/24 15:27, Pavel Begunkov wrote:
>>> ...
>>>>> Regardless, the rule with sth like that should be simpler,
>>>>> i.e. a ctx is getting killed => everything is run from fallback/kthread.
>>>>
>>>> I like it, and now there's another reason to do it. Can you out the
>>>> patch?
>>>
>>> Let's see if it works, hopefully will try today.
>>
>> I already tried it here fwiw, does fix the issue (as expected) and it
>> passes the full testing too.
> 
> Forgot to include the basic reproducer I wrote for this report, it's
> below.

Thanks for testing.

I was just looking at the patch, and the fun part is that it depends on
failing tw off PF_KTHREAD tasks... Otherwise when the fallback executes
a request the original task could still be alive =>
(req->task->flags & PF_EXITING) check passes and the request continues
doing stuff from the kthread (with no mm and so).

A side note that it should be marginally better than before on the
cancellation front because we're killing task_work more eagerly
for a dying ctx but alive tasks.

I'll send the patch properly.

-- 
Pavel Begunkov