linux-kernel - Re: [BUG] io_uring: defer logic based on shared data

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Message-ID: <31a7765b-bb6d-985a-454d-d998678100d1@gmail.com>
Date:   Fri, 25 Oct 2019 19:40:29 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [BUG] io_uring: defer logic based on shared data

On 25/10/2019 19:32, Jens Axboe wrote:
> On 10/25/19 10:27 AM, Jens Axboe wrote:
>> On 10/25/19 10:21 AM, Pavel Begunkov wrote:
>>> On 25/10/2019 19:03, Jens Axboe wrote:
>>>> On 10/25/19 3:55 AM, Pavel Begunkov wrote:
>>>>> I found 2 problems with __io_sequence_defer().
>>>>>
>>>>> 1. it uses @sq_dropped, but doesn't consider @cq_overflow
>>>>> 2. @sq_dropped and @cq_overflow are write-shared with userspace, so
>>>>> it can be maliciously changed.
>>>>>
>>>>> see sent liburing test (test/defer *_hung()), which left an unkillable
>>>>> process for me
>>>>
>>>> OK, how about the below. I'll split this in two, as it's really two
>>>> separate fixes.
>>> cached_sq_dropped is good, but I was concerned about cached_cq_overflow.
>>> io_cqring_fill_event() can be called in async, so shouldn't we do some
>>> synchronisation then?
>>
>> We should probably make it an atomic just to be on the safe side, I'll
>> update the series.
> 
> Here we go, patch 1:
> 
> http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=f2a241f596ed9e12b7c8f960e79ccda8053ea294
> 
> patch 2:
> 
> http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=b7d0297d2df5bfa0d1ecf9d6c66d23676751ef6a
> 
1. submit rqs (not yet completed)
2. poll_list is empty, inflight = 0
3. async completed and placed into poll_list

So, poll_list is not empty, but we won't get to polling again.
At least until someone submitted something.

-- 
Yours sincerely,
Pavel Begunkov



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