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Subject: Re: [BUG] io_uring: defer logic based on shared data
To:     Pavel Begunkov <asml.silence@gmail.com>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <b4e1f03c-e044-b09f-d943-cad3ab5b4969@kernel.dk>
Date:   Fri, 25 Oct 2019 10:44:35 -0600
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On 10/25/19 10:40 AM, Pavel Begunkov wrote:
> 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.

But if they are issued, the will sit in ->poll_list as well. That list
holds both "submitted, but pending" and completed entries.

-- 
Jens Axboe