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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: <f3b7a9fb-1a95-5419-3408-669d8b466d48@kernel.dk>
Date:   Fri, 25 Oct 2019 12:17:35 -0600
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On 10/25/19 12:13 PM, Pavel Begunkov wrote:
> On 25/10/2019 19:57, Jens Axboe wrote:
>> On 10/25/19 10:55 AM, Pavel Begunkov wrote:
>>> On 25/10/2019 19:44, Jens Axboe wrote:
>>>> 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.
>>>>
>>> Missed it, then should work. Thanks!
>>
>> Glad we agree :-)
>>
>>>> + ret = iters = 0;
>>> A small suggestion, could we just initialise it in declaration
>>> to be a bit more concise?
>>> e.g. int ret = 0, iters = 0;
>>>
>>> Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
>>> And let me test it as both patches are ready.
>>
>> Sure, I'll make that change and add your reviewed-by. Thanks!
>>
> Stress tested, works well!
> 
> Tested-by: Pavel Begunkov <asml.silence@gmail.com>

Great, thanks for finding these, sending patches, and testing the ones
that I fixed!

-- 
Jens Axboe