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

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Message-ID: <436eb658-582d-c752-f20a-2f2c43d741a3@gmail.com>
Date:   Fri, 25 Oct 2019 21:13:38 +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 19:57, 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@...il.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@...il.com>

-- 
Yours sincerely,
Pavel Begunkov



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