linux-kernel - Re: [PATCH] io_uring: remove wait loop spurious wakeups

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Message-ID: <51f54701-7cef-1ba1-e928-f427790ebfe4@gmail.com>
Date:   Wed, 9 Oct 2019 12:19:32 +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: [PATCH] io_uring: remove wait loop spurious wakeups

On 10/9/2019 5:54 AM, Jens Axboe wrote:
>>>>>> BTW, is there a reason for ref-counting in struct io_kiocb? I understand
>>>>>> the idea behind submission reference, but don't see any actual part
>>>>>> needing it.
>>>>>
>>>>> In short, it's to prevent the completion running before we're done with
>>>>> the iocb on the submission side.
>>>>
>>>> Yep, that's what I expected. Perhaps I missed something, but what I've
>>>> seen following code paths all the way down, it either
>>>> 1. gets error / completes synchronously and then frees req locally
>>>> 2. or passes it further (e.g. async list) and never accesses it after
>>>
>>> As soon as the IO is passed on, it can complete. In fact, it can complete
>>> even _before_ that call returns. That's the issue. Obviously this isn't
>>> true for purely polled IO, but it is true for IRQ based IO.
>>
>> And the idea was to not use io_kiocb after submission. Except when we know,
>> that it won't complete asynchronously (e.g. error), that could be checked
>> with return code, I guess.
> 
> I think you're still missing the point. During the submission it can go
> away, it can be deep in a call chain. So it's not enough to say "we
> won't touch it after completion returns", we need to hold a reference to
> ensure it doesn't go away WHILE being submitted.
> 
> Hope that helps!

Now I get it, thanks Jens!

-- 
Yours sincerely,
Pavel Begunkov