Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups
To:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <936cd758d6c694fe1b8b9de050e24cfecdc2e60d.1570489620.git.asml.silence@gmail.com>
 <e11a0716-eb18-4ce3-9902-3247beafe65a@kernel.dk>
 <d035bb1b-e6f0-77db-a434-1761b0a7a142@gmail.com>
 <62a8a6c7-9c5b-c9a4-9c73-c77db87c6637@kernel.dk>
 <99bfb7aa-6980-fc14-32f7-a479dea63eb4@gmail.com>
 <a1f8de23-fcad-7252-cbd4-8f5e617056cd@kernel.dk>
 <4004b5bc-edf0-cc8f-8efc-7f848c95f0ba@gmail.com>
 <adc20cef-bc8e-2e67-399c-30c2f4af1a93@kernel.dk>
From:   Pavel Begunkov <asml.silence@gmail.com>
Message-ID: <51f54701-7cef-1ba1-e928-f427790ebfe4@gmail.com>
Date:   Wed, 9 Oct 2019 12:19:32 +0300
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.0
MIME-Version: 1.0
In-Reply-To: <adc20cef-bc8e-2e67-399c-30c2f4af1a93@kernel.dk>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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