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

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Date:   Tue, 8 Oct 2019 20:54:38 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups

On 10/8/19 4:05 PM, Pavel Begunkov wrote:
> On 09/10/2019 00:22, Jens Axboe wrote:
>> On 10/8/19 2:58 PM, Pavel Begunkov wrote:
>>> On 08/10/2019 20:00, Jens Axboe wrote:
>>>> On 10/8/19 10:43 AM, Pavel Begunkov wrote:
>>>>> On 08/10/2019 06:16, Jens Axboe wrote:
>>>>>> On 10/7/19 5:18 PM, Pavel Begunkov (Silence) wrote:
>>>>>>> From: Pavel Begunkov <asml.silence@...il.com>
>>>>>>>
>>>>>>> Any changes interesting to tasks waiting in io_cqring_wait() are
>>>>>>> commited with io_cqring_ev_posted(). However, io_ring_drop_ctx_refs()
>>>>>>> also tries to do that but with no reason, that means spurious wakeups
>>>>>>> every io_free_req() and io_uring_enter().
>>>>>>>
>>>>>>> Just use percpu_ref_put() instead.
>>>>>>
>>>>>> Looks good, this is a leftover from when the ctx teardown used
>>>>>> the waitqueue as well.
>>>>>>
>>>>> 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!

-- 
Jens Axboe