Re: [PATCH] io_uring: remove wait loop spurious wakeups

From: Pavel Begunkov
Date: Tue Oct 08 2019 - 18:06:01 EST

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@xxxxxxxxx>
>>>>>> 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.

Anyway, thanks for the explanation!

Yours sincerely,
Pavel Begunkov

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