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

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Message-ID: <a1f8de23-fcad-7252-cbd4-8f5e617056cd@kernel.dk>
Date:   Tue, 8 Oct 2019 15:22:43 -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 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.

-- 
Jens Axboe