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

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Message-ID: <99bfb7aa-6980-fc14-32f7-a479dea63eb4@gmail.com>
Date:   Tue, 8 Oct 2019 23:58:21 +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 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


-- 
Yours sincerely,
Pavel Begunkov



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