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

From: Pavel Begunkov
Date: Tue Oct 08 2019 - 16:58:32 EST

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

Yours sincerely,
Pavel Begunkov

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