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    SubjectRe: [PATCH] io_uring: remove wait loop spurious wakeups
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    On 10/9/2019 5:54 AM, Jens Axboe wrote:
    >>>>>> 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!

    Now I get it, thanks Jens!

    --
    Yours sincerely,
    Pavel Begunkov

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