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Subject: Re: [PATCH v3] io_uring: don't wait when under-submitting
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
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 <900dbb63-ae9e-40e6-94f9-8faa1c14389e@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <9b422273-cee6-8fdb-0108-dc304e4b5ccb@kernel.dk>
Date:   Sun, 15 Dec 2019 14:33:32 -0700
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On 12/15/19 8:48 AM, Pavel Begunkov wrote:
> On 15/12/2019 08:42, Jens Axboe wrote:
>> On 12/14/19 11:43 AM, Jens Axboe wrote:
>>> On 12/14/19 7:53 AM, Pavel Begunkov wrote:
>>>> There is no reliable way to submit and wait in a single syscall, as
>>>> io_submit_sqes() may under-consume sqes (in case of an early error).
>>>> Then it will wait for not-yet-submitted requests, deadlocking the user
>>>> in most cases.
>>>>
>>>> In such cases adjust min_complete, so it won't wait for more than
>>>> what have been submitted in the current call to io_uring_enter(). It
>>>> may be less than totally in-flight including previous submissions,
>>>> but this shouldn't do harm and up to a user.
>>>
>>> Thanks, applied.
>>
>> This causes a behavioral change where if you ask to submit 1 but
>> there's nothing in the SQ ring, then you would get 0 before. Now
>> you get -EAGAIN. This doesn't make a lot of sense, since there's no
>> point in retrying as that won't change anything.
>>
>> Can we please just do something like the one I sent, instead of trying
>> to over-complicate it?
>>
> 
> Ok, when I get to a compiler.

Great, thanks. BTW, I noticed when a regression test failed.

-- 
Jens Axboe