linux-kernel - Re: [PATCH v3] io_uring: don't wait when under-submitting

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Message-ID: <ff49ace9-9d9a-a760-7fc0-325631a8b87c@kernel.dk>
Date:   Mon, 16 Dec 2019 09:51:59 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] io_uring: don't wait when under-submitting

On 12/16/19 9:47 AM, Pavel Begunkov wrote:
> On 16/12/2019 00:33, Jens Axboe wrote:
>> 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.
>>
> 
> Yeah, I properly tested only the first one. Clearly, not as easy as
> I thought, and there were more to consider.
> 
> I sent the next version, but that's odd basically taking your code.
> Probably, it would have been easier for you to just commit it yourself.

Nah, I'll keep you attribution, the hard part is finding/spotting the
issue, not the actual fix. I've applied v4, thanks Pavel!

-- 
Jens Axboe