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

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Message-ID: <3a102881-3cc3-ba05-2f86-475145a87566@kernel.dk>
Date:   Sat, 14 Dec 2019 22:42:15 -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 12/14/19 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?

-- 
Jens Axboe