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From:   Jens Axboe <axboe@kernel.dk>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH 1/1] io_uring: don't wait when under-submitting
Date:   Fri, 13 Dec 2019 14:48:28 -0700
Message-Id: <B50496AA-BE78-45B7-A17D-9740FF42A8EF@kernel.dk>
References: <47a5641b-af81-0edf-1d71-4e3063ce8517@gmail.com>
Cc:     io-uring@vger.kernel.org, linux-kernel@vger.kernel.org
In-Reply-To: <47a5641b-af81-0edf-1d71-4e3063ce8517@gmail.com>
To:     Pavel Begunkov <asml.silence@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Dec 13, 2019, at 2:32 PM, Pavel Begunkov <asml.silence@gmail.com> wrote=
:
>=20
> =EF=BB=BFOn 13/12/2019 21:32, Jens Axboe wrote:
>>> On 12/13/19 11:22 AM, Jens Axboe wrote:
>>> On 12/13/19 12:51 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.
>>>=20
>>> Why not just cap the wait_nr? If someone does to_submit =3D 8, wait_nr =3D=
 8,
>>> and we only submit 4, just wait for 4? Ala:
>>>=20
>=20
> Is it worth entangling the code? I don't expect anyone trying to recover,
> maybe except full reset/restart. So, failing ASAP seemed to me as the
> right thing to do. It may also mean nothing to the user if e.g.
> submit(1), submit(1), ..., submit_and_wait(1, n)
>=20
> Anyway, this shouldn't even happen in a not buggy code, so I'm fine with
> any version as long as it doesn't lock up. I'll resend if you still prefer=

> to cap it.

I like the cap version a lot better, and in fact we did used to have that bu=
t lost it early. I like it behaviorally a lot better, too.=20

Can you resend? Thanks!