linux-kernel - Re: [PATCH 1/1] io_uring: don't wait when under-submitting

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Message-Id: <B50496AA-BE78-45B7-A17D-9740FF42A8EF@kernel.dk>
Date:   Fri, 13 Dec 2019 14:48:28 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] io_uring: don't wait when under-submitting


> On Dec 13, 2019, at 2:32 PM, Pavel Begunkov <asml.silence@...il.com> wrote:
> 
> On 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.
>>> 
>>> Why not just cap the wait_nr? If someone does to_submit = 8, wait_nr = 8,
>>> and we only submit 4, just wait for 4? Ala:
>>> 
> 
> 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)
> 
> 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 but lost it early. I like it behaviorally a lot better, too. 

Can you resend? Thanks!