linux-kernel - Re: [PATCH v2 0/5] io_uring: avoid uring_lock for IORING_SETUP_SINGLE

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Message-ID: <0ea91968-f0fc-4e6d-9910-c9802b4227aa@gmail.com>
Date: Thu, 11 Sep 2025 11:36:57 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Jens Axboe <axboe@...nel.dk>,
 Caleb Sander Mateos <csander@...estorage.com>
Cc: io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] io_uring: avoid uring_lock for
 IORING_SETUP_SINGLE_ISSUER

On 9/10/25 16:36, Jens Axboe wrote:
> On 9/10/25 5:57 AM, Pavel Begunkov wrote:
>> On 9/9/25 14:35, Jens Axboe wrote:
>>>
>>> On Thu, 04 Sep 2025 11:08:57 -0600, Caleb Sander Mateos wrote:
>>>> As far as I can tell, setting IORING_SETUP_SINGLE_ISSUER when creating
>>>> an io_uring doesn't actually enable any additional optimizations (aside
>>>> from being a requirement for IORING_SETUP_DEFER_TASKRUN). This series
>>>> leverages IORING_SETUP_SINGLE_ISSUER's guarantee that only one task
>>>> submits SQEs to skip taking the uring_lock mutex in the submission and
>>>> task work paths.
>>>>
>>>> [...]
>>>
>>> Applied, thanks!
>>>
>>> [1/5] io_uring: don't include filetable.h in io_uring.h
>>>         commit: 5d4c52bfa8cdc1dc1ff701246e662be3f43a3fe1
>>> [2/5] io_uring/rsrc: respect submitter_task in io_register_clone_buffers()
>>>         commit: 2f076a453f75de691a081c89bce31b530153d53b
>>> [3/5] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
>>>         commit: 6f5a203998fcf43df1d43f60657d264d1918cdcd
>>> [4/5] io_uring: factor out uring_lock helpers
>>>         commit: 7940a4f3394a6af801af3f2bcd1d491a71a7631d
>>> [5/5] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER
>>>         commit: 4cc292a0faf1f0755935aebc9b288ce578d0ced2
>>
>> FWIW, from a glance that should be quite broken, there is a bunch of
>> bits protected from parallel use by the lock. I described this
>> optimisation few years back around when first introduced SINGLE_ISSUER
>> and the DEFER_TASKRUN locking model, but to this day think it's not
>> worth it as it'll be a major pain for any future changes. It would've
>> been more feasible if links wasn't a thing. Though, none of it is
>> my problem anymore, and I'm not insisting.
> 
> Hmm yes, was actually pondering this last night as well and was going
> to take a closer look today as I have a flight coming up. I'll leave
> them in there for now just to see if syzbot finds anything, and take

Better not to? People are already nervous about security / bugs,
and there is no ambiguity how and where it's wrong. Also because
it breaks uapi in a couple places, with at least one of them
being a security hole.

> that closer look and see if it's salvageable for now or if we just need
> a new revised take on this.

-- 
Pavel Begunkov