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

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Message-ID: <fe312d71-c546-4250-a730-79c23a92e028@gmail.com>
Date: Wed, 10 Sep 2025 12:57:42 +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 9/9/25 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.

-- 
Pavel Begunkov