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

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Message-ID: <CADUfDZpH+6mx=rYb9uoL2z3-9DCmLLnF8vTBR8DX-PQBH+nqaw@mail.gmail.com>
Date: Thu, 11 Sep 2025 09:14:06 -0700
From: Caleb Sander Mateos <csander@...estorage.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>, 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 Wed, Sep 10, 2025 at 4:56 AM Pavel Begunkov <asml.silence@...il.com> 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.

If you have a link to this prior discussion, I'd appreciate it. I had
tried searching through the io-uring lore archives, but apparently I
don't know the magic keywords. If there are specific issues with this
locking model, I'd like to understand and address them. But opaque
references to known "bugs" are not helpful in improving these patches.

Thanks,
Caleb