linux-kernel - Re: [PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP

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Message-ID: <CAJnrk1YiZ6NuUavG86ZGpZ0nz8+fqi_SYkqx=UQWdWhTPj7mWQ@mail.gmail.com>
Date: Tue, 16 Dec 2025 08:50:29 +0800
From: Joanne Koong <joannelkoong@...il.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL

On Tue, Dec 16, 2025 at 4:10 AM Caleb Sander Mateos
<csander@...estorage.com> wrote:
>
> IORING_SETUP_SINGLE_ISSUER doesn't currently enable any optimizations,
> but it will soon be used to avoid taking io_ring_ctx's uring_lock when
> submitting from the single issuer task. If the IORING_SETUP_SQPOLL flag
> is set, the SQ thread is the sole task issuing SQEs. However, other
> tasks may make io_uring_register() syscalls, which must be synchronized
> with SQE submission. So it wouldn't be safe to skip the uring_lock
> around the SQ thread's submission even if IORING_SETUP_SINGLE_ISSUER is
> set. Therefore, clear IORING_SETUP_SINGLE_ISSUER from the io_ring_ctx
> flags if IORING_SETUP_SQPOLL is set.

If i'm understanding this correctly, these params are set by the user
and passed through the "struct io_uring_params" arg to the
io_uring_setup() syscall. Do you think it makes sense to return
-EINVAL if the user sets both IORING_SETUP_SQPOLL and
IORING_SETUP_SINGLE_ISSUER? That seems clearer to me than silently
unsetting IORING_SETUP_SINGLE_ISSUER where the user may set
IORING_SETUP_SINGLE_ISSUER expecting certain optimizations but be
unaware that IORING_SETUP_SQPOLL effectively overrides it.

Thanks,
Joanne

>
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> ---
>  io_uring/io_uring.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 761b9612c5b6..44ff5756b328 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3478,10 +3478,19 @@ static int io_uring_sanitise_params(struct io_uring_params *p)
>          */
>         if ((flags & (IORING_SETUP_SQE128|IORING_SETUP_SQE_MIXED)) ==
>             (IORING_SETUP_SQE128|IORING_SETUP_SQE_MIXED))
>                 return -EINVAL;
>
> +       /*
> +        * If IORING_SETUP_SQPOLL is set, only the SQ thread issues SQEs,
> +        * but other threads may call io_uring_register() concurrently.
> +        * We still need ctx uring lock to synchronize these io_ring_ctx
> +        * accesses, so disable the single issuer optimizations.
> +        */
> +       if (flags & IORING_SETUP_SQPOLL)
> +               p->flags &= ~IORING_SETUP_SINGLE_ISSUER;
> +
>         return 0;
>  }
>
>  static int io_uring_fill_params(struct io_uring_params *p)
>  {
> --
> 2.45.2
>