Re: [PATCH v2 3/5] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL

[Jens Axboe <axboe@kernel.dk>]

On 9/4/25 11:09 AM, Caleb Sander Mateos 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.
> 
> Signed-off-by: Caleb Sander Mateos <csander@purestorage.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 42f6bfbb99d3..c7af9dc3d95a 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3724,10 +3724,19 @@ static int io_uring_sanitise_params(struct io_uring_params *p)
>  	 */
>  	if ((flags & (IORING_SETUP_CQE32|IORING_SETUP_CQE_MIXED)) ==
>  	    (IORING_SETUP_CQE32|IORING_SETUP_CQE_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 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;
> +

As mentioned I think this is fine. Just for posterity, one solution
here would be to require that the task doing eg io_uring_register() on a
setup with SINGLE_ISSUER|SQPOLL would be required to park and unpark the
SQ thread before doing what it needs to do. That should get us most/all
of the way there to enabling it with SQPOLL as well.

-- 
Jens Axboe