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[PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL

Caleb Sander Mateos posted 6 patches 18 hours ago
[PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
Posted by Caleb Sander Mateos 18 hours ago 
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 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
Re: [PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
Posted by Joanne Koong 13 hours ago 
On Tue, Dec 16, 2025 at 4:10 AM Caleb Sander Mateos
<csander@purestorage.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@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 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
>
Re: [PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
Posted by Caleb Sander Mateos 12 hours ago 
On Mon, Dec 15, 2025 at 4:50 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Tue, Dec 16, 2025 at 4:10 AM Caleb Sander Mateos
> <csander@purestorage.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

Yes, that is correct.

> -EINVAL if the user sets both IORING_SETUP_SQPOLL and
> IORING_SETUP_SINGLE_ISSUER? That seems clearer to me than silently

We can't break existing userspace applications that are setting both
flags. It may not be a recommendation combination, but it is currently
valid. (It enforces that a single thread is making io_uring_enter() +
io_uring_register() syscalls for io_uring, but the kernel SQ thread is
the one actually issuing the io_uring requests.)

Best,
Caleb

> 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@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 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
> >

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