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

[Caleb Sander Mateos <csander@purestorage.com>]

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
> >