Re: [PATCHv5 1/1] io_uring: add support for IORING_SETUP_SQE_MIXED

[Keith Busch <kbusch@kernel.org>]

On Tue, Oct 14, 2025 at 03:33:19PM -0700, Caleb Sander Mateos wrote:
> On Mon, Oct 13, 2025 at 11:00 AM Keith Busch <kbusch@meta.com> wrote:
> > +               /*
> > +                * A 128b op on a non-128b SQ requires mixed SQE support as
> > +                * well as 2 contiguous entries.
> > +                */
> > +               if (!(ctx->flags & IORING_SETUP_SQE_MIXED) || *left < 2 ||
> > +                   !(ctx->cached_sq_head & (ctx->sq_entries - 1)))
> > +                       return io_init_fail_req(req, -EINVAL);
> > +               /*
> > +                * A 128b operation on a mixed SQ uses two entries, so we have
> > +                * to increment the head and decrement what's left.
> > +                */
> > +               ctx->cached_sq_head++;
> > +               (*left)--;
> 
> Hmm, io_submit_sqes() calls io_get_task_refs() at the start to
> decrement cached_refs by the number of SQEs (counting 128-byte SQEs
> twice) but io_put_task() only increments it once for each completed
> request (counting 128-byte SQEs once). Does that mean there's a
> refcount leak? Perhaps io_submit_sqes() or this block needs to
> increment cached_refs to account for each 128-byte SQE?

It looks like you're right. I think the increment option is the easiest
way to deal with it, just adding this line into the above:

+ 	current->io_uring->cached_refs++;

I'm going to take a moment to figure out a good way to test this because
I don't think I'm hitting any problem with the admittedly simple tests
I've offered to liburing, so I may be missing something.