Re: [PATCH 4/6] io_uring: switch normal task_work to a mpscq

On Fri, Jun 12, 2026 at 12:37 PM Jens Axboe <[email protected]> wrote:
>
> On 6/12/26 12:59 PM, Caleb Sander Mateos wrote:
> >> @@ -236,10 +262,14 @@ void io_req_normal_work_add(struct io_kiocb *req)
> >>                 return;
> >>         }
> >>
> >> +       /* task_work must only be added once */
> >> +       if (test_and_set_bit(0, &tctx->tw_pending))
> >> +               return;
> >
> > Is tw_pending necessary? How come the task_work_add() exclusivity
> > isn't already provided by the mpscq_push() check above?
>
> It is, because the transition from empty -> not-empty no longer works
> for that, as the mpscq emtpies one-by-one rather than with a delete-all
> kind of primitive.

Sorry, I'm still not following why the empty check doesn't suffice.
It's true that mpscq elements can be removed from the head one at a
time, but mpscq_push() will continue to return false until the
consumer pops all the elements and successfully sets tail back to
&stub. mpscq_push() will return true once when tail transitions away
from &stub, and then not again until the task work runs and sets tail
back to &stub.

Thanks,
Caleb