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

On Mon, Jun 15, 2026 at 2:51 PM Jens Axboe <[email protected]> wrote:
>
> On 6/15/26 2:40 PM, Caleb Sander Mateos wrote:
> >> @@ -34,10 +34,6 @@ void io_tctx_fallback_work(struct work_struct *work)
> >>                                                   fallback_work);
> >>         unsigned int count = 0;
> >>
> >> -       /* see tctx_task_work() - a set bit must always have a run coming */
> >> -       clear_bit(0, &tctx->tw_pending);
> >> -       smp_mb__after_atomic();
> >> -
> >>         /*
> >>          * Run the entries directly. We're in PF_KTHRED context, hence
> >>          * io_should_terminate_tw() is true and they will be marked as
> >> @@ -101,6 +97,13 @@ void tctx_task_work_run(struct io_uring_task *tctx, unsigned int max_entries,
> >>                                 io_poll_task_func, io_req_rw_complete,
> >>                                 (struct io_tw_req){req}, ts);
> >>                 (*count)++;
> >> +               /*
> >> +                * Break if most recent pop emptied the queue. This helps
> >> +                * bound task_work run, and also protects the regular
> >> +                * task_work addition.
> >> +                */
> >> +               if (mpscq_pop_emptied(&tctx->task_list, tctx->task_head))
> >> +                       break;
> >
> > I think we can now remove the "if (mpscq_empty(&tctx->task_list))
> > break;" above? The queue must be nonempty initially, otherwise the
> > task work wouldn't have been scheduled. And if the queue is empty
> > after an attempted pop, the previous iteration of this loop must have
> > successfully marked the queue as empty.
>
> We could, but then we'd need to special case the SQPOLL side. I think
> it's better if we just leave it somewhat defensive as-is, it's just a
> single compare anyway, non-atomic.

Fine by me.

Best,
Caleb