Re: [PATCH 2/2] io_uring: add memory barrier to synchronize io_kiocb's result and iopoll_completed

Jens Axboe
2020-06-15T14:36:41Z

On 6/15/20 3:24 AM, Xiaoguang Wang wrote:
> In io_complete_rw_iopoll(), stores to io_kiocb's result and iopoll
> completed are two independent store operations, to ensure that once
> iopoll_completed is ture and then req->result must been perceived by
> the cpu executing io_do_iopoll(), proper memory barrier should be used.
>
> And in io_do_iopoll(), we check whether req->result is EAGAIN, if it is,
> we'll need to issue this io request using io-wq again. In order to just
> issue a single smp_rmb() on the completion side, move the re-submit work
> to io_iopoll_complete().

Did you actually test this one?

> @@ -1736,11 +1748,20 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
> {
> struct req_batch rb;
> struct io_kiocb *req;
> + LIST_HEAD(again);
> +
> + /* order with ->result store in io_complete_rw_iopoll() */
> + smp_rmb();
>
> rb.to_free = rb.need_iter = 0;
> while (!list_empty(done)) {
> int cflags = 0;
>
> + if (READ_ONCE(req->result) == -EAGAIN) {
> + req->iopoll_completed = 0;
> + list_move_tail(&req->list, &again);
> + continue;
> + }
> req = list_first_entry(done, struct io_kiocb, list);
> list_del(&req->list);
>

You're using 'req' here before you initialize it...

--
Jens Axboe