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

[Jens Axboe <axboe@kernel.dk>]

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