Re: [Qemu-devel] [PATCH v4 4/9] block/io_uring: implements interfaces for io_uring

[Stefan Hajnoczi]

On Mon, Jun 03, 2019 at 06:08:18PM +0530, Aarushi Mehta wrote:
> +static void qemu_luring_process_completions(LuringState *s)
> +{
> +    struct io_uring_cqe *cqes;
> +    int ret;
> +
> +    /*
> +     * Request completion callbacks can run the nested event loop.
> +     * Schedule ourselves so the nested event loop will "see" remaining
> +     * completed requests and process them.  Without this, completion
> +     * callbacks that wait for other requests using a nested event loop
> +     * would hang forever.
> +     */
> +    qemu_bh_schedule(s->completion_bh);
> +
> +    while (io_uring_peek_cqe(&s->ring, &cqes) == 0) {
> +        if (!cqes) {
> +            break;
> +        }
> +        LuringAIOCB *luringcb = io_uring_cqe_get_data(cqes);
> +        read_barrier();

What is the purpose of this barrier?

> +static void qemu_luring_process_completions_and_submit(LuringState *s)
> +{
> +    aio_context_acquire(s->aio_context);
> +    qemu_luring_process_completions(s);
> +
> +    if (!s->io_q.plugged && !s->io_q.in_queue) {

Submit when there are queued requesting waiting for submission:

s/!s->io_q.in_queue/s->io_q.in_queue > 0/

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