Re: [Qemu-devel] [RFC PATCH] async: Fix aio_notify_accept

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/08/2018 17:49, Fam Zheng wrote:
>  void aio_notify_accept(AioContext *ctx)
>  {
> -    if (atomic_xchg(&ctx->notified, false)) {
> +    /* If ctx->notify_me >= 2, another aio_poll() is waiting which may need the
> +     * ctx->notifier event to wake up, so don't already clear it just because "we" are
> +     * done iterating. */
> +    if (atomic_read(&ctx->notify_me) < 2
> +        && atomic_xchg(&ctx->notified, false)) {
>          event_notifier_test_and_clear(&ctx->notifier);
>      }
>  }

I'm worried that this would this cause a busy wait, and I don't
understand the issue.

When aio_poll()s are nested, outer calls are in the "dispatch" phase and
therefore do not need notification.

In your situation is notify_me actually ever >2?

Paolo