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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <4ac9be27-7775-1863-c34c-cdce85eab53e@redhat.com>
Date: Fri, 3 Aug 2018 18:51:35 +0200
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Subject: Re: [Qemu-devel] [RFC PATCH] async: Fix aio_notify_accept
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To: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org
Cc: Stefan Hajnoczi <stefanha@redhat.com>, qemu-block@nongnu.org

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 >=3D 2, another aio_poll() is waiting which m=
ay 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