Re: [Qemu-devel] [PATCH 1/1] virtio: fallback from irqfd to non-irqfd notify

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/03/2017 12:50, Halil Pasic wrote:
> The commits 03de2f527 "virtio-blk: do not use vring in dataplane"  and
> 9ffe337c08 "virtio-blk: always use dataplane path if ioeventfd is active"
> changed how notifications are done for virtio-blk substantially. Due to a
> race condition interrupts are lost when irqfd is torn down after
> notify_guest_bh was scheduled but before it actually runs.

I don't think the non-irqfd notification mechanism is thread safe, and
that would be a problem for this patch.

What is the path that causes the irqfd to be torn down?  Only something
like a reset should cause it (the only call in virtio-blk is from
virtio_blk_data_plane_stop), and then the guest doesn't care anymore
about interrupts.

That path also does a qemu_bh_delete, so the notify_guest_bh should not
be invoked at all.

Paolo

>  Furthermore
> virtio_notify_irqfd ignores the value returned by event_notifier_set
> which correctly indicates that notification has failed due to bad file
> descriptor.
> 
> Let's fix this by making virtio_notify_irqfd fall back to the non-irqfd
> notification mechanism if event_notifier_set fails.
> 
> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> ---
> 
> This is probably not the only way to fix this: suggestions welcome. I
> did not use a fixes tag because I'm not sure yet where exactly things got
> broken. Maybe guys more familiar with dataplane an coroutines can help
> (Paolo, Stefan).
> ---
>  hw/virtio/virtio.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 23483c7..8e1c1e9 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1581,7 +1581,9 @@ void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
>       * to an atomic operation.
>       */
>      virtio_set_isr(vq->vdev, 0x1);
> -    event_notifier_set(&vq->guest_notifier);
> +    if (event_notifier_set(&vq->guest_notifier)) {
> +        virtio_notify_vector(vdev, vq->vector);
> +    }
>  }
>  
>  static void virtio_irq(VirtQueue *vq)
>