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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 01, 2017 at 12:50:04PM +0100, 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.  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);
> +    }

Does this fail because the underlying fd got closed?
Then there's a problem: trying to write to a closed
fd might corrupt an unrelated fd.
If you want to use this way we need to set fds to -1 when we close.

>  }
>  
>  static void virtio_irq(VirtQueue *vq)
> -- 
> 2.8.4