Linux Virtualization - Aug 2020 - [PATCH -next v3] virtio_ring: Avoid loop when vq is broken in virtqueue_poll

On 2020/8/2 ??3:44, Mao Wenan wrote:
> The loop may exist if vq->broken is true,
> virtqueue_get_buf_ctx_packed or virtqueue_get_buf_ctx_split
> will return NULL, so virtnet_poll will reschedule napi to
> receive packet, it will lead cpu usage(si) to 100%.
>
> call trace as below:
> virtnet_poll
> 	virtnet_receive
> 		virtqueue_get_buf_ctx
> 			virtqueue_get_buf_ctx_packed
> 			virtqueue_get_buf_ctx_split
> 	virtqueue_napi_complete
> 		virtqueue_poll           //return true
> 		virtqueue_napi_schedule //it will reschedule napi
>
> to fix this, return false if vq is broken in virtqueue_poll.
>
> Signed-off-by: Mao Wenan <wenan.mao at linux.alibaba.com>
> Acked-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>   v2->v3: change subject, original is : "virtio_net: Avoid loop in
virtnet_poll"
>   v1->v2: fix it in virtqueue_poll suggested by Michael S. Tsirkin
<mst at redhat.com>
>   drivers/virtio/virtio_ring.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 58b96ba..4f7c73e 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1960,6 +1960,9 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned
last_used_idx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> +	if (unlikely(vq->broken))
> +		return false;
> +
>   	virtio_mb(vq->weak_barriers);
>   	return vq->packed_ring ? virtqueue_poll_packed(_vq, last_used_idx) :
>   				 virtqueue_poll_split(_vq, last_used_idx);

Acked-by: Jason Wang <jasowang at redhat.com>