[PATCH -next] virtio_net: Avoid loop in virtnet_poll

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_napi_schedule //it will reschedule napi

Signed-off-by: Mao Wenan <redacted>
---
 drivers/net/virtio_net.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ba38765..a058da1 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -327,7 +327,8 @@ static void virtqueue_napi_complete(struct napi_struct *napi,
 
 	opaque = virtqueue_enable_cb_prepare(vq);
 	if (napi_complete_done(napi, processed)) {
-		if (unlikely(virtqueue_poll(vq, opaque)))
+		if (unlikely(virtqueue_poll(vq, opaque)) &&
+		    unlikely(!virtqueue_is_broken(vq)))
 			virtqueue_napi_schedule(napi, vq);
 	} else {
 		virtqueue_disable_cb(vq);

-- 
1.8.3.1

On Sun, Aug 02, 2020 at 11:41:23AM +0800, 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_napi_schedule //it will reschedule napi

Signed-off-by: Mao Wenan <redacted>

I think it's more a bug in virtqueue_poll : virtqueue_get_buf reports
NULL on broken, so virtqueue_poll should report false for consistency.