[PATCH net V2] virtio-net: correctly enable callback during start_xmit

[Jason Wang <jasowang@redhat.com>]

Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
virtqueue callback via the following statement:

        do {
           ......
	} while (use_napi && kick &&
               unlikely(!virtqueue_enable_cb_delayed(sq->vq)));

When NAPI is used and kick is false, the callback won't be enabled
here. And when the virtqueue is about to be full, the tx will be
disabled, but we still don't enable tx interrupt which will cause a TX
hang. This could be observed when using pktgen with burst enabled.

Fixing this by trying to enable tx interrupt after we disable TX when
we're not using napi or kick is false.

Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
The patch is needed for -stable.
Changes since V1:
- enable tx interrupt after we disable tx
---
 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 86e52454b5b5..dcf3a536d78a 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1873,7 +1873,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	 */
 	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
 		netif_stop_subqueue(dev, qnum);
-		if (!use_napi &&
+		if ((!use_napi || !kick) &&
 		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
 			/* More just got used, free them then recheck. */
 			free_old_xmit_skbs(sq, false);
-- 
2.25.1