virtio_net: avoid using netif_tx_disable() for serializing tx routine

Commit 713a98d90c5e ("virtio-net: serialize tx routine during reset")
introduces netif_tx_disable() after netif_device_detach() in order to
avoid use-after-free of tx queues. However, there are two issues.

1) Its operation is redundant with netif_device_detach() in case the
   interface is running.
2) In case of the interface is not running before suspending and
   resuming, the tx does not get resumed by netif_device_attach().
   This results in losing network connectivity.

It is better to use netif_tx_lock_bh()/netif_tx_unlock_bh() instead for
serializing tx routine during reset. This also preserves the symmetry
of netif_device_detach() and netif_device_attach().

Fixes commit 713a98d90c5e ("virtio-net: serialize tx routine during reset")
Signed-off-by: Ake Koomsin <ake@igel.co.jp>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -2218,8 +2218,9 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
 	/* Make sure no work handler is accessing the device */
 	flush_work(&vi->config_work);
 
+	netif_tx_lock_bh(vi->dev);
 	netif_device_detach(vi->dev);
-	netif_tx_disable(vi->dev);
+	netif_tx_unlock_bh(vi->dev);
 	cancel_delayed_work_sync(&vi->refill);
 
 	if (netif_running(vi->dev)) {
@@ -2255,7 +2256,9 @@ static int virtnet_restore_up(struct virtio_device *vdev)
 		}
 	}
 
+	netif_tx_lock_bh(vi->dev);
 	netif_device_attach(vi->dev);
+	netif_tx_unlock_bh(vi->dev);
 	return err;
 }