hv_netvsc: Fix race condition on Multi-Send Data field

BugLink: http://bugs.launchpad.net/bugs/1583357

In commit 2a04ae8a ("hv_netvsc: remove locking in netvsc_send()"), the
locking for MSD (Multi-Send Data) field was removed. This could cause a
race condition between RNDIS control messages and data packets processing,
because these two types of traffic are not synchronized.
This patch fixes this issue by sending control messages out directly
without reading MSD field.
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit cf8190e4)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/net/hyperv/netvsc.c

@@ -867,6 +867,14 @@ int netvsc_send(struct hv_device *device,
 	packet->send_buf_index = NETVSC_INVALID_INDEX;
 	packet->cp_partial = false;
 
+	/* Send control message directly without accessing msd (Multi-Send
+	 * Data) field which may be changed during data packet processing.
+	 */
+	if (!skb) {
+		cur_send = packet;
+		goto send_now;
+	}
+
 	msdp = &net_device->msd[q_idx];
 
 	/* batch packets in send buffer if possible */
@@ -939,6 +947,7 @@ int netvsc_send(struct hv_device *device,
 		}
 	}
 
+send_now:
 	if (cur_send)
 		ret = netvsc_send_pkt(cur_send, net_device, pb, skb);