hv_netvsc: use start_remove flag to protect netvsc_link_change()

netvsc_link_change() can race with netvsc_change_mtu() or
netvsc_set_channels() as these functions destroy struct netvsc_device and
rndis filter. Use start_remove flag for syncronization. As
netvsc_change_mtu()/netvsc_set_channels() are called with rtnl lock held
we need to take it before checking start_remove value in
netvsc_link_change().
Reported-by: NHaiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc_drv.c

@@ -838,6 +838,8 @@ static int netvsc_set_channels(struct net_device *net,
  out:
 	netvsc_open(net);
 	net_device_ctx->start_remove = false;
+	/* We may have missed link change notifications */
+	schedule_delayed_work(&net_device_ctx->dwork, 0);
 
 	return ret;
 
@@ -946,6 +948,9 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
 	netvsc_open(ndev);
 	ndevctx->start_remove = false;
 
+	/* We may have missed link change notifications */
+	schedule_delayed_work(&ndevctx->dwork, 0);
+
 	return ret;
 }
 
@@ -1066,6 +1071,11 @@ static void netvsc_link_change(struct work_struct *w)
 	unsigned long flags, next_reconfig, delay;
 
 	ndev_ctx = container_of(w, struct net_device_context, dwork.work);
+
+	rtnl_lock();
+	if (ndev_ctx->start_remove)
+		goto out_unlock;
+
 	net_device = hv_get_drvdata(ndev_ctx->device_ctx);
 	rdev = net_device->extension;
 	net = net_device->ndev;
@@ -1079,7 +1089,7 @@ static void netvsc_link_change(struct work_struct *w)
 		delay = next_reconfig - jiffies;
 		delay = delay < LINKCHANGE_INT ? delay : LINKCHANGE_INT;
 		schedule_delayed_work(&ndev_ctx->dwork, delay);
-		return;
+		goto out_unlock;
 	}
 	ndev_ctx->last_reconfig = jiffies;
 
@@ -1093,9 +1103,7 @@ static void netvsc_link_change(struct work_struct *w)
 	spin_unlock_irqrestore(&ndev_ctx->lock, flags);
 
 	if (!event)
-		return;
-
-	rtnl_lock();
+		goto out_unlock;
 
 	switch (event->event) {
 		/* Only the following events are possible due to the check in
@@ -1144,6 +1152,11 @@ static void netvsc_link_change(struct work_struct *w)
 	 */
 	if (reschedule)
 		schedule_delayed_work(&ndev_ctx->dwork, LINKCHANGE_INT);
+
+	return;
+
+out_unlock:
+	rtnl_unlock();
 }
 
 static void netvsc_free_netdev(struct net_device *netdev)