netvsc: fix rtnl deadlock on unregister of vf

With new transparent VF support, it is possible to get a deadlock
when some of the deferred work is running and the unregister_vf
is trying to cancel the work element. The solution is to use
trylock and reschedule (similar to bonding and team device).

Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Fixes: 0c195567a8f6 ("netvsc: transparent VF management")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc_drv.c

@@ -1601,7 +1601,11 @@ static void netvsc_vf_setup(struct work_struct *w)
 	struct net_device *ndev = hv_get_drvdata(ndev_ctx->device_ctx);
 	struct net_device *vf_netdev;
 
-	rtnl_lock();
+	if (!rtnl_trylock()) {
+		schedule_work(w);
+		return;
+	}
+
 	vf_netdev = rtnl_dereference(ndev_ctx->vf_netdev);
 	if (vf_netdev)
 		__netvsc_vf_setup(ndev, vf_netdev);
@@ -1655,7 +1659,11 @@ static void netvsc_vf_update(struct work_struct *w)
 	struct net_device *vf_netdev;
 	bool vf_is_up;
 
-	rtnl_lock();
+	if (!rtnl_trylock()) {
+		schedule_work(w);
+		return;
+	}
+
 	vf_netdev = rtnl_dereference(ndev_ctx->vf_netdev);
 	if (!vf_netdev)
 		goto unlock;