tun: Fix device unregister race

It is currently possible for an asynchronous device unregister
to cause the same tun device to be unregistered twice.  This
is because the unregister in tun_chr_close only checks whether
__tun_get(tfile) != NULL.  This however has nothing to do with
whether the device has already been unregistered.  All it tells
you is whether __tun_detach has been called.

This patch fixes this by using the most obvious thing to test
whether the device has been unregistered.

It also moves __tun_detach outside of rtnl_unlock since nothing
that it does requires that lock.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1324,20 +1324,22 @@ static int tun_chr_close(struct inode *inode, struct file *file)
 	struct tun_file *tfile = file->private_data;
 	struct tun_struct *tun;
 
-
-	rtnl_lock();
 	tun = __tun_get(tfile);
 	if (tun) {
-		DBG(KERN_INFO "%s: tun_chr_close\n", tun->dev->name);
+		struct net_device *dev = tun->dev;
+
+		DBG(KERN_INFO "%s: tun_chr_close\n", dev->name);
 
 		__tun_detach(tun);
 
 		/* If desireable, unregister the netdevice. */
-		if (!(tun->flags & TUN_PERSIST))
-			unregister_netdevice(tun->dev);
-
-	}
+		if (!(tun->flags & TUN_PERSIST)) {
+			rtnl_lock();
+			if (dev->reg_state == NETREG_REGISTERED)
+				unregister_netdevice(dev);
 			rtnl_unlock();
+		}
+	}
 
 	tun = tfile->tun;
 	if (tun)