Staging: batman-adv: Don't remove interface with spinlock held

We call a lot of the netdevice code when holding if_list_lock which will
spin the whole time. This is not necessary because we only want to
protect the access to the list to be serialized. An extra queue can be
used which hold all interfaces which should be removed and then use that
queue without any locks for netdevice cleanup.

We create a "scheduling while atomic" Oops when calling different
netdevice related functions inside a spinlock protected area on a
preemtible kernel.
Reported-by: Rafal Lesniak <lesniak@eresi-project.org>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/batman-adv/hard-interface.c

@@ -463,9 +463,6 @@ static void hardif_remove_interface(struct batman_if *batman_if)
 		return;
 
 	batman_if->if_status = IF_TO_BE_REMOVED;
-
-	/* caller must take if_list_lock */
-	list_del_rcu(&batman_if->list);
 	synchronize_rcu();
 	sysfs_del_hardif(&batman_if->hardif_obj);
 	hardif_put(batman_if);
@@ -474,13 +471,21 @@ static void hardif_remove_interface(struct batman_if *batman_if)
 void hardif_remove_interfaces(void)
 {
 	struct batman_if *batman_if, *batman_if_tmp;
+	struct list_head if_queue;
+
+	INIT_LIST_HEAD(&if_queue);
 
-	rtnl_lock();
 	spin_lock(&if_list_lock);
 	list_for_each_entry_safe(batman_if, batman_if_tmp, &if_list, list) {
-		hardif_remove_interface(batman_if);
+		list_del_rcu(&batman_if->list);
+		list_add_tail(&batman_if->list, &if_queue);
 	}
 	spin_unlock(&if_list_lock);
+
+	rtnl_lock();
+	list_for_each_entry_safe(batman_if, batman_if_tmp, &if_queue, list) {
+		hardif_remove_interface(batman_if);
+	}
 	rtnl_unlock();
 }
 
@@ -507,8 +512,10 @@ static int hard_if_event(struct notifier_block *this,
 		break;
 	case NETDEV_UNREGISTER:
 		spin_lock(&if_list_lock);
-		hardif_remove_interface(batman_if);
+		list_del_rcu(&batman_if->list);
 		spin_unlock(&if_list_lock);
+
+		hardif_remove_interface(batman_if);
 		break;
 	case NETDEV_CHANGEMTU:
 		if (batman_if->soft_iface)