configfs: configfs_mkdir() failed to cleanup linkage.

If configfs_mkdir() errored in certain ways after the parent<->child
linkage was already created, it would not undo the linkage.  Also,
comment the reference counting for clarity.
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

fs/configfs/dir.c

@@ -505,13 +505,15 @@ static int populate_groups(struct config_group *group)
 	int i;
 
 	if (group->default_groups) {
-		/* FYI, we're faking mkdir here
+		/*
+		 * FYI, we're faking mkdir here
 		 * I'm not sure we need this semaphore, as we're called
 		 * from our parent's mkdir.  That holds our parent's
 		 * i_mutex, so afaik lookup cannot continue through our
 		 * parent to find us, let alone mess with our tree.
 		 * That said, taking our i_mutex is closer to mkdir
-		 * emulation, and shouldn't hurt. */
+		 * emulation, and shouldn't hurt.
+		 */
 		mutex_lock(&dentry->d_inode->i_mutex);
 
 		for (i = 0; group->default_groups[i]; i++) {
@@ -546,20 +548,34 @@ static void unlink_obj(struct config_item *item)
 
 		item->ci_group = NULL;
 		item->ci_parent = NULL;
+
+		/* Drop the reference for ci_entry */
 		config_item_put(item);
 
+		/* Drop the reference for ci_parent */
 		config_group_put(group);
 	}
 }
 
 static void link_obj(struct config_item *parent_item, struct config_item *item)
 {
-	/* Parent seems redundant with group, but it makes certain
-	 * traversals much nicer. */
+	/*
+	 * Parent seems redundant with group, but it makes certain
+	 * traversals much nicer.
+	 */
 	item->ci_parent = parent_item;
+
+	/*
+	 * We hold a reference on the parent for the child's ci_parent
+	 * link.
+	 */
 	item->ci_group = config_group_get(to_config_group(parent_item));
 	list_add_tail(&item->ci_entry, &item->ci_group->cg_children);
 
+	/*
+	 * We hold a reference on the child for ci_entry on the parent's
+	 * cg_children
+	 */
 	config_item_get(item);
 }
 
@@ -684,6 +700,10 @@ static void client_drop_item(struct config_item *parent_item,
 	type = parent_item->ci_type;
 	BUG_ON(!type);
 
+	/*
+	 * If ->drop_item() exists, it is responsible for the
+	 * config_item_put().
+	 */
 	if (type->ct_group_ops && type->ct_group_ops->drop_item)
 		type->ct_group_ops->drop_item(to_config_group(parent_item),
 						item);
@@ -694,14 +714,14 @@ static void client_drop_item(struct config_item *parent_item,
 
 static int configfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
 {
-	int ret;
+	int ret, module_got = 0;
 	struct config_group *group;
 	struct config_item *item;
 	struct config_item *parent_item;
 	struct configfs_subsystem *subsys;
 	struct configfs_dirent *sd;
 	struct config_item_type *type;
-	struct module *owner;
+	struct module *owner = NULL;
 	char *name;
 
 	if (dentry->d_parent == configfs_sb->s_root) {
@@ -754,43 +774,63 @@ static int configfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
 
 	kfree(name);
 	if (!item) {
+		/*
+		 * If item == NULL, then link_obj() was never called.
+		 * There are no extra references to clean up.
+		 */
 		ret = -ENOMEM;
 		goto out_put;
 	}
 
-	ret = -EINVAL;
+	/*
+	 * link_obj() has been called (via link_group() for groups).
+	 * From here on out, errors must clean that up.
+	 */
+
 	type = item->ci_type;
-	if (type) {
-		owner = type->ct_owner;
-		if (try_module_get(owner)) {
-			if (group) {
-				ret = configfs_attach_group(parent_item,
-							    item,
-							    dentry);
-			} else {
-				ret = configfs_attach_item(parent_item,
-							   item,
-							   dentry);
-			}
+	if (!type) {
+		ret = -EINVAL;
+		goto out_unlink;
+	}
 
-			if (ret) {
-				down(&subsys->su_sem);
-				if (group)
-					unlink_group(group);
-				else
-					unlink_obj(item);
-				client_drop_item(parent_item, item);
-				up(&subsys->su_sem);
+	owner = type->ct_owner;
+	if (!try_module_get(owner)) {
+		ret = -EINVAL;
+		goto out_unlink;
+	}
 
-				module_put(owner);
-			}
-		}
+	/*
+	 * I hate doing it this way, but if there is
+	 * an error,  module_put() probably should
+	 * happen after any cleanup.
+	 */
+	module_got = 1;
+
+	if (group)
+		ret = configfs_attach_group(parent_item, item, dentry);
+	else
+		ret = configfs_attach_item(parent_item, item, dentry);
+
+out_unlink:
+	if (ret) {
+		/* Tear down everything we built up */
+		down(&subsys->su_sem);
+		if (group)
+			unlink_group(group);
+		else
+			unlink_obj(item);
+		client_drop_item(parent_item, item);
+		up(&subsys->su_sem);
+
+		if (module_got)
+			module_put(owner);
 	}
 
 out_put:
 	/*
-	 * link_obj()/link_group() took a reference from child->parent.
-	 * Drop our working ref
+	 * link_obj()/link_group() took a reference from child->parent,
+	 * so the parent is safely pinned.  We can drop our working
+	 * reference.
 	 */
 	config_item_put(parent_item);