configfs: accessing item hierarchy during rmdir(2)

Add a notification callback, ops->disconnect_notify(). It has the same
prototype as ->drop_item(), but it will be called just before the item
linkage is broken. This way, configfs users who want to do work while
the object is still in the heirarchy have a chance.

Client drivers will still need to config_item_put() in their
->drop_item(), if they implement it.  They need do nothing in
->disconnect_notify().  They don't have to provide it if they don't
care.  But someone who wants to be notified before ci_parent is set to
NULL can now be notified.
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

Documentation/filesystems/configfs/configfs.txt

@@ -238,6 +238,8 @@ config_item_type.
 		struct config_group *(*make_group)(struct config_group *group,
 						   const char *name);
 		int (*commit_item)(struct config_item *item);
+		void (*disconnect_notify)(struct config_group *group,
+					  struct config_item *item);
 		void (*drop_item)(struct config_group *group,
 				  struct config_item *item);
 	};
@@ -268,6 +270,16 @@ the item in other threads, the memory is safe.  It may take some time
 for the item to actually disappear from the subsystem's usage.  But it
 is gone from configfs.
 
+When drop_item() is called, the item's linkage has already been torn
+down.  It no longer has a reference on its parent and has no place in
+the item hierarchy.  If a client needs to do some cleanup before this
+teardown happens, the subsystem can implement the
+ct_group_ops->disconnect_notify() method.  The method is called after
+configfs has removed the item from the filesystem view but before the
+item is removed from its parent group.  Like drop_item(),
+disconnect_notify() is void and cannot fail.  Client subsystems should
+not drop any references here, as they still must do it in drop_item().
+
 A config_group cannot be removed while it still has child items.  This
 is implemented in the configfs rmdir(2) code.  ->drop_item() will not be
 called, as the item has not been dropped.  rmdir(2) will fail, as the

fs/configfs/dir.c

@@ -713,6 +713,28 @@ static void configfs_detach_group(struct config_item *item)
 	configfs_detach_item(item);
 }
 
+/*
+ * After the item has been detached from the filesystem view, we are
+ * ready to tear it out of the hierarchy.  Notify the client before
+ * we do that so they can perform any cleanup that requires
+ * navigating the hierarchy.  A client does not need to provide this
+ * callback.  The subsystem semaphore MUST be held by the caller, and
+ * references must be valid for both items.  It also assumes the
+ * caller has validated ci_type.
+ */
+static void client_disconnect_notify(struct config_item *parent_item,
+				     struct config_item *item)
+{
+	struct config_item_type *type;
+
+	type = parent_item->ci_type;
+	BUG_ON(!type);
+
+	if (type->ct_group_ops && type->ct_group_ops->disconnect_notify)
+		type->ct_group_ops->disconnect_notify(to_config_group(parent_item),
+						      item);
+}
+
 /*
  * Drop the initial reference from make_item()/make_group()
  * This function assumes that reference is held on item
@@ -842,11 +864,14 @@ static int configfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
 	if (ret) {
 		/* Tear down everything we built up */
 		mutex_lock(&subsys->su_mutex);
+
+		client_disconnect_notify(parent_item, item);
 		if (group)
 			unlink_group(group);
 		else
 			unlink_obj(item);
 		client_drop_item(parent_item, item);
+
 		mutex_unlock(&subsys->su_mutex);
 
 		if (module_got)
@@ -911,11 +936,13 @@ static int configfs_rmdir(struct inode *dir, struct dentry *dentry)
 		configfs_detach_group(item);
 
 		mutex_lock(&subsys->su_mutex);
+		client_disconnect_notify(parent_item, item);
 		unlink_group(to_config_group(item));
 	} else {
 		configfs_detach_item(item);
 
 		mutex_lock(&subsys->su_mutex);
+		client_disconnect_notify(parent_item, item);
 		unlink_obj(item);
 	}
 

include/linux/configfs.h

@@ -169,6 +169,7 @@ struct configfs_group_operations {
 	struct config_item *(*make_item)(struct config_group *group, const char *name);
 	struct config_group *(*make_group)(struct config_group *group, const char *name);
 	int (*commit_item)(struct config_item *item);
+	void (*disconnect_notify)(struct config_group *group, struct config_item *item);
 	void (*drop_item)(struct config_group *group, struct config_item *item);
 };