kernfs: implement kernfs_{de|re}activate[_self]()

This patch implements four functions to manipulate deactivation state
- deactivate, reactivate and the _self suffixed pair.  A new fields
kernfs_node->deact_depth is added so that concurrent and nested
deactivations are handled properly.  kernfs_node->hash is moved so
that it's paired with the new field so that it doesn't increase the
size of kernfs_node.

A kernfs user's lock would normally nest inside active ref but during
removal the user may want to perform kernfs_remove() while holding the
said lock, which would introduce a reverse locking dependency.  This
function can be used to break such reverse dependency by allowing
deactivation step to performed separately outside user's critical
section.

This will also be used implement kernfs_remove_self().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 9b5a4bb..ac86930 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -80,6 +80,8 @@ struct kernfs_elem_attr {
 struct kernfs_node {
 	atomic_t		count;
 	atomic_t		active;
+	int			deact_depth;
+	unsigned int		hash;	/* ns + name hash */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map	dep_map;
 #endif
@@ -90,7 +92,6 @@ struct kernfs_node {
 	struct rb_node		rb;
 
 	const void		*ns;	/* namespace tag */
-	unsigned int		hash;	/* ns + name hash */
 	union {
 		struct kernfs_elem_dir		dir;
 		struct kernfs_elem_symlink	symlink;
@@ -233,6 +234,10 @@ struct kernfs_node *__kernfs_create_file(struct kernfs_node *parent,
 struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
 				       const char *name,
 				       struct kernfs_node *target);
+void kernfs_deactivate(struct kernfs_node *kn);
+void kernfs_reactivate(struct kernfs_node *kn);
+void kernfs_deactivate_self(struct kernfs_node *kn);
+void kernfs_reactivate_self(struct kernfs_node *kn);
 void kernfs_remove(struct kernfs_node *kn);
 int kernfs_remove_by_name_ns(struct kernfs_node *parent, const char *name,
 			     const void *ns);