kernfs: implement kernfs_path_len()

Add a function to determine the path length of a kernfs node.  This
for now will be used by writeback tracepoint updates.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/kernfs/dir.c

@@ -91,6 +91,29 @@ int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
 	return ret;
 }
 
+/**
+ * kernfs_path_len - determine the length of the full path of a given node
+ * @kn: kernfs_node of interest
+ *
+ * The returned length doesn't include the space for the terminating '\0'.
+ */
+size_t kernfs_path_len(struct kernfs_node *kn)
+{
+	size_t len = 0;
+	unsigned long flags;
+
+	spin_lock_irqsave(&kernfs_rename_lock, flags);
+
+	do {
+		len += strlen(kn->name) + 1;
+		kn = kn->parent;
+	} while (kn && kn->parent);
+
+	spin_unlock_irqrestore(&kernfs_rename_lock, flags);
+
+	return len;
+}
+
 /**
  * kernfs_path - build full path of a given node
  * @kn: kernfs_node of interest

include/linux/kernfs.h

@@ -266,6 +266,7 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
 }
 
 int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen);
+size_t kernfs_path_len(struct kernfs_node *kn);
 char * __must_check kernfs_path(struct kernfs_node *kn, char *buf,
 				size_t buflen);
 void pr_cont_kernfs_name(struct kernfs_node *kn);
@@ -332,6 +333,9 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
 static inline int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
 { return -ENOSYS; }
 
+static inline size_t kernfs_path_len(struct kernfs_node *kn)
+{ return 0; }
+
 static inline char * __must_check kernfs_path(struct kernfs_node *kn, char *buf,
 					      size_t buflen)
 { return NULL; }