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 <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/kernfs/dir.c

···
 }
 
 /**
+ * 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
  * @buf: buffer to copy @kn's name into

include/linux/kernfs.h

···
 }
 
 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);
···
 
 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)