klist: implement klist_prev()

klist_prev() gets the previous element in the list. It is useful to traverse
through the list in reverse order, for example, to provide LIFO (last in first
out) variant of access.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

lib/klist.c

@@ -323,6 +323,47 @@ static struct klist_node *to_klist_node(struct list_head *n)
 	return container_of(n, struct klist_node, n_node);
 }
 
+/**
+ * klist_prev - Ante up prev node in list.
+ * @i: Iterator structure.
+ *
+ * First grab list lock. Decrement the reference count of the previous
+ * node, if there was one. Grab the prev node, increment its reference
+ * count, drop the lock, and return that prev node.
+ */
+struct klist_node *klist_prev(struct klist_iter *i)
+{
+	void (*put)(struct klist_node *) = i->i_klist->put;
+	struct klist_node *last = i->i_cur;
+	struct klist_node *prev;
+
+	spin_lock(&i->i_klist->k_lock);
+
+	if (last) {
+		prev = to_klist_node(last->n_node.prev);
+		if (!klist_dec_and_del(last))
+			put = NULL;
+	} else
+		prev = to_klist_node(i->i_klist->k_list.prev);
+
+	i->i_cur = NULL;
+	while (prev != to_klist_node(&i->i_klist->k_list)) {
+		if (likely(!knode_dead(prev))) {
+			kref_get(&prev->n_ref);
+			i->i_cur = prev;
+			break;
+		}
+		prev = to_klist_node(prev->n_node.prev);
+	}
+
+	spin_unlock(&i->i_klist->k_lock);
+
+	if (put && last)
+		put(last);
+	return i->i_cur;
+}
+EXPORT_SYMBOL_GPL(klist_prev);
+
 /**
  * klist_next - Ante up next node in list.
  * @i: Iterator structure.