Introduce rb_replace_node_rcu()

Implement an RCU-safe variant of rb_replace_node() and rearrange
rb_replace_node() to do things in the same order.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

1 files changed, 24 insertions, 2 deletions

diff --git a/lib/rbtree.c b/lib/rbtree.c
index 1356454e36de9..eb8a19fee1100 100644
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -539,17 +539,39 @@ void rb_replace_node(struct rb_node *victim, struct rb_node *new,
 {
 	struct rb_node *parent = rb_parent(victim);
 
+	/* Copy the pointers/colour from the victim to the replacement */
+	*new = *victim;
+
 	/* Set the surrounding nodes to point to the replacement */
-	__rb_change_child(victim, new, parent, root);
 	if (victim->rb_left)
 		rb_set_parent(victim->rb_left, new);
 	if (victim->rb_right)
 		rb_set_parent(victim->rb_right, new);
+	__rb_change_child(victim, new, parent, root);
+}
+EXPORT_SYMBOL(rb_replace_node);
+
+void rb_replace_node_rcu(struct rb_node *victim, struct rb_node *new,
+			 struct rb_root *root)
+{
+	struct rb_node *parent = rb_parent(victim);
 
 	/* Copy the pointers/colour from the victim to the replacement */
 	*new = *victim;
+
+	/* Set the surrounding nodes to point to the replacement */
+	if (victim->rb_left)
+		rb_set_parent(victim->rb_left, new);
+	if (victim->rb_right)
+		rb_set_parent(victim->rb_right, new);
+
+	/* Set the parent's pointer to the new node last after an RCU barrier
+	 * so that the pointers onwards are seen to be set correctly when doing
+	 * an RCU walk over the tree.
+	 */
+	__rb_change_child_rcu(victim, new, parent, root);
 }
-EXPORT_SYMBOL(rb_replace_node);
+EXPORT_SYMBOL(rb_replace_node_rcu);
 
 static struct rb_node *rb_left_deepest_node(const struct rb_node *node)
 {