[PATCH] Introduce rb_replace_node_rcu()

[David Howells <dhowells@redhat.com>]

Hi Peter,

How about the attached patch?  Should I also reorder rb_replace_node() whilst
I'm at it so that the new node is initialised first (it shouldn't make a
difference, I know)?

David
---
commit 812667d2a82a6a8fe35a44e951e8b1515b04696a
Author: David Howells <dhowells@redhat.com>
Date:   Fri Jul 1 07:53:51 2016 +0100

    Introduce rb_replace_node_rcu()
    
    Implement an RCU-safe variant of rb_replace_node().
    
    Signed-off-by: David Howells <dhowells@redhat.com>
    cc: Peter Zijlstra <peterz@infradead.org>

diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
index b6900099ea81..e585018498d5 100644
--- a/include/linux/rbtree.h
+++ b/include/linux/rbtree.h
@@ -76,6 +76,8 @@ extern struct rb_node *rb_next_postorder(const struct rb_node *);
 /* Fast replacement of a single node without remove/rebalance/add/rebalance */
 extern void rb_replace_node(struct rb_node *victim, struct rb_node *new,
 			    struct rb_root *root);
+extern void rb_replace_node_rcu(struct rb_node *victim, struct rb_node *new,
+				struct rb_root *root);
 
 static inline void rb_link_node(struct rb_node *node, struct rb_node *parent,
 				struct rb_node **rb_link)
diff --git a/include/linux/rbtree_augmented.h b/include/linux/rbtree_augmented.h
index 14d7b831b63a..d076183e49be 100644
--- a/include/linux/rbtree_augmented.h
+++ b/include/linux/rbtree_augmented.h
@@ -130,6 +130,19 @@ __rb_change_child(struct rb_node *old, struct rb_node *new,
 		WRITE_ONCE(root->rb_node, new);
 }
 
+static inline void
+__rb_change_child_rcu(struct rb_node *old, struct rb_node *new,
+		      struct rb_node *parent, struct rb_root *root)
+{
+	if (parent) {
+		if (parent->rb_left == old)
+			rcu_assign_pointer(parent->rb_left, new);
+		else
+			rcu_assign_pointer(parent->rb_right, new);
+	} else
+		rcu_assign_pointer(root->rb_node, new);
+}
+
 extern void __rb_erase_color(struct rb_node *parent, struct rb_root *root,
 	void (*augment_rotate)(struct rb_node *old, struct rb_node *new));
 
diff --git a/lib/rbtree.c b/lib/rbtree.c
index 1356454e36de..59eb906c6c3b 100644
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -551,6 +551,25 @@ void rb_replace_node(struct rb_node *victim, struct rb_node *new,
 }
 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 onward pointer last with an RCU barrier */
+	__rb_change_child_rcu(victim, new, parent, root);
+}
+EXPORT_SYMBOL(rb_replace_node_rcu);
+
 static struct rb_node *rb_left_deepest_node(const struct rb_node *node)
 {
 	for (;;) {