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Subject[PATCH] Introduce rb_replace_node_rcu()
Date
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 (;;) {
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