Re: [PATCH] Introduce rb_replace_node_rcu()

From: David Howells
Date: Fri Jul 01 2016 - 04:08:18 EST


Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> One little niggle below, but:

How about the attached? I've updated that comment and rearranged
rb_replace_node_rcu().

David
---
commit 8bd38ef5a38728390348cc5b4a8797be16af493f
Author: David Howells <dhowells@xxxxxxxxxx>
Date: Fri Jul 1 07:53:51 2016 +0100

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@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

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..eb8a19fee110 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)
{