Subject: Re: [PATCH 1/2] remove rcu_assign_pointer(NULL) penalty with type/macro safety

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 13 Feb 2008 17:34:27 -0800

On Wed, Feb 13, 2008 at 04:53:56PM -0800, Stephen Hemminger wrote:
> On Wed, 13 Feb 2008 16:42:53 -0800
> "Paul E. McKenney" <[email-masked]> wrote:
> > On Wed, Feb 13, 2008 at 04:27:00PM -0800, Stephen Hemminger wrote:

[ . . . ]

> > > That is heading towards ugly...  Maybe not using the macro at all (for this case) would be best:
> > > 
> > > static inline void node_set_parent(struct node *node, struct tnode *ptr)
> > > {
> > > 	smp_wmb();
> > > 	node->parent = (unsigned long)ptr | NODE_TYPE(node);
> > > }
> > 
> > Or, alternatively, the rcu_assign_index() patch sent earlier to avoid
> > the bare memory barrier?
> > 
> > 							Thanx, Paul
> 
> I am fine with rcu_assign_index(), and add a comment in node_set_parent.

OK, how about the following?

Signed-off-by: Paul E. McKenney <[email-masked]>
---

 fib_trie.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff -urpNa -X dontdiff linux-2.6.25-rc1/net/ipv4/fib_trie.c linux-2.6.25-rc1-fib_trie-warn.compile/net/ipv4/fib_trie.c
--- linux-2.6.25-rc1/net/ipv4/fib_trie.c	2008-02-13 14:38:12.000000000 -0800
+++ linux-2.6.25-rc1-fib_trie-warn.compile/net/ipv4/fib_trie.c	2008-02-13 17:31:16.000000000 -0800
@@ -96,6 +96,14 @@ typedef unsigned int t_key;
 #define IS_TNODE(n) (!(n->parent & T_LEAF))
 #define IS_LEAF(n) (n->parent & T_LEAF)
 
+/*
+ * The "parent" fields in struct node and struct leaf are really pointers,
+ * but with the possibility that the T_LEAF bit is set.  Therefore, both
+ * the C compiler and RCU see them as integers rather than pointers.
+ * This in turn means that rcu_assign_index() must be used to assign
+ * values to these fields, rather than the usual rcu_assign_pointer().
+ */
+
 struct node {
 	unsigned long parent;
 	t_key key;
@@ -179,8 +187,7 @@ static inline struct tnode *node_parent_
 
 static inline void node_set_parent(struct node *node, struct tnode *ptr)
 {
-	rcu_assign_pointer(node->parent,
-			   (unsigned long)ptr | NODE_TYPE(node));
+	rcu_assign_index(node->parent, (unsigned long)ptr | NODE_TYPE(node));
 }
 
 static inline struct node *tnode_get_child(struct tnode *tn, unsigned int i)