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

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Date:	Wed, 13 Feb 2008 17:37:39 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Stephen Hemminger 
	<"stephen.hemminger@...tta.com"@mail.vyatta.com>,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	netdev@...r.kernel.org, dipankar@...ibm.com, ego@...ibm.com,
	herbert@...dor.apana.org.au, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2] remove rcu_assign_pointer(NULL) penalty with
 type/macro safety

On Wed, 13 Feb 2008 17:34:27 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> 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" <paulmck@...ux.vnet.ibm.com> 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 <paulmck@...ux.vnet.ibm.com>
> ---
> 
>  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)

Yes, thats great.
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