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

From: Stephen Hemminger <[email-masked]>
Date: Wed, 13 Feb 2008 16:53:56 -0800

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:
> > On Wed, 13 Feb 2008 16:14:04 -0800
> > "Paul E. McKenney" <[email-masked]> wrote:
> > > On Wed, Feb 13, 2008 at 03:51:58PM -0800, Stephen Hemminger wrote:
> 
> [ . . . ]
> 
> > > > Maybe cast both sides to void * in this case:
> > > > 
> > > > static inline void node_set_parent(struct node *node, struct tnode *ptr)
> > > > {
> > > >  	rcu_assign_pointer((void *) node->parent, (void *)((unsigned long)ptr | NODE_TYPE(node)));
> > > > }
> > > 
> > > That gets me the following:
> > > 
> > > 	net/ipv4/fib_trie.c: In function ‘node_set_parent’:
> > > 	net/ipv4/fib_trie.c:182: error: invalid lvalue in assignment
> > > 
> > > However, as with much in computing, an extra level of indirection fixes
> > > things.  Your call as to whether or not the cure is preferable to the
> > > disease.  ;-)
> > > 
> > > Signed-off-by: Paul E. McKenney <[email-masked]>
> > > ---
> > > 
> > >  fib_trie.c |    4 ++--
> > >  1 file changed, 2 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 16:10:07.000000000 -0800
> > > @@ -179,8 +179,8 @@ 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_pointer((*(void **)&node->parent),
> > > +			   (void *)((unsigned long)ptr | NODE_TYPE(node)));
> > >  }
> > 
> > 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.