[IPV4]: Kill redundant rcu_dereference on fa_info

This patch kills a redundant rcu_dereference on fa->fa_info in fib_trie.c.
As this dereference directly follows a list_for_each_entry_rcu line, we
have already taken a read barrier with respect to getting an entry from
the list.

This read barrier guarantees that all values read out of fa are valid.
In particular, the contents of structure pointed to by fa->fa_info is
initialised before fa->fa_info is actually set (see fn_trie_insert);
the setting of fa->fa_info itself is further separated with a write
barrier from the insertion of fa into the list.

Therefore by taking a read barrier after obtaining fa from the list
(which is given by list_for_each_entry_rcu), we can be sure that
fa->fa_info contains a valid pointer, as well as the fact that the
data pointed to by fa->fa_info is itself valid.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 0093ea08c7f5378c8bbd80e966007c9eaf87f382..66247f38b3716193637ab4c79020e22d9f65e1dd 100644 (file)
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -2404,7 +2404,7 @@ static int fib_route_seq_show(struct seq_file *seq, void *v)
                prefix = htonl(l->key);
 
                list_for_each_entry_rcu(fa, &li->falh, fa_list) {
-                       const struct fib_info *fi = rcu_dereference(fa->fa_info);
+                       const struct fib_info *fi = fa->fa_info;
                        unsigned flags = fib_flag_trans(fa->fa_type, mask, fi);
 
                        if (fa->fa_type == RTN_BROADCAST