[NET]: Add barriers for dst refcnt.

In light of the recent discussion about sk_buff, I think we need
the following patch for dst_entry.  This adds a memory barrier
before dst_release drops the refcnt, and a read memory barrier
before dst_destroy starts destroying the entry.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -147,6 +147,7 @@ void dst_release(struct dst_entry * dst)
 {
 	if (dst) {
 		WARN_ON(atomic_read(&dst->__refcnt) < 1);
+		smp_mb__before_atomic_dec();
 		atomic_dec(&dst->__refcnt);
 	}
 }

net/core/dst.c

@@ -169,6 +169,8 @@ struct dst_entry *dst_destroy(struct dst_entry * dst)
 	struct neighbour *neigh;
 	struct hh_cache *hh;
 
+	smp_rmb();
+
 again:
 	neigh = dst->neighbour;
 	hh = dst->hh;