rhashtable: Fix reader/rehash race

There is a potential race condition between readers and the rehasher.
In particular, the rehasher could have started a rehash while the
reader finishes a scan of the old table but fails to see the new
table pointer.

This patch closes this window by adding smp_wmb/smp_rmb.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/rhashtable.c

@@ -271,6 +271,9 @@ static void rhashtable_rehash(struct rhashtable *ht,
 	 */
 	rcu_assign_pointer(ht->future_tbl, new_tbl);
 
+	/* Ensure the new table is visible to readers. */
+	smp_wmb();
+
 	for (old_hash = 0; old_hash < old_tbl->size; old_hash++)
 		rhashtable_rehash_chain(ht, old_hash);
 
@@ -618,6 +621,9 @@ void *rhashtable_lookup_compare(struct rhashtable *ht, const void *key,
 		return rht_obj(ht, he);
 	}
 
+	/* Ensure we see any new tables. */
+	smp_rmb();
+
 	old_tbl = tbl;
 	tbl = rht_dereference_rcu(ht->future_tbl, ht);
 	if (unlikely(tbl != old_tbl))