rhashtable: use cmpxchg() to protect ->future_tbl.

Rather than borrowing one of the bucket locks to
protect ->future_tbl updates, use cmpxchg().
This gives more freedom to change how bucket locking
is implemented.
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/rhashtable.c

@@ -297,21 +297,14 @@ static int rhashtable_rehash_attach(struct rhashtable *ht,
 				    struct bucket_table *old_tbl,
 				    struct bucket_table *new_tbl)
 {
-	/* Protect future_tbl using the first bucket lock. */
-	spin_lock_bh(old_tbl->locks);
-
-	/* Did somebody beat us to it? */
-	if (rcu_access_pointer(old_tbl->future_tbl)) {
-		spin_unlock_bh(old_tbl->locks);
-		return -EEXIST;
-	}
-
 	/* Make insertions go into the new, empty table right away. Deletions
 	 * and lookups will be attempted in both tables until we synchronize.
+	 * As cmpxchg() provides strong barriers, we do not need
+	 * rcu_assign_pointer().
 	 */
-	rcu_assign_pointer(old_tbl->future_tbl, new_tbl);
 
-	spin_unlock_bh(old_tbl->locks);
+	if (cmpxchg(&old_tbl->future_tbl, NULL, new_tbl) != NULL)
+		return -EEXIST;
 
 	return 0;
 }