bn_read_lock
int bn_read_lock(struct dm_btree_info *info, dm_block_t b,
r = bn_read_lock(s->info, new_child, s->nodes + s->count);
r = bn_read_lock(info, block, &node);
r = bn_read_lock(c->info, b, &n->b);
r = bn_read_lock(info, root, &node);
r = bn_read_lock(info, b, &block);