CI框架session跨页面失效

大家好，请问为什么【CI框架session跨页面失效】？？？我的config.php已经设置足够的时间啦！它每次都是我输入用户名和密码之后跳到controllers层的set_session方法，这时候能正常显示，但是一旦我回到index方法之后本应该也能读取出之前存放的session啊，但是最后显示的效果是【session为空1。请大家帮帮忙~~~~谢谢
代码如下：
        function index(){
                if($this->session->userdata('userSession')){
                        $data=$this->session->userdata('userSession');
                        $this->load->view('mysession',$data);
                }
                else{
                        $data['username']='session为空1';
                        $this->load->view('mysession',$data);
                }
        }
        
        function set_session(){
                if($this->input->post('btn_submit')){
                        $userSession=array('username'=>$this->input->post('txt_username'), 'pwd'=>$this->input->post('txt_pwd'));
                        $this->session->set_userdata('userSession',$userSession);
                        $this->session->unset_userdata('mysession');
                }
                if($this->session->userdata('userSession')){
                        $data=$this->session->userdata('userSession');
                        $this->load->view('mysession',$data);
                }
                else{
                        $data['username']='session为空2';
                        $this->load->view('mysession',$data);
                }
        }