php+ajax登陆问题

服务端代码:
<?php
include_once('./global.php');
if($_GET['out']){
    unset($_SESSION['name']);
}
if(!isset($_SESSION['name'])){
    if(isset($_POST['usn'])){
        $db->setTable('user_info');
        $row=$db->select("*",array('user_account'=>$_POST['usn'],'user_password'=>md5($_POST['pwd']),'user_mark'=>0));
        if(isset($row)){
            $_SESSION['name']=$row[0]['user_names'];
            echo "<script language=\"javascript\">location.href='index.php';</script>";
        }else{
            echo 'faild';
        }
    }
    else{
        $smarty->display("login.htm");
    }
}
else{
    echo "<script language=\"javascript\">location.href='index.php';</script>";
}
?>

客户端代码:

<script>
    $(document).ready(function(){
        $("#myform").submit(function(){
            login();
            return false;
        });
        function login(){
            var user=$("#usn").val();
            var pwd=$("#pwd").val();
            if(user==""){
                $("#waring").text("请输入用户名");
                $("#usn").focus();
                return false;
            }
            if(pwd==""){
                $("#waring").text("请输入密码");
                $("#pwd").focus();
                return false;
            }
            $.ajax({
                url:'login.php',
                type:'post',
                data:{usn:user,pwd:pwd},
                success:function(msg){
                    if(msg=="faild"){
                        alert("bbb");
                        $("#waring").text("登陆失败！");
                    }else{
                        alert("aaa");
                    }
                }
            });
        }
    });
  </script>

<form name="myform" id="myform" method="post">
        <ul>
            <li>用户名:<input type="text" name="usn" id="usn"></li>
            <li>密  码:<input type="password" name="pwd" id="pwd"></li>
            <li><input type="submit" name="tj" id="tj" value="提交">  <input type="reset" name="reset" id="reset" value="重置"></li>
            <li id="waring"></li>
        </ul>
        </form>

现在遇到的问题是，不管输入正确或错误的用户名密码，都alert("aaa")，没有达到想要的要求！
就是输入正确的用户名密码直接跳转首页，错误的用户名密码在#waring这里提示信息

$.ajax({
                var user=$("#usn").val();
                var pwd=$("#pwd").val();
                url:'login.php',
                type:'post',
                data:"username="+user+"&password="+pwd,
                success:function(msg){
                    if(msg=="faild"){
                        alert("bbb");
                        $("#waring").text("登陆失败！");
                    }else{
                        alert("aaa");
                    }
                }
            });
这么写试试