如何处理这个代码的竞争条件

Question

.save()有竞争条件。那么，如何应对呢？。 并且，数据库无法保存tokenUser 值。

login.js

var crypto = require('crypto');
var express = require('express');
var jwt = require('jsonwebtoken');
var router = express.Router();
var User = require('../modules/user');
var Token = require('../modules/token');

router.get('/', function(req, res) {
res.render('login', {
    title: '登录界面'
});
});

router.post('/', function(req, res) {
User.findOne({
        tel: req.body.tel,
    })
    .exec(function(err, user) {
        var md5 = crypto.createHash('md5'),
            password = md5.update(req.body.password)
            .digest('hex');
        if (!user) {
            return res.json({
                success: false,
                message: '认证失败，用户名找不到'
            });
        } else if (user.hash_password === password) {
            return;
        } else {
            return res.json({
                status: 007,
                success: false,
                message: '没有当前用户'
            });
        }
    });
var tel = req.body.tel;
var user = new User({
    tel: tel
});
var md5 = crypto.createHash('md5'),
    password = md5.update(req.body.password)
    .digest('hex');

user.set("hash_password", password);
user.set("name", req.body.name);
user.set("created_at", Date.now());

var token = jwt.sign(user, 'whuteditor', {
    expiresIn: "30 days" // 设置过期时间
});

var tokenUser = new Token({
    token: token
});

tokenUser.set("nickname", req.body.name);

tokenUser.save(function(err, token) { //how to deal with??
    if (err) {
        return res.json({
            status: 120,
            success: false,
            message: "Bad Storage"
        });
    } else {
        res.json({
            status: 100,
            success: true,
            message: 'Enjoy your token!',
            token: token
        });
    }
});
});

module.exports = router;