串行队列中添加同步会造成死锁，互相等待同步任务执行完才能执行下一个

  let quenue = DispatchQueue.main
       quenue.async {
            print("xxxxxxxxx")
            quenue.sync {
               print("bbbbbbbbbb")
         }
    }

全局的并行队列不会死锁

    let quenue = DispatchQueue.global()
    quenue.async {
        print("xxxxxxxxx")
        quenue.sync {
            print("bbbbbbbbbb")
        }
    }