sem_post 最安全
sem 有序,会卡顿 阻塞
mutex 无序,不能同步
http://blog.chinaunix.net/uid-20671208-id-4935154.html
https://www.cnblogs.com/ngnetboy/p/3521547.html
http://www.jianshu.com/p/1e59f0970bf5
http://blog.csdn.net/jenny8080/article/details/52094140