First execute process1 wait(s) decrements the count of semaphore s to zero, then execute process2 wait(s) ,the count of s becomes negative and the second process will be blocked.

• 信号量的机制中，有这么一个规则：每当信号加1的同时，将会释放一个等待该信号量队列中的进程（或线程），并不是等到信号量为正时才释放的。