(1)  是临界区，需要使用到共享资源B
(2)
semaphore over = 0;
P1
{
C1;
signal(over);
}
P2
{
wait(over);
C2;
}
(2)
semaphore mutex = 1;
P1
{
wait(mutex);
C3:
signal(mutex);
}
P2
{
wait(mutex);
C3:
signal(mutex);
}