down_interruptible(struct semaphore *sem)

int down_interruptible(struct semaphore *sem)
如果得不到信号量，此时没有信号打断，那么进入睡眠。
主要用来进程间的互斥同步


你如果看函数的注释，也可以知道大概的
  66/**
  67 * down_interruptible - acquire the semaphore unless interrupted
  68 * @sem: the semaphore to be acquired
  69 *
  70 * Attempts to acquire the semaphore. If no more tasks are allowed to
  71 * acquire the semaphore, calling this function will put the task to sleep.
  72 * If the sleep is interrupted by a signal, this function will return -EINTR.
  73 * If the semaphore is successfully acquired, this function returns 0.
  74 */
  75int down_interruptible(struct semaphore *sem)
传入的信号量为1好比天亮，如果当前信号量为0，进程睡眠，直到（信号量为1）天亮才醒，但是可能中途有个闹铃（信号）把你闹醒。