wait_event_timeout的理解

wait_event_timeout(wq, condition, timeout)

表示的有以下两个意思：

（1）当condition为真的时候，会返回

（2）当timeout到达时也会返回，不管此时condition为真为假都会返回

此时接着执行wait_event_timeout之后的代码，只要退出wait_event_timeout，进程就被置为TASK_RUNNING（因为源码中，在退出函数时，会调用到__set_current_state(TASK_RUNNING);）

这个函数是将进程睡眠，置状态为TASK_UNINTERRUPTIBLE直到condition为真，每一次调用唤 醒函数wake_up时都会检查condition，condition为假就继续等待，每次改变任何会导致condition变化的变量时候，都会调用wake_up()

函数返回0：表示timeout超时

       返回一个正数：表示还没有超时，但condition变为真，返回剩余的时间

/**
 * wait_event_timeout - sleep until a condition gets true or a timeout elapses
 * @wq: the waitqueue to wait on
 * @condition: a C expression for the event to wait for
 * @timeout: timeout, in jiffies
 *
 * The process is put to sleep (TASK_UNINTERRUPTIBLE) until the
 * @condition evaluates to true. The @condition is checked each time
 * the waitqueue @wq is woken up.
 *
 * wake_up() has to be called after changing any variable that could
 * change the result of the wait condition.
 *
 * The function returns 0 if the @timeout elapsed, or the remaining
 * jiffies (at least 1) if the @condition evaluated to %true before
 * the @timeout elapsed.
 */

查看源码，wait_event_timeout( )的返回值，是调用schedule_timeout()返回的

schedule_timeout()表示的进程睡眠直到时间超时，函数就会立即返回，除非进程状态被设置

有以下两种情况：

TASK_UNINTERRUPTIBLE：此时函数就会返回0，需要等待超时时间到才行

TASK_INTERRUPTIBLE：如果一个信号唤醒这个进程，函数就会提前返回，返回值为剩余的jiffies值；也可能是返回0，此时是超时时间刚好到

/**
 * schedule_timeout - sleep until timeout
 * @timeout: timeout value in jiffies
 *
 * Make the current task sleep until @timeout jiffies have
 * elapsed. The routine will return immediately unless
 * the current task state has been set (see set_current_state()).
 *
 * You can set the task state as follows -
 *
 * %TASK_UNINTERRUPTIBLE - at least @timeout jiffies are guaranteed to
 * pass before the routine returns. The routine will return 0
 *
 * %TASK_INTERRUPTIBLE - the routine may return early if a signal is
 * delivered to the current task. In this case the remaining time
 * in jiffies will be returned, or 0 if the timer expired in time
 *
 * The current task state is guaranteed to be TASK_RUNNING when this
 * routine returns.
 *
 * Specifying a @timeout value of %MAX_SCHEDULE_TIMEOUT will schedule
 * the CPU away without a bound on the timeout. In this case the return
 * value will be %MAX_SCHEDULE_TIMEOUT.
 *
 * In all cases the return value is guaranteed to be non-negative.
 */