wait_event 类接口详解
1、引言
在 Linux 内核中，等待某个条件成立是驱动开发与内核线程编程中最常见的场景之一。为了避免不必要的轮询和资源浪费，内核提供了以等待队列（wait_queue_head_t）为基础的一套机制，用于在条件未满足时挂起当前进程，并在条件成立或事件发生后将其唤醒。
其中，wait_event()、wait_event_timeout() 和 wait_event_interruptible() 是最常用的三个宏：
- wait_event()：当前进程将进入不可中断睡眠状态，直到指定条件为真
- wait_event_timeout()：在 wait_event() 的基础上增加了超时机制，即使条件不满足，也会在超时后返回
- wait_event_interruptible()：等待期间允许信号中断（如 SIGINT），进程进入可中断睡眠状态，可提前响应信号退出等待。
这三者在行为、适用场景以及异常处理上存在细微而关键的差异。下面通过实际代码示例，结合内核调度机制与条件判断时机，深入分析它们的使用方式及常见误区。
2、wait_event
#define __wait_event(wq_head, condition) \(void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0, \schedule())
/*** wait_event - sleep until a condition gets true* @wq_head: the waitqueue to wait on* @condition: a C expression for the event to wait for** The process is put to sleep (TASK_UNINTERRUPTIBLE) until the* @condition evaluates to true. The @condition is checked each time* the waitqueue @wq_head is woken up.** wake_up() has to be called after changing any variable that could* change the result of the wait condition.*/
#define wait_event(wq_head, condition) \
do { \might_sleep(); \if (condition) \break; \__wait_event(wq_head, condition); \
} while (0)
由上述代码可见，wait_event 函数入口，会先检测 condition 是否为 true，如果为 true 则返回；否则调用 __wait_event 进入睡眠状态。 __wait_event 宏最终调用的是 schedule() 调度器接口让调用者进入睡眠状态。
这其中，最重要的函数是：
/** The below macro ___wait_event() has an explicit shadow of the __ret* variable when used from the wait_event_*() macros.** This is so that both can use the ___wait_cond_timeout() construct* to wrap the condition.** The type inconsistency of the wait_event_*() __ret variable is also* on purpose; we use long where we can return timeout values and int* otherwise.*/#define ___wait_event(wq_head, condition, state, exclusive, ret, cmd) \
({ \__label__ __out; \struct wait_queue_entry __wq_entry; \long __ret = ret; /* explicit shadow */ \\init_wait_entry(&__wq_entry, exclusive ? WQ_FLAG_EXCLUSIVE : 0); \for (;;) { \long __int = prepare_to_wait_event(&wq_head, &__wq_entry, state);\\//条件满足退出循环 if (condition) \break; \\ //如果设置了 interruptible，并且有信号打断（__int 不为 0），直接无视 condition，退出等待 if (___wait_is_interruptible(state) && __int) { \__ret = __int; \goto __out; \} \\cmd; \} \finish_wait(&wq_head, &__wq_entry); \
__out: __ret; \
})
从 TASK_UNINTERRUPTIBLE 标志可以看出，wait_event 接口，默认是不可以被信号打断的。也就是说，一旦 wait_event 接口进入睡眠状态，只有调用 wake_up 接口才能将其唤醒。但是被唤醒后，会再次检查 condition，如果 condition 为 false 的话，会接着进入睡眠状态…
| 进程状态 | 说明 |
|---|---|
| TASK_RUNNING | 可运行状态。未必正在使用CPU，也许是在等待调度 |
| TASK_INTERRUPTIBLE | 可中断的睡眠状态。正在等待某个条件满足 |
| TASK_UNINTERRUPTIBLE | 不可中断的睡眠状态。不会被信号中断 |
3、wait_event_interruptible
#define __wait_event_interruptible(wq_head, condition) \___wait_event(wq_head, condition, TASK_INTERRUPTIBLE, 0, 0, \schedule())/*** wait_event_interruptible - sleep until a condition gets true* @wq_head: the waitqueue to wait on* @condition: a C expression for the event to wait for** The process is put to sleep (TASK_INTERRUPTIBLE) until the* @condition evaluates to true or a signal is received.* The @condition is checked each time the waitqueue @wq_head is woken up.** wake_up() has to be called after changing any variable that could* change the result of the wait condition.** The function will return -ERESTARTSYS if it was interrupted by a* signal and 0 if @condition evaluated to true.*/
#define wait_event_interruptible(wq_head, condition) \
({ \int __ret = 0; \might_sleep(); \if (!(condition)) \__ret = __wait_event_interruptible(wq_head, condition); \__ret; \
})
该接口和 wait_event 接口唯一的区别，就是可以被信号打断的。从 TASK_INTERRUPTIBLE 标志可以看出。
wait_event_interruptible 宏最终调用的是 schedule() 调度器接口让调用者进入睡眠状态。
4、wait_event_timeout
#define ___wait_cond_timeout(condition) \
({ \bool __cond = (condition); \if (__cond && !__ret) \__ret = 1; \__cond || !__ret; \
})#define __wait_event_timeout(wq_head, condition, timeout) \___wait_event(wq_head, ___wait_cond_timeout(condition), \TASK_UNINTERRUPTIBLE, 0, timeout, \__ret = schedule_timeout(__ret))/**- wait_event_timeout - sleep until a condition gets true or a timeout elapses- @wq_head: 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_head is woken up.- * wake_up() has to be called after changing any variable that could- change the result of the wait condition.- * Returns:- 0 if the @condition evaluated to %false after the @timeout elapsed,- 1 if the @condition evaluated to %true after the @timeout elapsed,- or the remaining jiffies (at least 1) if the @condition evaluated- to %true before the @timeout elapsed.*/
#define wait_event_timeout(wq_head, condition, timeout) \
({ \long __ret = timeout; \might_sleep(); \if (!___wait_cond_timeout(condition)) \__ret = __wait_event_timeout(wq_head, condition, timeout); \__ret; \
})
该接口的实现，相比 wait_event 接口，多了一个 “timeout” 参数。该参数是为了控制不让调用者一直睡眠下去。
wait_event_timeout 宏最终调用的是 schedule_timeout() 调度器接口让调用者进入睡眠状态。
只有两种情况，调用者会被唤醒：
- 主动调用
wake_up接口，唤醒被wait_event_timeout接口阻塞的任务wait_event_timeout返一个正数。表示还没有超时，但 condition 变为真，返回剩余的时间
- timeout 超时时间达到，被定时器唤醒。不管这时候 condition 是否为真，都不会再继续睡眠
wait_event_timeout返回 0，一直到 timeout 时间超时，condition 都是 falsewait_event_timeout返回 1，到 timeout 时间超时时刻，condition 是 true