阻塞操作是指在执行设备操作时，若不能获得资源，则挂起进程直到满足可操作的条件后再进行操作。被挂起的进程进入休眠状态，被从调度器的运行队列移走，知道等待的条件被满足。而非阻塞的进程在不能进行设备操作时，并不挂起，它或者放弃，或者不停地查询，直到可以操作为止。
在linux驱动程序中，可以使用等待队列（wait queue）来实现阻塞进程的唤醒。
1. 等待队列头
一个等待队列有一个“等待队列头”来管理，wait_queue_head_t定义在linux/wait.h，实现在kernel/wait.c中。
struct __wait_queue_head {
spinlock_t lock;
struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_t;
DECLARE_WAIT_QUEUE_HEAD(name); //静态
wait_queue_head_t my_queue;
init_waitqueue_head(&my_queue);
2. 定义等待队列
typedef struct __wait_queue wait_queue_t;
typedef int (*wait_queue_func_t)(wait_queue_t *wait, unsigned mode, int flags, void *key);
int default_wake_function(wait_queue_t *wait, unsigned mode, int flags, void *key); struct __wait_queue {
unsigned int flags;
#define WQ_FLAG_EXCLUSIVE 0x01
void *private;
wait_queue_func_t func;
struct list_head task_list;
};
DECLARE_WAIT_QUEUE(name, tsk);
该宏用于定义并初始化一个名为name的等待队列。
#define __WAITQUEUE_INITIALIZER(name, tsk) { \
.private = tsk, \
.func = default_wake_function, \
.task_list = { NULL, NULL } }
#define DECLARE_WAITQUEUE(name, tsk) \
wait_queue_t name = __WAITQUEUE_INITIALIZER(name, tsk)
3. 移除和添加等待队列
extern void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
extern void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait);
extern void remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
4. 等待事件
#define wait_event(wq, condition)
#define wait_event_timeout(wq, condition, timeout)
#define wait_event_interruptible(wq, condition)
#define wait_event_interruptible_timeout(wq, condition, timeout)
/**
* wait_event_interruptible_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_INTERRUPTIBLE) until the
* @condition evaluates to true or a signal is received.
* 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.
*
* Returns:
* 0 if the @timeout elapsed, -%ERESTARTSYS if it was interrupted by
* a signal, or the remaining jiffies (at least 1) if the @condition
* evaluated to %true before the @timeout elapsed.
*/
等待第一个参数wq作为等待队列头的等待队列被唤醒，而且第2个参数condition必须满足，否则继续阻塞。
timeout以jiffy为单位。
5. 唤醒队列
void wake_up(wait_queue_head_t *queue);
void wake_up_interruptible(wait_queue_head_t *queue);
6. 在等待队列上睡眠
extern void sleep_on(wait_queue_head_t *q);
extern long sleep_on_timeout(wait_queue_head_t *q, signed long timeout);
extern void interruptible_sleep_on(wait_queue_head_t *q);
extern long interruptible_sleep_on_timeout(wait_queue_head_t *q, signed long timeout);
将当前进程添加到等待队列中，从而在等待队列上睡眠。当超时发生时，进程被唤醒。
参考：
1. 等待队列