linux内核阻塞IO

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