LDD scull例子中的scullpipe模块分析

/* 有多少空间被释放 */
static int spacefreee(struct scull_pipe *dev)
{
    if (dev->rp == dev->wp)
    {
        return dev->buffersize - 1;
    }

    /* 这条语句很好，空间为写指针位置之后，
     * 读指针位置之前的这段空间
     */
    return ( (dev->rp - dev->wp + dev->buffersize) % dev->buffersize ) -1 ;
}

下面看手工休眠的过程。
第一步是建立并初始化一个等待队列入口。

DEFINE_WAIT(my_wait);

其中name是等待队列入口变量的名称。也可以通过下面两个步骤完成这个工作：

wait_queue_t my_wait;
init_wait(&my_wait);

下一个步骤是将等待队列入口添加到队列中，并设置进程的状态，前面的两种方法都可通过下面的函数完成:

void prepare_to_wait(wait_queue_head_t *queue, wait_queue_t *wait, int state);

queue 和 wait分别是等待队列头和进程入口。state是进程的新状态；它应该是TASK_INTERRUPTIBLE 或TASK_UNINTERRUPTIBLE


这里先看下,结构体wait_queue_head_t和wait_queue_t 的具体定义:

typedef struct __wait_queue_head wait_queue_head_t;
struct __wait_queue_head
{
    spinlock_t lock;
    struct list_head task_list;
};

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;
};

在调用了prepare_to_wait 之后,进程即可调用schedule，当然在这之前，应确保仍有必要等待。
一旦schedule返回，就到了清理时间了，这个工作也可通过下面的特殊函数完成：

void finish_wait(wait_queue_head_t *queue, wait_queue_t *wait);

/*
 * finish_wait - clean up after waiting in a queue
 * @q: waitqueue waited on
 * @wait: wait descriptor
 *
 * Sets current thread back to running state and removes
 * the wait descriptor from the given waitqueue if still
 * queued.
 */
void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
{
    unsigned long flags;

    __set_current_state(TASK_RUNNING);
    /*
     * We can check for list emptiness outside the lock
     * IFF:
     *  - we use the "careful" check that verifies both
     *    the next and prev pointers, so that there cannot
     *    be any half-pending updates in progress on other
     *    CPU's that we haven't seen yet (and that might
     *    still change the stack area.
     * and
     *  - all other users take the lock (ie we can only
     *    have _one_ other CPU that looks at or modifies
     *    the list).
     */
    if (!list_empty_careful(&wait->task_list)) {
        spin_lock_irqsave(&q->lock, flags);
        list_del_init(&wait->task_list);
        spin_unlock_irqrestore(&q->lock, flags);
    }
}

之后，代码可测试其状态，并判断是否需要重新等待。

好了，让我们来看看DEFINE_WAIT(my_wait)等得定义吧，跟踪下。

而init_wait(&my_wait)的定义如下:

#define init_wait(wait)                            \
do {                                            \
    (wait)->private = current;                    \
    (wait)->func = autoremove_wake_function;    \
    INIT_LIST_HEAD(&(wait)->task_list);            \
} while (0)

static inline void INIT_LIST_HEAD(struct list_head *list)
{
    list->next = list;
    list->prev = list;
}

由上面看来DEFINE_WAIT(my_wait)同
wait_queue_t my_wait;
init_wait(my_wait);
确实做得是一样的事情。

再来看看prepare_to_wait()函数吧.

void prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait, int state)
{
    unsigned long flags;

    wait->flags &= ~WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&q->lock, flags);
    if (list_empty(&wait->task_list))
        __add_wait_queue(q, wait);
    set_current_state(state);
    spin_unlock_irqrestore(&q->lock, flags);
}

static inline void __add_wait_queue(wait_queue_head_t *head, wait_queue_t *new)
{
    list_add(&new->task_list, &head->task_list);
}

static inline void __list_add(struct list_head *new,
                  struct list_head *prev,
                  struct list_head *next)
{
    next->prev = new;
    new->next = next;
    new->prev = prev;
    prev->next = new;
}

再来看finish_wait 函数

/*
 * finish_wait - clean up after waiting in a queue
 * @q: waitqueue waited on
 * @wait: wait descriptor
 *
 * Sets current thread back to running state and removes
 * the wait descriptor from the given waitqueue if still
 * queued.
 */
void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
{
    unsigned long flags;

    __set_current_state(TASK_RUNNING);
    /*
     * We can check for list emptiness outside the lock
     * IFF:
     *  - we use the "careful" check that verifies both
     *    the next and prev pointers, so that there cannot
     *    be any half-pending updates in progress on other
     *    CPU's that we haven't seen yet (and that might
     *    still change the stack area.
     * and
     *  - all other users take the lock (ie we can only
     *    have _one_ other CPU that looks at or modifies
     *    the list).
     */
    if (!list_empty_careful(&wait->task_list)) {
        spin_lock_irqsave(&q->lock, flags);
        list_del_init(&wait->task_list);
        spin_unlock_irqrestore(&q->lock, flags);
    }
}

wait_event(queue, condition) 其实里面的内容就是我们上面说的东西，看代码：
/**
 * wait_event - sleep until a condition gets true
 * @wq: 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 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, condition)                     \
do {                                    \
    if (condition)                             \
        break;                            \
    __wait_event(wq, condition);                    \
} while (0)
#define __wait_event(wq, condition)                     \
do {                                    \
    DEFINE_WAIT(__wait);                        \
                                    \
    for (;;) {                            \
        prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE);    \
        if (condition)                        \
            break;                        \
        schedule();                        \
    }                                \
    finish_wait(&wq, &__wait);                    \
} while (0)