wait _event_interruptible()函数分析

wait _event_interruptible()函数分析

 (2011-11-03 11:32:51)

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标签：  杂谈

分类： linuxkernel

Linux-2.6提供如下关于等待队列的操作:

    (1) 定义"等待队列头", 

 

        wait_queue_head_t my_queue;
defined in linux/wait.h
  50 struct __wait_queue_head {

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

        
(2) 初始化"等待队列头"
        init_waitqueue_head(&my_queue);
defined in linux/wait.c header file
 13 void init_waitqueue_head(wait_queue_head_t *q)

 {
 spin_lock_init(&q->lock);
 INIT_LIST_HEAD(&q->task_list);
 }

   
        定义和初始化的快捷方式:
        DECLARE_WAIT_QUEUE_HEAD(my_queue);   
linux/wait.h

 #define __WAIT_QUEUE_HEAD_INITIALIZER(name) { \
 .lock = __SPIN_LOCK_UNLOCKED(name.lock), \
 .task_list = { &(name).task_list, &(name).task_list } }

 #define DECLARE_WAIT_QUEUE_HEAD(name) \
 wait_queue_head_t name = __WAIT_QUEUE_HEAD_INITIALIZER(name)

    (3) 定义等待队列
        DECLARE_WAITQUEUE(name, tsk);
        定义并初始化一个名为name的等待队列(wait_queue_t);
linux/wait.h

 struct __wait_queue {
 unsigned int flags;
 #define WQ_FLAG_EXCLUSIVE 0x01
 void *private;
 wait_queue_func_t func;
 struct list_head task_list;
 };
 typedef struct __wait_queue wait_queue_t;

 #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)
2 specific analysis
wait_event_interruptible()。该函数修改task的状态为TASK_INTERRUPTIBLE，意味着改进程将不会继续运行直到被唤醒，然后被添加到等待队列wq中。
在wait_event_interruptible()中首先判断condition是不是已经满足，如果是则直接返回0，否则调用__wait_event_interruptible()，并用__ret来存放返回值
---------------------------------------------------------------
#define wait_event_interruptible(wq, condition)          \
({                                                       \
    int __ret = 0;                                       \
    if (!(condition))                                    \
        __wait_event_interruptible(wq, condition, __ret);\
    __ret;                                               \
})
wait_event_interruptible() --> __wait_event_interruptible()
__wait_event_interruptible()首先定义并初始化一个wait_queue_t变量__wait，其中数据为当前进程current，并把__wait入队。
   
在无限循环中，__wait_event_interruptible()将本进程置为可中断的挂起状态，反复检查condition是否成立，如果成立
则退出，如果不成立则继续休眠；条件满足后，即把本进程运行状态置为运行态，并将__wait从等待队列中清除掉，从而进程能够调度运行。如果进程当前有
异步信号(POSIX的)，则返回-ERESTARTSYS。
----------------------------------------------------------------
#define __wait_event_interruptible(wq, condition, ret)      \
do {                                                        \
    DEFINE_WAIT(__wait);                                    \
    for (;;) {                                              \
        prepare_to_wait(&wq, &__wait, TASK_INTERRUPTIBLE); \
        if (condition)                                      \
            break;                                          \
        if (!signal_pending(current)) {                     \
            schedule();                                     \
            continue;                                       \
        }                                                   \
        ret = -ERESTARTSYS;                                 \
        break;                                              \
    }                                                       \
    finish_wait(&wq, &__wait);                              \
} while (0)
__wait_event_interruptible() --> DEFINE_WAIT(name)
/usr/src/linux-2.6.21.5/include/linux/wait.h
---------------------------------------------------------
#define DEFINE_WAIT(name)                               \
    wait_queue_t name = {                               \
        .private    = current,                          \
        .func       = autoremove_wake_function,         \
        .task_list = LIST_HEAD_INIT((name).task_list), -\
    }
wait_queue_t
---------------------------------------------------------
typedef struct __wait_queue wait_queue_t;
struct __wait_queue {
    unsigned int      flags;
#define WQ_FLAG_EXCLUSIVE   0x01
    void              *private;
    wait_queue_func_t func;
    struct list_head task_list;
};

__wait_event_interruptible() --> prepare_to_wait()
void fastcall
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);
    if (is_sync_wait(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);
 }
here, we can see clearly that that function is just simply to link the variable
wait (type of wait_queue_t) into the variable p(type of wait_queue_head_t).
description of list_add:
that essentially invokes the __list_add(new , head, head->next), which implements the task
of inserting new between head and head->next
 void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
 {
 unsigned long flags;
 
 __set_current_state(TASK_RUNNING);
 
 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);
 }