wait _event_interruptible()函数分析

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本文详细剖析了wait_event_interruptible()函数的工作原理，包括其在信号处理和中断上下文中的行为，以及在系统调用和进程调度中的角色。

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wait _event_interruptible()函数分析

(2011-11-03 11:32:51)

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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 {

51 spinlock_t lock;
52 struct list_head task_list;
53 };
54 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)

14 {
15 spin_lock_init(&q->lock);
16 INIT_LIST_HEAD(&q->task_list);
17 }

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

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

74 #define DECLARE_WAIT_QUEUE_HEAD(name) \
75 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

32 struct __wait_queue {
33 unsigned int flags;
34 #define WQ_FLAG_EXCLUSIVE 0x01
35 void *private;
36 wait_queue_func_t func;
37 struct list_head task_list;
38 };
28 typedef struct __wait_queue wait_queue_t;

62 #define __WAITQUEUE_INITIALIZER(name, tsk) { \
63 .private = tsk, \
64 .func = default_wake_function, \
65 .task_list = { NULL, NULL } }
66 
67 #define DECLARE_WAITQUEUE(name, tsk) \
68 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);
}

124 static inline void __add_wait_queue(wait_queue_head_t *head, wait_queue_t *new)
125 {
126 list_add(&new->task_list, &head->task_list);
127 }
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
104 void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
105 {
106 unsigned long flags;
107 
108 __set_current_state(TASK_RUNNING);
109 
122 if (!list_empty_careful(&wait->task_list)) {
123 spin_lock_irqsave(&q->lock, flags);
124 list_del_init(&wait->task_list);
125 spin_unlock_irqrestore(&q->lock, flags);
126 }