等待队列
定义：
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include/linux/wait.h
struct __wait_queue_head {
spinlock_t lock;
struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_t;
include/linux/wait.h
struct __wait_queue_head {
spinlock_t lock;
struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_t;
二、作用：
在内核里面，等待队列是有很多用处的，尤其是在中断处理、进程同步、定时等场合。可以使用等待队列在实现阻塞进程的唤醒。它以队列为基础数据结构，与进程调度机制紧密结合，能够用于实现内核中的异步事件通知机制，同步对系统资源的访问等。
三、字段详解：
1、spinlock_t lock;
在对task_list与操作的过程中，使用该锁实现对等待队列的互斥访问。
2、srtuct list_head_t task_list;
双向循环链表，存放等待的进程。
三、操作：
1、定义并初始化：
(1)
wait_queue_head_t my_queue;
init_waitqueue_head(&my_queue);
直接定义并初始化。init_waitqueue_head()函数会将自旋锁初始化为未锁，等待队列初始化为空的双向循环链表。
(2)
DECLARE_WAIT_QUEUE_HEAD(my_queue);
定义并初始化，相当于(1)。
(3)定义等待队列：
DECLARE_WAITQUEUE(name,tsk);
注意此处是定义一个wait_queue_t类型的变量name，并将其private与设置为tsk。wait_queue_t类型定义如下：view plaincopy to clipboardprint?
struct __wait_queue {
unsigned int flags;
#define WQ_FLAG_EXCLUSIVE 0x01
void *private;
wait_queue_func_t func;
struct list_head task_list;
};
struct __wait_queue {
unsigned int flags;
#define WQ_FLAG_EXCLUSIVE 0x01
void *private;
wait_queue_func_t func;
struct list_head task_list;
};
其中flags域指明该等待的进程是互斥进程还是非互斥进程。 其中0是非互斥进程，WQ_FLAG_EXCLUSIVE(0x01)是互斥进程。等待队列(wait_queue_t)和等待对列头 (wait_queue_head_t)的区别是等待队列是等待队列头的成员。也就是说等待队列头的task_list域链接的成员就是等待队列类型的 (wait_queue_t)。
2、(从等待队列头中)添加／移出等待队列：
(1)add_wait_queue()函数：
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void fastcall add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
wait->flags &= ~WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
__add_wait_queue(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
void fastcall add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
wait->flags &= ~WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
__add_wait_queue(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
设置等待的进程为非互斥进程，并将其添加进等待队列头(q)的队头中。
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void fastcall add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
wait->flags |= WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
__add_wait_queue_tail(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
void fastcall add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
wait->flags |= WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
__add_wait_queue_tail(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
该函数也和add_wait_queue()函数功能基本一样，只不过它是将等待的进程(wait)设置为互斥进程。
(2)remove_wait_queue()函数：
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void fastcall remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
spin_lock_irqsave(&q->lock, flags);
__remove_wait_queue(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
void fastcall remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
spin_lock_irqsave(&q->lock, flags);
__remove_wait_queue(q, wait);
spin_unlock_irqrestore(&q->lock, flags);
}
在等待的资源或事件满足时，进程被唤醒，使用该函数被从等待头中删除。
3、等待事件：
(1)wait_event()宏：
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#define wait_event(wq, condition) /
do { /
if (condition) /
break; /
__wait_event(wq, condition); /
} while (0)
#define __wait_event_timeout(wq, condition, ret) /
do { /
DEFINE_WAIT(__wait); /
/
for (;;) { /
prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); /
if (condition) /
break; /
ret = schedule_timeout(ret); /
if (!ret) /
break; /
} /
finish_wait(&wq, &__wait); /
} while (0)
#define wait_event(wq, condition) /
do { /
if (condition) /
break; /
__wait_event(wq, condition); /
} while (0)
#define __wait_event_timeout(wq, condition, ret) /
do { /
DEFINE_WAIT(__wait); /
/
for (;;) { /
prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); /
if (condition) /
break; /
ret = schedule_timeout(ret); /
if (!ret) /
break; /
} /
finish_wait(&wq, &__wait); /
} while (0)
在等待会列中睡眠直到condition为真。在等待的期间，进程会被置为TASK_UNINTERRUPTIBLE进入睡眠，直到condition变量变为真。每次进程被唤醒的时候都会检查condition的值.
(2)wait_event_interruptible()函数:
和wait_event()的区别是调用该宏在等待的过程中当前进程会被设置为TASK_INTERRUPTIBLE状态.在每次被唤醒的时候,首先检查condition是否为真,如果为真则返回,否则检查如果进程是被信号唤醒,会返回-ERESTARTSYS错误码.如果是condition为真,则返回0.
(3)wait_event_timeout()宏:
也与wait_event()类似.不过如果所给的睡眠时间为负数则立即返回.如果在睡眠期间被唤醒,且condition为真则返回剩余的睡眠时间,否则继续睡眠直到到达或超过给定的睡眠时间,然后返回0.
(4)wait_event_interruptible_timeout()宏:
与wait_event_timeout()类似,不过如果在睡眠期间被信号打断则返回ERESTARTSYS错误码.
(5) wait_event_interruptible_exclusive()宏
同样和wait_event_interruptible()一样,不过该睡眠的进程是一个互斥进程.
5、唤醒队列:
(1)wake_up()函数:
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#define wake_up(x) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 1, NULL)
void fastcall __wake_up(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, void *key)
{
unsigned long flags;
spin_lock_irqsave(&q->lock, flags);
__wake_up_common(q, mode, nr_exclusive, 0, key);
spin_unlock_irqrestore(&q->lock, flags);
}
static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, int sync, void *key)
{
struct list_head *tmp, *next;
list_for_each_safe(tmp, next, &q->task_list) {
wait_queue_t *curr = list_entry(tmp, wait_queue_t, task_list);
unsigned flags = curr->flags;
if (curr->func(curr, mode, sync, key) &&
(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
break;
}
}
#define wake_up(x) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 1, NULL)
void fastcall __wake_up(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, void *key)
{
unsigned long flags;
spin_lock_irqsave(&q->lock, flags);
__wake_up_common(q, mode, nr_exclusive, 0, key);
spin_unlock_irqrestore(&q->lock, flags);
}
static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, int sync, void *key)
{
struct list_head *tmp, *next;
list_for_each_safe(tmp, next, &q->task_list) {
wait_queue_t *curr = list_entry(tmp, wait_queue_t, task_list);
unsigned flags = curr->flags;
if (curr->func(curr, mode, sync, key) &&
(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
break;
}
}
唤醒等待队列.可唤醒处于TASK_INTERRUPTIBLE和TASK_UNINTERUPTIBLE状态的进程,和wait_event/wait_event_timeout成对使用.
(2)wake_up_interruptible()函数:
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#define wake_up_interruptible(x) __wake_up(x, TASK_INTERRUPTIBLE, 1, NULL)
#define wake_up_interruptible(x) __wake_up(x, TASK_INTERRUPTIBLE, 1, NULL)
和wake_up()唯一的区别是它只能唤醒TASK_INTERRUPTIBLE状态的进程.,与wait_event_interruptible/wait_event_interruptible_timeout/ wait_event_interruptible_exclusive成对使用.
(3)
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#define wake_up_all(x) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 0, NULL)
#define wake_up_interruptible_nr(x, nr) __wake_up(x, TASK_INTERRUPTIBLE, nr, NULL)
#define wake_up_interruptible_all(x) __wake_up(x, TASK_INTERRUPTIBLE, 0, NULL)
#define wake_up_all(x) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 0, NULL)
#define wake_up_interruptible_nr(x, nr) __wake_up(x, TASK_INTERRUPTIBLE, nr, NULL)
#define wake_up_interruptible_all(x) __wake_up(x, TASK_INTERRUPTIBLE, 0, NULL)
这些也基本都和wake_up/wake_up_interruptible一样.
6、在等待队列上睡眠：
(1)sleep_on()函数:
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void __sched sleep_on(wait_queue_head_t *q)
{
unsigned long flags;
wait_queue_t wait;
init_waitqueue_entry(&wait, current);
current->state = TASK_UNINTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
schedule();
sleep_on_tail(q, &wait, &flags);
}
void __sched sleep_on(wait_queue_head_t *q)
{
unsigned long flags;
wait_queue_t wait;
init_waitqueue_entry(&wait, current);
current->state = TASK_UNINTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
schedule();
sleep_on_tail(q, &wait, &flags);
}
该函数的作用是定义一个等待队列(wait)，并将当前进程添加到等待队列中(wait)，然后将当前进程的状态置为TASK_UNINTERRUPTIBLE，并将等待队列(wait)添加到等待队列头(q)中。之后就被挂起直到资源可以获取，才被从等待队列头(q)中唤醒，从等待队列头中移出。在被挂起等待资源期间，该进程不能被信号唤醒。
(2)sleep_on_timeout()函数：
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long __sched sleep_on_timeout(wait_queue_head_t *q, long timeout)
{
unsigned long flags;
wait_queue_t wait
init_waitqueue_entry(&wait, current);
current->state = TASK_UNINTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
timeout = schedule_timeout(timeout);
sleep_on_tail(q, &wait, &flags);
return timeout;
}
long __sched sleep_on_timeout(wait_queue_head_t *q, long timeout)
{
unsigned long flags;
wait_queue_t wait
init_waitqueue_entry(&wait, current);
current->state = TASK_UNINTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
timeout = schedule_timeout(timeout);
sleep_on_tail(q, &wait, &flags);
return timeout;
}
与sleep_on()函数的区别在于调用该函数时，如果在指定的时间内(timeout)没有获得等待的资源就会返回。实际上是调用schedule_timeout()函数实现的。值得注意的是如果所给的睡眠时间(timeout)小于0，则不会睡眠。该函数返回的是真正的睡眠时间。
(3)interruptible_sleep_on()函数：
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void __sched interruptible_sleep_on(wait_queue_head_t *q)
{
unsigned long flags;
wait_queue_t wait;
init_waitqueue_entry(&wait, current);
current->state = TASK_INTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
schedule();
sleep_on_tail(q, &wait, &flags);
}
void __sched interruptible_sleep_on(wait_queue_head_t *q)
{
unsigned long flags;
wait_queue_t wait;
init_waitqueue_entry(&wait, current);
current->state = TASK_INTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
schedule();
sleep_on_tail(q, &wait, &flags);
}
该函数和sleep_on()函数唯一的区别是将当前进程的状态置为TASK_INTERRUPTINLE，这意味在睡眠如果该进程收到信号则会被唤醒。
(4)interruptible_sleep_on_timeout()函数：
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long __sched
interruptible_sleep_on_timeout(wait_queue_head_t *q, long timeout)
{
unsigned long flags;
wait_queue_t wait;
init_waitqueue_entry(&wait, current);
current->state = TASK_INTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
timeout = schedule_timeout(timeout);
sleep_on_tail(q, &wait, &flags);
return timeout;
}
long __sched
interruptible_sleep_on_timeout(wait_queue_head_t *q, long timeout)
{
unsigned long flags;
wait_queue_t wait;
init_waitqueue_entry(&wait, current);
current->state = TASK_INTERRUPTIBLE;
sleep_on_head(q, &wait, &flags);
timeout = schedule_timeout(timeout);
sleep_on_tail(q, &wait, &flags);
return timeout;
}
类似于sleep_on_timeout()函数。进程在睡眠中可能在等待的时间没有到达就被信号打断而被唤醒，也可能是等待的时间到达而被唤醒。
以上四个函数都是让进程在等待队列上睡眠，不过是小有诧异而已。在实际用的过程中，根据需要选择合适的函数使用就是了。例如在对软驱数据的读写中，如果设备没有就绪则调用sleep_on()函数睡眠直到数据可读(可写)，在打开串口的时候，如果串口端口处于关闭状态则调用interruptible_sleep_on()函数尝试等待其打开。在声卡驱动中，读取声音数据时，如果没有数据可读，就会等待足够常的时间直到可读取。
Alarm.c (kernel/drivers/rtc):static DECLARE_WAIT_QUEUE_HEAD(alarm_wait_queue);
Alarm.c (kernel/drivers/rtc): rv = wait_event_interruptible(alarm_wait_queue, alarm_pending);
Alarm.c (kernel/drivers/rtc): wake_up(&alarm_wait_queue);