linux wait queue详解

#include <linux/wait.h> 

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_head {
    spinlock_t lock;
    struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_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) 

#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) 

#define __WAIT_BIT_KEY_INITIALIZER(word, bit)                \
    { .flags = word, .bit_nr = bit, } 

extern void __init_waitqueue_head(wait_queue_head_t *q, struct lock_class_key *); 

#define init_waitqueue_head(q)                \
    do {                        \
        static struct lock_class_key __key;    \
                            \
        __init_waitqueue_head((q), &__key);    \
    } while (0) 

void __init_waitqueue_head(wait_queue_head_t *q, struct lock_class_key *key)
{
    spin_lock_init(&q->lock);
    lockdep_set_class(&q->lock, key);
    INIT_LIST_HEAD(&q->task_list);
} 

操作:
1. 定义"等待队列头"

wait_queue_head_t my_queue;

2. 初始化"等待队列头"

init_waitqueue_head(&my_queue);

同时, DECLARE_WAIT_QUEUE_HEAD(name)宏可以定义并初始化等待队列头.

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

3. 定义等待队列

DECLARE_WAITQUEUE(name, tsk)

tsk一般为当前进行current. 这个宏定义并初始化一个名为name的等待队列.

#define DECLARE_WAITQUEUE(name, tsk)                    \
    wait_queue_t name = __WAITQUEUE_INITIALIZER(name, tsk)
#define __WAITQUEUE_INITIALIZER(name, tsk) {                \
    .private    = tsk,                        \
    .func        = default_wake_function,            \
    .task_list    = { NULL, NULL } }

这时的private将会在wake_up()及其变种中使用, 最终将会默认调用default_wake_function()函数, 此函数实际上为try_to_wake_up(curr->private, mode, wake_flags);

/***
 * try_to_wake_up - wake up a thread
 * @p: the to-be-woken-up thread
 * @state: the mask of task states that can be woken
 * @sync: do a synchronous wakeup?
 *
 * Put it on the run-queue if it's not already there. The "current"
 * thread is always on the run-queue (except when the actual
 * re-schedule is in progress), and as such you're allowed to do
 * the simpler "current->state = TASK_RUNNING" to mark yourself
 * runnable without the overhead of this.
 *
 * returns failure only if the task is already active.
 */
static int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags);

4. 添加/删除等待队列

void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait);
void remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);

5. 等待事件

wait_event(wq, condition)
wait_event_timeout(wq, condition, timeout)
wait_event_interruptible(wq, condition)
wait_event_interruptible_timeout(wq, condition, timeout)

6. 唤醒等待队列

#define wake_up(x)            __wake_up(x, TASK_NORMAL, 1, NULL)
#define wake_up_nr(x, nr)        __wake_up(x, TASK_NORMAL, nr, NULL)
#define wake_up_all(x)            __wake_up(x, TASK_NORMAL, 0, NULL)
#define wake_up_locked(x)        __wake_up_locked((x), TASK_NORMAL) 

#define wake_up_interruptible(x)    __wake_up(x, TASK_INTERRUPTIBLE, 1, 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_interruptible_sync(x)    __wake_up_sync((x), TASK_INTERRUPTIBLE, 1) 

/**
 * __wake_up - wake up threads blocked on a waitqueue.
 * @q: the waitqueue
 * @mode: which threads
 * @nr_exclusive: how many wake-one or wake-many threads to wake up
 * @key: is directly passed to the wakeup function
 *
 * It may be assumed that this function implies a write memory barrier before
 * changing the task state if and only if any tasks are woken up.
 */
void __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);
}

7. 在等待队列上睡眠

void __sched sleep_on(wait_queue_head_t *q)
long __sched sleep_on_timeout(wait_queue_head_t *q, long timeout);
void __sched interruptible_sleep_on(wait_queue_head_t *q);
long __sched interruptible_sleep_on_timeout(wait_queue_head_t *q, long timeout);
#define __sched        __attribute__((__section__(".sched.text")))

sleep_on()函数应该与wake_up()成对使用, interrupt_sleep_on()应该与wake_up_interruptible()成对使用

#include <linux/wait.h>