3.6.1. linux wait queue
涉及源码目录
kernel/include/linux/wait.h
kernel/kernel/sched/wait.c
kernel/include/linux/sched.h
kernel/kernel/sched/core.c
3.6.1.1. 概述
linux内核的等待队列事非常重要的数据结构,在内核驱动中广为使用,它是以双向循环链表为基础的数据结构,与进程的休眠唤醒机制紧密相连，是实现异步事件通知 、跨进程通信、同步资源访问等技术的底层技术支撑。
等待队列有两种数据结构：等待队列头(wait_queue_head)和等待队列项(wait_queue_entry),两者都有一个list_head类型的task_list. 双向链表通过task_list将等待队列头和一系列的等待队列项串起来
3.6.1.2. 等待队列
3.6.1.2.1. struct wait_queue_entry
struct wait_queue_entry {
unsigned int flags;
void *private;
wait_queue_func_t func;
struct list_head entry;
};
可以通过DECLARE_WAITQUEUE(name)来创建类型为wait_queue_entry类型的等待队列name
#define __WAITQUEUE_INITIALIZER(name, tsk) { \
.private = tsk, \
.func = default_wake_function, \
.entry = { NULL, NULL } }
#define DECLARE_WAITQUEUE(name, tsk) \
struct wait_queue_entry name = __WAITQUEUE_INITIALIZER(name, tsk)
3.6.1.2.2. struct wait_queue_head
struct wait_queue_head {
spinlock_t lock;
struct list_head head;
};
typedef struct wait_queue_head wait_queue_head_t;
可以通过DECLARE_WAITQUEUE_HEAD(name)
#define __WAIT_QUEUE_HEAD_INITIALIZER(name) { \
.lock = __SPIN_LOCK_UNLOCKED(name.lock), \
.head = { &(name).head, &(name).head } }
#define DECLARE_WAIT_QUEUE_HEAD(name) \
struct wait_queue_head name = __WAIT_QUEUE_HEAD_INITIALIZER(name)
3.6.1.2.3. add_wait_queue
void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
unsigned long flags;
wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&wq_head->lock, flags);
__add_wait_queue(wq_head, wq_entry);
spin_unlock_irqrestore(&wq_head->lock, flags);
}
static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
list_add(&wq_entry->entry, &wq_head->head);
}
该方法用于将wait_entry等待队列项挂到等待队列头wq_head中
3.6.1.3. 休眠与唤醒
3.6.1.3.1. wait_event
#define ___wait_event(wq_head, condition, state, exclusive, ret, cmd) \
({ \
__label__ __out; \
struct wait_queue_entry __wq_entry; \
long __ret = ret; /* explicit shadow */ \
\
init_wait_entry(&__wq_entry, exclusive ? WQ_FLAG_EXCLUSIVE : 0); \
for (;;) { \
long __int = prepare_to_wait_event(&wq_head, &__wq_entry, state);/*检测当前进程是否有待处理的信号*/ \
\
if (condition) \
break; \
\
if (___wait_is_interruptible(state) && __int) { \
__ret = __int; \
goto __out; \
} \
\
cmd; /*schedule(), 进入睡眠，从进程就绪队列中选择一个高优先级的进程来代替当前进程运行*/ \
} \
finish_wait(&wq_head, &__wq_entry); \
__out: __ret; \
})
#define __wait_event(wq_head, condition) \
(void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0, \
schedule())
/**
* wait_event - sleep until a condition gets true
* @wq_head: 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_head 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_head, condition) \
do { \
might_sleep(); \
if (condition) \
break; \
__wait_event(wq_head, condition); \
} while (0)
3.6.1.3.2. prepare_to_wait_event
long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
unsigned long flags;
long ret = 0;
spin_lock_irqsave(&wq_head->lock, flags);
if (signal_pending_state(state, current)) {
list_del_init(&wq_entry->entry);
ret = -ERESTARTSYS;
} else {
if (list_empty(&wq_entry->entry)) {
if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
__add_wait_queue_entry_tail(wq_head, wq_entry);
else
__add_wait_queue(wq_head, wq_entry);
}
set_current_state(state);
}
spin_unlock_irqrestore(&wq_head->lock, flags);
return ret;
}
3.6.1.3.3. wake_up
#define wake_up(x) __wake_up(x, TASK_NORMAL, 1, NULL)
void __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
int nr_exclusive, void *key)
{
__wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
}
static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
int nr_exclusive, int wake_flags, void *key,
wait_queue_entry_t *bookmark)
{
wait_queue_entry_t *curr, *next;
int cnt = 0;
lockdep_assert_held(&wq_head->lock);
if (bookmark && (bookmark->flags & WQ_FLAG_BOOKMARK)) {
curr = list_next_entry(bookmark, entry);
list_del(&bookmark->entry);
bookmark->flags = 0;
} else
curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);
if (&curr->entry == &wq_head->head)
return nr_exclusive;
list_for_each_entry_safe_from(curr, next, &wq_head->head, entry) {
unsigned flags = curr->flags;
int ret;
if (flags & WQ_FLAG_BOOKMARK)
continue;
//调用唤醒函数,一般唤醒函数为default_wake_function
ret = curr->func(curr, mode, wake_flags, key);
if (ret < 0)
break;
if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
break;
if (bookmark && (++cnt > WAITQUEUE_WALK_BREAK_CNT) &&
(&next->entry != &wq_head->head)) {
bookmark->flags = WQ_FLAG_BOOKMARK;
list_add_tail(&bookmark->entry, &next->entry);
break;
}
}
return nr_exclusive;
}
static void __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode,
int nr_exclusive, int wake_flags, void *key)
{
unsigned long flags;
wait_queue_entry_t bookmark;
bookmark.flags = 0;
bookmark.private = NULL;
bookmark.func = NULL;
INIT_LIST_HEAD(&bookmark.entry);
do {
spin_lock_irqsave(&wq_head->lock, flags);
nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive,
wake_flags, key, &bookmark);
spin_unlock_irqrestore(&wq_head->lock, flags);
} while (bookmark.flags & WQ_FLAG_BOOKMARK);
}
3.6.1.3.4. default_wake_function
此函数为默认的唤醒函数
int default_wake_function(wait_queue_entry_t *curr, unsigned mode, int wake_flags,
void *key)
{
return try_to_wake_up(curr->private, mode, wake_flags);
}