Linux kernel的wait queue機(jī)制

1. 介紹

當(dāng)編寫Linux驅(qū)動(dòng)程序、模塊或內(nèi)核程序時(shí)，一些進(jìn)程會(huì)等待或休眠一些事件。Linux中有幾種處理睡眠和醒來(lái)的方法，每種方法對(duì)應(yīng)不同的需求，而wait queue便是其中一種。

每當(dāng)進(jìn)程必須等待一個(gè)事件（例如數(shù)據(jù)的到達(dá)或進(jìn)程的終止）時(shí)，它都應(yīng)該進(jìn)入睡眠狀態(tài)。睡眠會(huì)導(dǎo)致進(jìn)程暫停執(zhí)行，從而釋放處理器以供其他用途。一段時(shí)間后，該過程將被喚醒，并在我們等待的事件到達(dá)時(shí)繼續(xù)其工作。

等待隊(duì)列是內(nèi)核提供的一種機(jī)制，用于實(shí)現(xiàn)等待。顧名思義，wait queue是等待事件的進(jìn)程列表。換句話說(shuō)，當(dāng)某個(gè)條件成立時(shí)，等待隊(duì)列用于等待有人叫醒你。它們必須小心使用，以確保沒有競(jìng)爭(zhēng)條件的存在。

實(shí)現(xiàn)wait queue的步驟如下：

初始化等待隊(duì)列

排隊(duì)（將任務(wù)置于睡眠狀態(tài)，直到事件發(fā)生）

喚醒排隊(duì)的任務(wù)

以下逐步介紹每個(gè)步驟的實(shí)現(xiàn)方式。

2. 初始化等待隊(duì)列

若使用wait queue功能，需要包含/linux/wait.h頭文件?？苫趧?dòng)態(tài)和靜態(tài)兩種方式實(shí)現(xiàn)等待隊(duì)列的初始化。

靜態(tài)方式：

?

DECLARE_WAIT_QUEUE_HEAD(wq);

?

其中，wq是要將任務(wù)置于睡眠狀態(tài)的隊(duì)列的名稱。

動(dòng)態(tài)方式：

?

wait_queue_head_t?wq;
init_waitqueue_head?(&wq);

?

除了創(chuàng)建等待隊(duì)列的方式不同之外，其他操作對(duì)于靜態(tài)和動(dòng)態(tài)方法都是相同的。

3. 排隊(duì)

一旦聲明并初始化了等待隊(duì)列，進(jìn)程就可以使用它進(jìn)入睡眠狀態(tài)。有幾個(gè)宏可用于不同的用途。我們將逐一說(shuō)明。

wait_event

wait_event_timeout

wait_event_cmd

wait_event_interruptible

wait_event_interruptible_timeout

wait_event_killable

每當(dāng)我們使用上面的宏時(shí)，它會(huì)將該任務(wù)添加到我們創(chuàng)建的等待隊(duì)列中。然后它會(huì)等待事件。

wait_event

進(jìn)程進(jìn)入休眠狀態(tài)(TASK_UNINTERUPTIBLE)，直到條件評(píng)估為true。每次喚醒等待隊(duì)列wq時(shí)，都會(huì)檢查該條件。

?

/*?wq?–?等待隊(duì)列
?*?condition?-?要等待的C表達(dá)式的事件?
?*/
wait_event(wq,?condition);

?

wait_event_timeout

進(jìn)程進(jìn)入休眠狀態(tài)(TASK_UNINTERUPTIBLE)，直到條件評(píng)估為true或超時(shí)。每次喚醒等待隊(duì)列wq時(shí)，都會(huì)檢查該條件。

如果超時(shí)后條件評(píng)估為false，則返回0；如果超時(shí)后情況評(píng)估為true，則返回1；如果超時(shí)前情況評(píng)估為true，則返回剩余的jiffies（至少1）。

?

/*?wq?–?等待隊(duì)列
?*?condition?-?要等待的C表達(dá)式的事件?
?*?timeout?–??超時(shí)時(shí)間，單位jiffies?
?*/
wait_event_timeout(wq,?condition,?timeout);

?

wait_event_cmd

進(jìn)程進(jìn)入休眠狀態(tài)(TASK_UNINTERUPTIBLE)，直到條件評(píng)估為true。每次喚醒等待隊(duì)列wq時(shí)，都會(huì)檢查該條件。

?

/*?wq?–?等待隊(duì)列
?*?condition?-?要等待的C表達(dá)式的事件?
?*?cmd1–該命令將在睡眠前執(zhí)行
?*?cmd2–該命令將在睡眠后執(zhí)行
?*/
wait_event_cmd(wq,?condition,?cmd1,?cmd2);

?

wait_event_interruptible

進(jìn)程進(jìn)入休眠狀態(tài)(TASK_INTERRUPTIBLE)，直到條件評(píng)估為真或接收到信號(hào)。每次喚醒等待隊(duì)列wq時(shí)，都會(huì)檢查該條件。

如果被信號(hào)中斷，函數(shù)將返回-ERESTARTSYS，如果條件評(píng)估為true，則返回0。

?

/*?wq?–?等待隊(duì)列
?*?condition?-?要等待的C表達(dá)式的事件?
?*/
wait_event_interruptible(wq,?condition);

?

wait_event_interruptible_timeout

進(jìn)程進(jìn)入休眠狀態(tài)(TASK_INTERRUPTIBLE)，直到條件評(píng)估為真或接收到信號(hào)或超時(shí)。每次喚醒等待隊(duì)列wq時(shí)，都會(huì)檢查該條件。

如果超時(shí)后條件評(píng)估為false，則返回0；如果超時(shí)后情況評(píng)估為true，則返回1；如果超時(shí)前情況評(píng)估為true，則返回剩余的jiffies（至少1）；如果被信號(hào)中斷，則返回-ERESTARTSYS。

?

/*?wq?–?等待隊(duì)列
?*?condition?-?要等待的C表達(dá)式的事件?
?*?timeout?–??超時(shí)時(shí)間，單位jiffies?
?*/
wait_event_interruptible_timeout(wq,?condition,?timeout);

?

wait_event_killable

進(jìn)程進(jìn)入休眠狀態(tài)（TASK_KILLABLE），直到條件評(píng)估為真或收到信號(hào)。每次喚醒等待隊(duì)列wq時(shí)，都會(huì)檢查該條件。

如果被信號(hào)中斷，函數(shù)將返回-ERESTARTSYS，如果條件評(píng)估為true，則返回0。

?

/*?wq?–?等待隊(duì)列
?*?condition?-?要等待的C表達(dá)式的事件?
?*/
wait_event_killable(wq,?condition);

?

4. 喚醒排隊(duì)的任務(wù)

當(dāng)一些任務(wù)由于等待隊(duì)列而處于睡眠模式時(shí)，我們可以使用下面的函數(shù)來(lái)喚醒這些任務(wù)。

wake_up

wake_up_all

wake_up_interruptible

wake_up_sync and wake_up_interruptible_sync

通常，調(diào)用wake_up會(huì)立即觸發(fā)重新調(diào)度，這意味著在wake_up返回之前可能會(huì)運(yùn)行其他進(jìn)程。“同步”變體使任何喚醒的進(jìn)程都可以運(yùn)行，但不會(huì)重新調(diào)度CPU。這用于避免在已知當(dāng)前進(jìn)程進(jìn)入睡眠狀態(tài)時(shí)重新調(diào)度，從而強(qiáng)制重新調(diào)度。注意，被喚醒的進(jìn)程可以立即在不同的處理器上運(yùn)行，因此不應(yīng)期望這些函數(shù)提供互斥

5. 實(shí)踐

我們?cè)趦蓚€(gè)地方發(fā)送了一個(gè)wake_up。一個(gè)來(lái)自讀取功能，另一個(gè)來(lái)自驅(qū)動(dòng)退出。

首先創(chuàng)建了一個(gè)線程（wait_function）。該線程將始終等待該事件。它會(huì)一直睡到接到喚醒事件。當(dāng)它得到wake_up調(diào)用時(shí)，它將檢查條件。如果條件為1，則喚醒來(lái)自讀取功能。如果是2，則喚醒來(lái)自退出功能。如果wake_up來(lái)自讀取功能，它將打印讀取計(jì)數(shù)，并再次等待。如果它來(lái)自exit函數(shù)，那么它將從線程中退出。

靜態(tài)創(chuàng)建wait queue

?

/***************************************************************************//**
*??file???????driver.c
*
*??details????Simple?linux?driver?(Waitqueue?Static?method)
*
*??author?????xxx
*******************************************************************************/
#include?
#include?
#include?
#include?
#include?
#include?
#include?
#include??????????????????//kmalloc()
#include???????????????//copy_to/from_user()
#include?
#include??????????????????//?Required?for?the?wait?queues
#include?
?
?
uint32_t?read_count?=?0;
static?struct?task_struct?*wait_thread;
?
DECLARE_WAIT_QUEUE_HEAD(wait_queue_etx);
?
dev_t?dev?=?0;
static?struct?class?*dev_class;
static?struct?cdev?etx_cdev;
int?wait_queue_flag?=?0;

/*
**?Function?Prototypes
*/
static?int??????__init?etx_driver_init(void);
static?void?????__exit?etx_driver_exit(void);
?
/***************?Driver?functions?**********************/
static?int??????etx_open(struct?inode?*inode,?struct?file?*file);
static?int??????etx_release(struct?inode?*inode,?struct?file?*file);
static?ssize_t??etx_read(struct?file?*filp,?char?__user?*buf,?size_t?len,loff_t?*?off);
static?ssize_t??etx_write(struct?file?*filp,?const?char?*buf,?size_t?len,?loff_t?*?off);

/*
**?File?operation?sturcture
*/
static?struct?file_operations?fops?=
{
????????.owner??????????=?THIS_MODULE,
????????.read???????????=?etx_read,
????????.write??????????=?etx_write,
????????.open???????????=?etx_open,
????????.release????????=?etx_release,
};

/*
**?Thread?function
*/
static?int?wait_function(void?*unused)
{
????????
????????while(1)?{
????????????????pr_info("Waiting?For?Event...
");
????????????????wait_event_interruptible(wait_queue_etx,?wait_queue_flag?!=?0?);
????????????????if(wait_queue_flag?==?2)?{
????????????????????????pr_info("Event?Came?From?Exit?Function
");
????????????????????????return?0;
????????????????}
????????????????pr_info("Event?Came?From?Read?Function?-?%d
",?++read_count);
????????????????wait_queue_flag?=?0;
????????}
????????do_exit(0);
????????return?0;
}

/*
**?This?function?will?be?called?when?we?open?the?Device?file
*/
static?int?etx_open(struct?inode?*inode,?struct?file?*file)
{
????????pr_info("Device?File?Opened...!!!
");
????????return?0;
}

/*
**?This?function?will?be?called?when?we?close?the?Device?file
*/
static?int?etx_release(struct?inode?*inode,?struct?file?*file)
{
????????pr_info("Device?File?Closed...!!!
");
????????return?0;
}

/*
**?This?function?will?be?called?when?we?read?the?Device?file
*/
static?ssize_t?etx_read(struct?file?*filp,?char?__user?*buf,?size_t?len,?loff_t?*off)
{
????????pr_info("Read?Function
");
????????wait_queue_flag?=?1;
????????wake_up_interruptible(&wait_queue_etx);
????????return?0;
}

/*
**?This?function?will?be?called?when?we?write?the?Device?file
*/
static?ssize_t?etx_write(struct?file?*filp,?const?char?__user?*buf,?size_t?len,?loff_t?*off)
{
????????pr_info("Write?function
");
????????return?len;
}
?
/*
**?Module?Init?function
*/
static?int?__init?etx_driver_init(void)
{
????????/*Allocating?Major?number*/
????????if((alloc_chrdev_region(&dev,?0,?1,?"etx_Dev"))?<0){
????????????????pr_info("Cannot?allocate?major?number
");
????????????????return?-1;
????????}
????????pr_info("Major?=?%d?Minor?=?%d?
",MAJOR(dev),?MINOR(dev));
?
????????/*Creating?cdev?structure*/
????????cdev_init(&etx_cdev,&fops);
????????etx_cdev.owner?=?THIS_MODULE;
????????etx_cdev.ops?=?&fops;
?
????????/*Adding?character?device?to?the?system*/
????????if((cdev_add(&etx_cdev,dev,1))?

	?

	動(dòng)態(tài)創(chuàng)建wait queue

	?
/****************************************************************************//**
*??file???????driver.c
*
*??details????Simple?linux?driver?(Waitqueue?Dynamic?method)
*
*??author?????xxx
*******************************************************************************/
#include?
#include?
#include?
#include?
#include?
#include?
#include?
#include??????????????????//kmalloc()
#include???????????????//copy_to/from_user()
#include?
#include??????????????????//?Required?for?the?wait?queues
#include?
?
?
uint32_t?read_count?=?0;
static?struct?task_struct?*wait_thread;
?
dev_t?dev?=?0;
static?struct?class?*dev_class;
static?struct?cdev?etx_cdev;
wait_queue_head_t?wait_queue_etx;
int?wait_queue_flag?=?0;
?
/*
**?Function?Prototypes
*/
static?int??????__init?etx_driver_init(void);
static?void?????__exit?etx_driver_exit(void);
?
/***************?Driver?functions?**********************/
static?int??????etx_open(struct?inode?*inode,?struct?file?*file);
static?int??????etx_release(struct?inode?*inode,?struct?file?*file);
static?ssize_t??etx_read(struct?file?*filp,?char?__user?*buf,?size_t?len,loff_t?*?off);
static?ssize_t??etx_write(struct?file?*filp,?const?char?*buf,?size_t?len,?loff_t?*?off);

/*
**?File?operation?sturcture
*/
static?struct?file_operations?fops?=
{
????????.owner??????????=?THIS_MODULE,
????????.read???????????=?etx_read,
????????.write??????????=?etx_write,
????????.open???????????=?etx_open,
????????.release????????=?etx_release,
};
?
/*
**?Thread?function
*/
static?int?wait_function(void?*unused)
{
????????
????????while(1)?{
????????????????pr_info("Waiting?For?Event...
");
????????????????wait_event_interruptible(wait_queue_etx,?wait_queue_flag?!=?0?);
????????????????if(wait_queue_flag?==?2)?{
????????????????????????pr_info("Event?Came?From?Exit?Function
");
????????????????????????return?0;
????????????????}
????????????????pr_info("Event?Came?From?Read?Function?-?%d
",?++read_count);
????????????????wait_queue_flag?=?0;
????????}
????????return?0;
}
?
/*
**?This?function?will?be?called?when?we?open?the?Device?file
*/?
static?int?etx_open(struct?inode?*inode,?struct?file?*file)
{
????????pr_info("Device?File?Opened...!!!
");
????????return?0;
}

/*
**?This?function?will?be?called?when?we?close?the?Device?file
*/
static?int?etx_release(struct?inode?*inode,?struct?file?*file)
{
????????pr_info("Device?File?Closed...!!!
");
????????return?0;
}

/*
**?This?function?will?be?called?when?we?read?the?Device?file
*/
static?ssize_t?etx_read(struct?file?*filp,?char?__user?*buf,?size_t?len,?loff_t?*off)
{
????????pr_info("Read?Function
");
????????wait_queue_flag?=?1;
????????wake_up_interruptible(&wait_queue_etx);
????????return?0;
}

/*
**?This?function?will?be?called?when?we?write?the?Device?file
*/
static?ssize_t?etx_write(struct?file?*filp,?const?char?__user?*buf,?size_t?len,?loff_t?*off)
{
????????pr_info("Write?function
");
????????return?len;
}

/*
**?Module?Init?function
*/
static?int?__init?etx_driver_init(void)
{
????????/*Allocating?Major?number*/
????????if((alloc_chrdev_region(&dev,?0,?1,?"etx_Dev"))?<0){
????????????????pr_info("Cannot?allocate?major?number
");
????????????????return?-1;
????????}
????????pr_info("Major?=?%d?Minor?=?%d?
",MAJOR(dev),?MINOR(dev));
?
????????/*Creating?cdev?structure*/
????????cdev_init(&etx_cdev,&fops);
?
????????/*Adding?character?device?to?the?system*/
????????if((cdev_add(&etx_cdev,dev,1))?

	?

	MakeFile

	?
obj-m?+=?driver.o
KDIR?=?/lib/modules/$(shell?uname?-r)/build
all:
????make?-C?$(KDIR)??M=$(shell?pwd)?modules
clean:
????make?-C?$(KDIR)??M=$(shell?pwd)?clean


	?

	編譯和測(cè)試

	使用Makefile（sudo make）構(gòu)建驅(qū)動(dòng)程序

	使用sudo insmod driver.ko加載驅(qū)動(dòng)程序

	然后檢查dmesg

	?
Major?=?246?Minor?=?0
Thread?Created?successfully
Device?Driver?Insert...Done!!!
Waiting?For?Event...


	?

	因此，該線程正在等待該事件?，F(xiàn)在，我們將通過使用sudo cat/dev/etx_device讀取驅(qū)動(dòng)程序來(lái)發(fā)送事件

	現(xiàn)在檢查dmesg

	?
Device?File?Opened...!!!
Read?Function
Event?Came?From?Read?Function?-?1
Waiting?For?Event...
Device?File?Closed...!!!


	?

	我們從讀取功能發(fā)送喚醒，因此它將打印讀取計(jì)數(shù)，然后再次休眠。現(xiàn)在通過sudo rmmod驅(qū)動(dòng)程序從退出功能發(fā)送事件

	?
Event?Came?From?Exit?Function
Device?Driver?Remove...Done!!!


	?

	現(xiàn)在條件是2。因此，它將從線程返回并刪除驅(qū)動(dòng)程序。

	?


	　　審核編輯：湯梓紅