linux设备驱动阻塞等待队列应用１

最新推荐文章于 2020-02-09 23:24:51 发布

Ian22l

最新推荐文章于 2020-02-09 23:24:51 发布

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分类专栏： linux编程文章标签： linux

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本文深入探讨了Linux设备驱动中使用等待队列实现FIFO的原理与实践，通过具体实例解析了进程如何在条件不满足时进行阻塞，以及在条件满足后如何被唤醒，重点介绍了等待队列的定义、添加、移除及唤醒操作。

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１.实例原理

期在回顾Linux设备驱动，看了宋宝华的书籍做了学习记录。利用等待队列实现一个fifo。
当fifo里面没有数据时，读进程Ａ　fifo要阻塞（所谓的阻塞就是当操作设备条件不满足时，进程进入休眠，直到条件满足后才访问设备并且成功返回），要等写进程B写进数据后才唤醒通知读进程Ａ可以读取;同样当fifo已经写满后，写进程B要阻塞，要等读进程A读取后才唤醒通知写进程Ｂ可以写入。

2.等待队列相关大概步骤(手动添加队列)

(1)定义一个队列头并且初始化

wait_queue_head_t my_queue;
init_waitqueue_head(&my_queue);
//或者利用宏定义
DECLARE_WAIT_QUEUE_HEAD(name);

(2)判断条件操作设备调价是否满足,不满足则阻塞等待。

DECLEAR_WAITQUEUE(name,tsk);//定义一个队列
add_wait_queue(wait_queue_head_t *q,wait_queue_t *wait);//把队列添加到队列链表中去
while(!condition){
	__set_current_state(TASK_INTERRUPTIBLE);
	schedule();
	if(signal_pending(current)){
		ret = -ERESTARTSYS;//信号意外唤醒，返回重新调用信号
		goto out;//跳出等待访问，
	}
	....
}
//访问设备代码段
...
...

//访问设备完成后要把等待队列进程移出等待队列，并且设置更新进程状态
out:
	remove_wait_queue(wait_queue_head_t  *q,wait_queue_t wait);
	__set_current_state(TASK_RUNNING);
	return ret;

(3)在条件满足的时候唤醒阻塞队列

condition = ture;
if(condition){
	wake_up_interruptible(wait_queue_head_t *q);//唤醒队列上的所有进程
}

３.实例源码

#include "globalmem.h"
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/types.h>
#include <linux/completion.h>
#include <linux/wait.h>
#include <linux/signal.h>
#include <linux/sched/signal.h>

#define GLOBALMEM_BUFFER_SIZE 4096
#define GLOBALMEM_MAJOR 255

struct globalmem_dev
{
	dev_t devno;
	struct cdev dev;
	unsigned char globalmem_buf[GLOBALMEM_BUFFER_SIZE];
	struct mutex globalmem_mutex;
	//(1)定义队列头
	wait_queue_head_t r_wait;//定义读取进程队列头
	wait_queue_head_t w_wait;//定义写进程队列头
	unsigned long current_len;
	//struct completion globalmem_complet;
};

struct globalmem_dev *globalmem_devp = NULL;

static int globalmem_open(struct inode *inode, struct file *filp)
{
	if(!globalmem_devp)
		return -ENODEV;

	filp->private_data = globalmem_devp;

	return 0;
}

static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)
{
	int ret = 0;
	unsigned long count = size;
	struct globalmem_dev *dev = filp->private_data;

	//（３）define and add a wait_queue，定义一个队列
	DECLARE_WAITQUEUE(wait,current);
	mutex_lock(&dev->globalmem_mutex);

	//(4)添加队列到读队列头中去
	add_wait_queue(&dev->r_wait,&wait);

	//（５）条件不满足时，阻塞
	while(dev->current_len == 0){
		if(filp->f_flags & O_NONBLOCK){
			ret = -EAGAIN;
			goto out;
		}

		//second to set TASK_INTERRUPTIBLE　current state
		__set_current_state(TASK_INTERRUPTIBLE);
		mutex_unlock(&dev->globalmem_mutex);

		schedule();
		if(signal_pending(current)){
			ret = -ERESTARTSYS;
			goto out2;
		}

		mutex_lock(&dev->globalmem_mutex);
	}
	
	//(7)访问条件满足，访问设备资源
	if(count > dev->current_len)
		count = dev->current_len;
	
	if(copy_to_user(buf,dev->globalmem_buf,count)){
		ret = -EFAULT;
		goto out;
	}else{
		memcpy(&dev->globalmem_buf,&dev->globalmem_buf+count,GLOBALMEM_BUFFER_SIZE-count);
		dev->current_len -= count;
		wake_up_interruptible(&dev->w_wait);
		ret = count;
	}

out:
	mutex_unlock(&dev->globalmem_mutex);

out2:
	//（８）及时更新进程信息，等待下一次访问。
	remove_wait_queue(&dev->r_wait,&wait);
	__set_current_state(TASK_RUNNING);
	return ret;

}

static ssize_t globalmem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos)
{
	int ret = 0;
	unsigned long count = size;

	struct globalmem_dev *dev = filp->private_data;
	DECLARE_WAITQUEUE(wait,current);
	mutex_lock(&dev->globalmem_mutex);
	add_wait_queue(&dev->w_wait,&wait);

	while(dev->current_len == GLOBALMEM_BUFFER_SIZE){
		if(filp->f_flags & O_NONBLOCK){
			ret = -EAGAIN;
			goto out;
		}

		__set_current_state(TASK_INTERRUPTIBLE);
			mutex_unlock(&dev->globalmem_mutex);
		schedule();

		if(signal_pending(current)){
			ret = -ERESTARTSYS;
			goto out2;
		}

		mutex_lock(&dev->globalmem_mutex);

	}


	if(count > GLOBALMEM_BUFFER_SIZE -dev->current_len)
		count = GLOBALMEM_BUFFER_SIZE - dev->current_len;

	if(copy_from_user(dev->globalmem_buf+dev->current_len,buf,count)){
		ret = -EFAULT;
	}else{
	　	//(6)因为成功数据，fifo数据不为空，读取条件满足，唤醒读取进程
		dev->current_len += count;
		wake_up_interruptible(&dev->r_wait);
		ret = count;
	}

out:
	mutex_unlock(&dev->globalmem_mutex);

out2:
	remove_wait_queue(&dev->w_wait,&wait);
	__set_current_state(TASK_RUNNING);
	return ret;

}

struct file_operations globalmem_fops = {
	.open = globalmem_open,
	.read = globalmem_read,
	.write = globalmem_write
};



static int __init globalmem_init(void)
{
	int ret ;

	globalmem_devp = (struct globalmem_dev *)kzalloc(sizeof(struct globalmem_dev),GFP_KERNEL);
	if(!globalmem_devp)
		return -ENOMEM;

	globalmem_devp->devno = MKDEV(GLOBALMEM_MAJOR,0);

	if(GLOBALMEM_MAJOR)
		ret = register_chrdev_region(globalmem_devp->devno,1,"globalmem");
	else
		ret = alloc_chrdev_region(&globalmem_devp->devno,0,1,"globalmem");
	if(ret < 0)
		return ret;

	cdev_init(&globalmem_devp->dev,&globalmem_fops);
	globalmem_devp->dev.owner = THIS_MODULE;
	ret = cdev_add(&globalmem_devp->dev,globalmem_devp->devno,1);
	if(ret < 0 ){
		unregister_chrdev_region(globalmem_devp->devno,1);
		kfree(globalmem_devp);
		return ret;
	}

	globalmem_devp->current_len = 0;
	mutex_init(&globalmem_devp->globalmem_mutex);
	//(2)初始化队列头
	init_waitqueue_head(&globalmem_devp->r_wait);
	init_waitqueue_head(&globalmem_devp->w_wait);

	return 0;
}

static void __exit globalmem_exit(void)
{
	cdev_del(&globalmem_devp->dev);
	unregister_chrdev_region(globalmem_devp->devno,1);
	kfree(globalmem_devp);
}

module_init(globalmem_init);
module_exit(globalmem_exit);

MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("gentle_lenug");

该方式是通过手动通过

__set_current_state(TASK_INTERRUPTIBLE);
schedule();

使进程进入阻塞,比较灵活，但是容易犯错。下一次通过wait_queue等方式使进程阻塞。

参考:宋宝华书籍<linux设备驱动编程第三版>
代码程序参考宋宝华老师的驱动设备编程之阻塞章节，不敢说原创，只是说做了记录笔记总结。