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----  深入浅出Linux设备驱动之并发控制(2)  (http://sun4.cn/bbs/dispbbs.asp?boardid=33&id=1742)

--  作者:wangxinxin
--  发布时间:2010-11-24 11:46:56
--  深入浅出Linux设备驱动之并发控制(2)
下面进入对并发控制的实战。首先,在globalvar的驱动程序中,我们可以通过信号量来控制对int global_var的并发访问,下面给出源代码:

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <asm/semaphore.h>
MODULE_LICENSE("GPL");

#define MAJOR_NUM 254

static ssize_t globalvar_read(struct file *, char *, size_t, loff_t*);
static ssize_t globalvar_write(struct file *, const char *, size_t, loff_t*);

struct file_operations globalvar_fops =
{
 read: globalvar_read, write: globalvar_write,
};
static int global_var = 0;
static struct semaphore sem;

static int __init globalvar_init(void)
{
 int ret;
 ret = register_chrdev(MAJOR_NUM, "globalvar", &globalvar_fops);
 if (ret)
 {
  printk("globalvar register failure");
 }
 else
 {
  printk("globalvar register success");
  init_MUTEX(&sem);
 }
 return ret;
}

static void __exit globalvar_exit(void)
{
 int ret;
 ret = unregister_chrdev(MAJOR_NUM, "globalvar");
 if (ret)
 {
  printk("globalvar unregister failure");
 }
 else
 {
  printk("globalvar unregister success");
 }
}

static ssize_t globalvar_read(struct file *filp, char *buf, size_t len, loff_t *off)
{
 //获得信号量
 if (down_interruptible(&sem))
 {
  return - ERESTARTSYS;
 }

 //将global_var从内核空间复制到用户空间
 if (copy_to_user(buf, &global_var, sizeof(int)))
 {
  up(&sem);
  return - EFAULT;
 }

 //释放信号量
 up(&sem);

 return sizeof(int);
}

ssize_t globalvar_write(struct file *filp, const char *buf, size_t len, loff_t *off)
{
 //获得信号量
 if (down_interruptible(&sem))
 {
  return - ERESTARTSYS;
 }

 //将用户空间的数据复制到内核空间的global_var
 if (copy_from_user(&global_var, buf, sizeof(int)))
 {
  up(&sem);
  return - EFAULT;
 }

 //释放信号量
 up(&sem);
 return sizeof(int);
}

module_init(globalvar_init);
module_exit(globalvar_exit);

  接下来,我们给globalvar的驱动程序增加open()和release()函数,并在其中借助自旋锁来保护对全局变量int globalvar_count(记录打开设备的进程数)的访问来实现设备只能被一个进程打开(必须确保globalvar_count最多只能为1):

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <asm/semaphore.h>

MODULE_LICENSE("GPL");

#define MAJOR_NUM 254

static ssize_t globalvar_read(struct file *, char *, size_t, loff_t*);
static ssize_t globalvar_write(struct file *, const char *, size_t, loff_t*);
static int globalvar_open(struct inode *inode, struct file *filp);
static int globalvar_release(struct inode *inode, struct file *filp);

struct file_operations globalvar_fops =
{
 read: globalvar_read, write: globalvar_write, open: globalvar_open, release:
globalvar_release,
};

static int global_var = 0;
static int globalvar_count = 0;
static struct semaphore sem;
static spinlock_t spin = SPIN_LOCK_UNLOCKED;

static int __init globalvar_init(void)
{
 int ret;
 ret = register_chrdev(MAJOR_NUM, "globalvar", &globalvar_fops);
 if (ret)
 {
  printk("globalvar register failure");
 }
 else
 {
  printk("globalvar register success");
  init_MUTEX(&sem);
 }
 return ret;
}

static void __exit globalvar_exit(void)
{
 int ret;
 ret = unregister_chrdev(MAJOR_NUM, "globalvar");
 if (ret)
 {
  printk("globalvar unregister failure");
 }
 else
 {
  printk("globalvar unregister success");
 }
}

static int globalvar_open(struct inode *inode, struct file *filp)
{
 //获得自选锁
 spin_lock(&spin);

 //临界资源访问
 if (globalvar_count)
 {
  spin_unlock(&spin);
  return - EBUSY;
 }
 globalvar_count++;

 //释放自选锁
 spin_unlock(&spin);
 return 0;
}

static int globalvar_release(struct inode *inode, struct file *filp)
{
 globalvar_count--;
 return 0;
}

static ssize_t globalvar_read(struct file *filp, char *buf, size_t len, loff_t
*off)
{
 if (down_interruptible(&sem))
 {
  return - ERESTARTSYS;
 }
 if (copy_to_user(buf, &global_var, sizeof(int)))
 {
  up(&sem);
  return - EFAULT;
 }
 up(&sem);
 return sizeof(int);
}

static ssize_t globalvar_write(struct file *filp, const char *buf, size_t len,
loff_t *off)
{
 if (down_interruptible(&sem))
 {
  return - ERESTARTSYS;
 }
 if (copy_from_user(&global_var, buf, sizeof(int)))
 {
  up(&sem);
  return - EFAULT;
 }
 up(&sem);
 return sizeof(int);
}

module_init(globalvar_init);
module_exit(globalvar_exit);

  为了上述驱动程序的效果,我们启动两个进程分别打开/dev/globalvar。在两个终端中调用./globalvartest.o测试程序,当一个进程打开/dev/globalvar后,另外一个进程将打开失败,输出"device open failure",如下图:

图片点击可在新窗口打开查看
输出结果