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等級：青蜂俠
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注冊：2010-11-12 11:08:23
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深入淺出Linux設備驅動之并發控制(2)  Post By：2010-11-24 11:46:56
下面進入對并發控制的實戰。首先，在globalvar的驅動程序中，我們可以通過信號量來控制對int global_var的并發訪問，下面給出源代碼：
#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)
{
//獲得信號量
if (down_interruptible(&sem))
{
return - ERESTARTSYS;
}
//將global_var從內核空間復制到用戶空間
if (copy_to_user(buf, &global_var, sizeof(int)))
{
up(&sem);
return - EFAULT;
}
//釋放信號量
up(&sem);
return sizeof(int);
}
ssize_t globalvar_write(struct file *filp, const char *buf, size_t len, loff_t *off)
{
//獲得信號量
if (down_interruptible(&sem))
{
return - ERESTARTSYS;
}
//將用戶空間的數據復制到內核空間的global_var
if (copy_from_user(&global_var, buf, sizeof(int)))
{
up(&sem);
return - EFAULT;
}
//釋放信號量
up(&sem);
return sizeof(int);
}
module_init(globalvar_init);
module_exit(globalvar_exit); | 接下來，我們給globalvar的驅動程序增加open()和release()函數，并在其中借助自旋鎖來保護對全局變量int globalvar_count（記錄打開設備的進程數）的訪問來實現設備只能被一個進程打開（必須確保globalvar_count最多只能為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)
{
//獲得自選鎖
spin_lock(&spin);
//臨界資源訪問
if (globalvar_count)
{
spin_unlock(&spin);
return - EBUSY;
}
globalvar_count++;
//釋放自選鎖
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); | 為了上述驅動程序的效果，我們啟動兩個進程分別打開/dev/globalvar。在兩個終端中調用./globalvartest.o測試程序，當一個進程打開/dev/globalvar后，另外一個進程將打開失敗，輸出"device open failure"，如下圖：
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