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-- Linux驅動開發 (http://www.bgl88.cn/bbs/list.asp?boardid=33)
---- 深入淺出Linux設備驅動之并發控制(2) (http://www.bgl88.cn/bbs/dispbbs.asp?boardid=33&id=1742)
-- 發布時間：2010-11-24 11:46:56
-- 深入淺出Linux設備驅動之并發控制(2)
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：
 輸出結果 |