字符设备驱动-同步互斥阻塞

我们想来达到一个目的：同一时刻，只能有一个应用程序打开/dev/buttons

一般的，我们想达到目的会想到下面这种方法：

static int canopen = 1;static int sixth_drv_open(struct inode *inode,struct file *file){    if(--canopen != 0)    {        canopen++;        return -EBUSY;    }}static int sixth_drv_close (struct inode *inode, struct file *file){    canopen++;}

假设程序A来调用，那么进入open函数，canopen = 0,if条件不成立；如果A没有释放，程序B来调用时候,canopen = -1,if条件成立，return -EBUSY;

原则上是可以实现，但是我们进行–canopen操作实际上转化程汇编使用三条指令(读->改->写)执行的，由于Linux是多任务编程的。当我们程序A进行–canopen的读取后，程序B刚好进行那么就有可能发生程序AB都能调用驱动！看下图分析：

下面来介绍三种解决上面bug的方法

一、原子操作：

原子操作指的是在执行过程中不会被别的代码路径所中断的操作。
常用原子操作函数举例：

atomic_t v = ATOMIC_INIT(0);     //定义原子变量v并初始化为0atomic_read(atomic_t *v);        //返回原子变量的值void atomic_inc(atomic_t *v);    //原子变量增加1void atomic_dec(atomic_t *v);    //原子变量减少1int atomic_dec_and_test(atomic_t *v); //自减操作后测试其是否为0，为0则返回true，否则返回false。

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Makefile

KERN_DIR = /work/system/linux-2.6.22.6all:    make -C $(KERN_DIR) M=`pwd` modules clean:    make -C $(KERN_DIR) M=`pwd` modules clean    rm -rf modules.orderobj-m += sixth_drv.o

驱动函数:sixth_drv.c

#include <linux/module.h>  #include <linux/kernel.h>  #include <linux/fs.h>  #include <linux/init.h>  #include <linux/delay.h>  #include <linux/irq.h>#include <asm/uaccess.h>  #include <asm/irq.h>  #include <asm/io.h>  #include <asm/arch/regs-gpio.h>  #include <asm/hardware.h>#include <linux/poll.h>static struct fasync_struct *button_async;static struct class *sixthdrv_class;static struct class_device *sixthdrv_class_dev;static DECLARE_WAIT_QUEUE_HEAD(button_waitq);/* 中断事件标志, 中断服务程序将它置1，s3c24xx_sixth_read将它清0 */static volatile int ev_press = 0;volatile unsigned long *gpfcon;volatile unsigned long *gpfdat;volatile unsigned long *gpgcon;volatile unsigned long *gpgdat;struct pin_desc{    unsigned int pin;    unsigned int key_val;};/* 键值： 按下时，0x01、0x02、0x03 *//* 键值： 松开时，0x81、0x82、0x83 */static unsigned char key_val;struct pin_desc pin_desc[3] = {    {S3C2410_GPF0,0X01},    {S3C2410_GPF2,0X02},    {S3C2410_GPG3,0X03},};atomic_t canopen = ATOMIC_INIT(1);//定义原子变量canopen并初始化为1static irqreturn_t buttons_irq(int irq, void *dev_id){    struct pin_desc *pindesc = (struct pin_desc *)dev_id;    unsigned int pinval;    pinval =  s3c2410_gpio_getpin(pindesc->pin);    if(pinval)    {        /* 松开 */        key_val = 0x80 | (pindesc->key_val);        *gpfdat |= ((1<<4) | (1<<5) | (1<<6));    }    else    {        /* 按下 */        key_val = pindesc->key_val;        *gpfdat &= ~((1<<4) | (1<<5) | (1<<6));    }    ev_press = 1;                /* 表示中断发生了 */    wake_up_interruptible(&button_waitq);   /* 唤醒休眠的进程 */    kill_fasync (&button_async, SIGIO, POLL_IN);    return IRQ_RETVAL(IRQ_HANDLED);}static int sixth_drv_open(struct inode *inode,struct file *file){    if(!atomic_dec_and_test(&canopen))    {        atomic_inc(&canopen);         return -EBUSY;    }    /* 配置GPF0,2、GPG3为中断引脚 */    request_irq(IRQ_EINT0, buttons_irq,IRQT_BOTHEDGE,"s2",&pin_desc[0]);    request_irq(IRQ_EINT2, buttons_irq,IRQT_BOTHEDGE,"s3",&pin_desc[1]);    request_irq(IRQ_EINT11,buttons_irq,IRQT_BOTHEDGE,"s4",&pin_desc[2]);    /* 配置GPF4、5、6为输入引脚 */    *gpfcon &= ~((0x3<<4*2) | (0x3<<5*2) | (0x3<<6*2));    *gpfcon |=  ((1<<4*2) | (1<<5*2) | (1<<6*2));    return 0;}static ssize_t sixth_drv_read (struct file *file, char __user *buf, size_t count, loff_t *ppos){    //看用户需要读取的空间，和这里的是否相同    if(count != 1)        return -EINVAL;    /* 如果无按键动作发生，则进行休眠状态 */    /* 如果ev_press等于0，休眠 */    wait_event_interruptible(button_waitq,ev_press);    /* 如果有按键动作发生，则返回按键的值 */    copy_to_user(buf,&key_val,1);    ev_press = 0;    return 1;}static int sixth_drv_close (struct inode *inode, struct file *file){    atomic_inc(&canopen);     free_irq(IRQ_EINT0,  &pin_desc[0]);    free_irq(IRQ_EINT2,  &pin_desc[1]);    free_irq(IRQ_EINT11, &pin_desc[2]);    return 0;}static unsigned int sixth_drv_poll(struct file *file, struct poll_table_struct *wait){    unsigned int mask = 0;    poll_wait(file, &button_waitq, wait);    if (ev_press)        mask |= POLLIN | POLLRDNORM;       return mask;}static int sixth_drv_fasync (int fd, struct file *filp, int on){    printk("driver: sixth_drv_fasync\n");    return fasync_helper (fd, filp, on, &button_async);}static struct file_operations sixth_drv_fops = {    .owner   = THIS_MODULE, /* 这是一个宏，推向编译模块时自动创建的__this_module变量 */    .open    = sixth_drv_open,    .read    = sixth_drv_read,    .release = sixth_drv_close,    .poll    = sixth_drv_poll,    .fasync  = sixth_drv_fasync,};int major; static int sixth_drv_init(void){        major = register_chrdev(0,"sixth_drv",&sixth_drv_fops);        sixthdrv_class = class_create(THIS_MODULE,"sixthdrv");        sixthdrv_class_dev = class_device_create(sixthdrv_class,NULL,MKDEV(major,0),NULL,"buttons");        gpfcon = (volatile unsigned long *)ioremap(0x56000050,16);        gpfdat = gpfcon + 1;        gpgcon = (volatile unsigned long *)ioremap(0x56000060,16);        gpgdat = gpgcon + 1;        return 0;}static int sixth_drv_exit(void){        unregister_chrdev(major,"sixth_drv");        class_device_unregister(sixthdrv_class_dev);        class_destroy(sixthdrv_class);        iounmap(gpfcon);        iounmap(gpgcon);        return 0;}module_init(sixth_drv_init);module_exit(sixth_drv_exit);MODULE_LICENSE("GPL");

驱动测试函数：sixthdrvtest.c

#include <sys/types.h>  #include <sys/stat.h>  #include <fcntl.h>  #include <stdio.h>  #include <poll.h>#include <signal.h>#include <sys/types.h>#include <unistd.h>/*  * sixthdrvtest */int fd;void czg_signal_handler(int signum){    unsigned char key_val = 0;    read(fd,&key_val,1);    printf("key_val: 0x%x\n",key_val);}int main(int argc, char **argv){    int ret;    int oflags;    signal(SIGIO,czg_signal_handler);    fd = open("/dev/buttons",O_RDWR);    if(fd < 0)    {        printf("can't open!\n");        return -1;    }    fcntl(fd,F_SETOWN,getpid()); // 告诉内核，发给谁    oflags = fcntl(fd,F_GETFL);    fcntl(fd,F_SETFL,oflags | FASYNC); // 改变fasync标记，    //最终会调用到驱动的faync > fasync_helper：初始化/释放fasync_struct    //然后当按键按下时候，在irqreturn_t buttons_irq中断处理中调用kill_fasync    while(1)    {        sleep(1000);    }    return 0;}

测试：

makearm-linux-gcc -o sixthdrvtest sixthdrvtest.c cp sixthdrvtest sixth_drv.ko /work/nfs_root/czginsmod sixth_drv.ko./sixthdrvtest &

二、信号量：

信号量（semaphore）是用于保护临界区的一种常用方法，只有得到信号量的进程才能执行临界区代码。当获取不到信号量时，进程进入休眠等待状态。

//定义信号量struct semaphore sem;//初始化信号量void sema_init (struct semaphore *sem, int val);void init_MUTEX(struct semaphore *sem);//初始化为0static DECLARE_MUTEX(button_lock);     //定义互斥锁//获得信号量void down(struct semaphore * sem);int down_interruptible(struct semaphore * sem); int down_trylock(struct semaphore * sem);//释放信号量void up(struct semaphore * sem);

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代码只需要在上面的驱动程序sixth_drv.c稍做修改

static DECLARE_MUTEX(button_lock);     //定义互斥锁/* 获 取 信 号 量 */down(&button_lock);/* 释放信号量 */up(&button_lock);

三、阻塞：

阻塞操作：

是指在执行设备操作时若不能获得资源则挂起进程，直到满足可操作的条件后再进行操作。被挂起的进程进入休眠状态，被从调度器的运行队列移走，直到等待的条件被满足。

非阻塞操作：

进程在不能进行设备操作时并不挂起，它或者放弃，或者不停地查询，直至可以进行操作为止。

代码在上面的驱动程序sixth_drv.c稍做修改

static int sixth_drv_open(struct inode *inode,struct file *file){    if (file->f_flags &  O_NONBLOCK)    {        if (down_trylock(&button_lock))            return -EBUSY;    }    else    {        /* 获 取 信 号 量 */        down(&button_lock);    }}/*************************************************/static ssize_t sixth_drv_read (struct file *file, char __user *buf, size_t count, loff_t *ppos){    //看用户需要读取的空间，和这里的是否相同    if(count != 1)        return -EINVAL;    if(file->f_flags & O_NONBLOCK)    {        if(!ev_press)            return -EAGAIN;    }    else    {    /* 如果无按键动作发生，则进行休眠状态 */    /* 如果ev_press等于0，休眠 */    wait_event_interruptible(button_waitq,ev_press);    }}

阻塞：

驱动测试代码：sixthdrvtest.c

#include <sys/types.h>  #include <sys/stat.h>  #include <fcntl.h>  #include <stdio.h>  #include <poll.h>#include <signal.h>#include <sys/types.h>#include <unistd.h>/*  * sixthdrvtest */int fd;void czg_signal_handler(int signum){    unsigned char key_val = 0;    read(fd,&key_val,1);    printf("key_val: 0x%x\n",key_val);}int main(int argc, char **argv){    int ret;    int oflags;    unsigned char key_val = 0;    //signal(SIGIO,czg_signal_handler);    fd = open("/dev/buttons",O_RDWR);    if(fd < 0)    {        printf("can't open!\n");        return -1;    }    //fcntl(fd,F_SETOWN,getpid()); // 告诉内核，发给谁    //oflags = fcntl(fd,F_GETFL);    //fcntl(fd,F_SETFL,oflags | FASYNC); // 改变fasync标记，    //最终会调用到驱动的faync > fasync_helper：初始化/释放fasync_struct    //然后当按键按下时候，在irqreturn_t buttons_irq中断处理中调用kill_fasync    while(1)    {        read(fd,&key_val,1);        printf("key_val: 0x%x\n",key_val);        //sleep(1000);    }    return 0;}

非阻塞：

驱动测试代码：sixthdrvtest.c

#include <sys/types.h>  #include <sys/stat.h>  #include <fcntl.h>  #include <stdio.h>  #include <poll.h>#include <signal.h>#include <sys/types.h>#include <unistd.h>/*  * sixthdrvtest */int fd;void czg_signal_handler(int signum){    unsigned char key_val = 0;    read(fd,&key_val,1);    printf("key_val: 0x%x\n",key_val);}int main(int argc, char **argv){    int ret;    int oflags;    unsigned char key_val = 0;    //signal(SIGIO,czg_signal_handler);    fd = open("/dev/buttons",O_RDWR | O_NONBLOCK);    if(fd < 0)    {        printf("can't open!\n");        return -1;    }    //fcntl(fd,F_SETOWN,getpid()); // 告诉内核，发给谁    //oflags = fcntl(fd,F_GETFL);    //fcntl(fd,F_SETFL,oflags | FASYNC); // 改变fasync标记，    //最终会调用到驱动的faync > fasync_helper：初始化/释放fasync_struct    //然后当按键按下时候，在irqreturn_t buttons_irq中断处理中调用kill_fasync    while(1)    {        ret = read(fd,&key_val,1);        printf("key_val: 0x%x,ret = %d\n",key_val,ret);        sleep(4);    }    return 0;}