字符驱动第七课----原子操作

术语

可重入

当同一份代码产生两个实例（运行态）时，他们所操作的空间就是同一块空间，这会造成数据不按预想地被修改，这就是重入问题，避免了这种冲突的代码叫可重入代码。同一个进程内，一个函数还没有执行完就又开始第二次调用，这也会引起重入问题。如正在执行add()函数，此时来了个中断，中断服务子程序中也要调用add()函数，此时可能会有风险。

粒度

保护范围，细粒度即是保护得那段代码很精简

同步

强调代码的秩序，即谁先谁后

互斥

排他，资源性保护

中断屏蔽

最简单粗暴无脑的做法，容易造成系统性能下降，一般不用，屏蔽了中断的代码要求快速执行，不能休眠，执行完马上打开中断。

原子操作

不可被打断的操作叫做原子操作

1.出处：

asm/atomic.h类型: atomic_t

2.初始化:

初始化：atomic_t tv = ATOMIC_INIT（初值）；

3.读写:

<include/asm/atomic.h>/** 功能：读取原子量的值*/atomic_read(v)

/** 功能：设置原子量为某个值*/atomic_set(v, i)

4.加减:

源码目录下arch/tile/include/asm/     include/asm/atomic.h

/** 功能：将原子量加上某个值后返回*/atomic_add_return(int i, atomic_t *v)

/** 功能：将原子量加1后返回*/ atomic64_inc_return(v)

/** 功能：将原子量减掉某个值并且判断减后是否为0* 返回值：减后为0则返回ture（1），否则返回fase（0）*/ atomic64_sub_and_test(i, v)

工程实例

#include <linux/init.h>#include <linux/module.h>#include <linux/kernel.h>#include <linux/cdev.h>#include <linux/fs.h>#include <linux/errno.h>#include <asm/current.h>#include <linux/sched.h>#include <linux/uaccess.h>#include <asm/atomic.h>#include <linux/device.h>static struct class *cls = NULL;#include "mycmd.h"static int major = 0;static int minor = 0;const  int count = 6;#define DEVNAME "demo"static struct cdev *demop = NULL;#define KMAX 1024static int counter = 0;static char kbuf[KMAX];static atomic_t tv;//打开设备static int demo_open(struct inode *inode, struct file *filp){    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    //get major and minor from inode    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d\n",        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);    /*这里确保了不被重复打开*/    if(!atomic_dec_and_test(&tv)){//原子量自减后为0？        atomic_inc(&tv);//因为上一步自减了，这里要加回去        return -EBUSY;    }    memset(kbuf, 0, KMAX);    counter = 0;    return 0;}//关闭设备static int demo_release(struct inode *inode, struct file *filp){    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    //get major and minor from inode    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d\n",        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);    atomic_inc(&tv);//原子量自增1    return 0;}//读设备//ssize_t read(int fd, void *buf, size_t count)static ssize_t demo_read(struct file *filp, char __user *buf, size_t size, loff_t *offset){    struct inode *inode = filp->f_path.dentry->d_inode;    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    //get major and minor from inode    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d\n",        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);    if(counter < size){        size = counter;    }    if(copy_to_user(buf, kbuf, size)){//从内核空间复制到用户空间        return -EAGAIN;    }    counter = 0;//模拟已读空    return size;}//写设备static ssize_t demo_write(struct file *filp, const char __user *buf, size_t size, loff_t *offset){    struct inode *inode = filp->f_path.dentry->d_inode;    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    //get major and minor from inode    printk(KERN_INFO "(major=%d, minor=%d), %s : %s : %d\n",        imajor(inode), iminor(inode), __FILE__, __func__, __LINE__);    if(size > KMAX){        return -ENOMEM;    }    if(copy_from_user(kbuf, buf, size)){//从用户空间拷贝到内核空间        return -EAGAIN;    }    counter = size;     //模拟成功写入size个字节    return counter;}/* * read/write param * read status * contrl device */static long demo_ioctl(struct file *filp, unsigned int cmd, unsigned long arg){    static struct karg karg = {        .kval = 0,        .kbuf = {0},    };    struct karg *usrarg;    switch(cmd){    case CMDON:        printk(KERN_INFO "CMDON: %s : %s : %d\n", __FILE__, __func__, __LINE__);        break;    case CMDOFF:        printk(KERN_INFO "CMDOFF: %s : %s : %d\n", __FILE__, __func__, __LINE__);        break;    case CMDR:        printk(KERN_INFO "CMDR: %s : %s : %d\n", __FILE__, __func__, __LINE__);        if(_IOC_SIZE(cmd) != sizeof(karg)){            return -EINVAL;        }        usrarg = (struct karg *)arg;        if(copy_to_user(usrarg, &karg, sizeof(karg))){            return -EAGAIN;        }        printk(KERN_INFO "CMDR: %s : %s : %d ---done.\n", __FILE__, __func__, __LINE__);        break;    case CMDW:        printk(KERN_INFO "CMDW: %s : %s : %d\n", __FILE__, __func__, __LINE__);        if(_IOC_SIZE(cmd) != sizeof(karg)){            return -EINVAL;        }        usrarg = (struct karg *)arg;        if(copy_from_user(&karg, usrarg, sizeof(karg))){            return -EAGAIN;        }        printk(KERN_INFO "CMDW: %d : %s\n", karg.kval, karg.kbuf);        break;    default:        ;    };    return 0;}static struct file_operations fops = {    .owner  = THIS_MODULE,    .open   = demo_open,    .release= demo_release,    .read   = demo_read,    .write  = demo_write,    .unlocked_ioctl = demo_ioctl,};static int __init demo_init(void){    dev_t devnum;    int ret, i;    struct device *devp = NULL;    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    //1. alloc cdev obj    demop = cdev_alloc();    if(NULL == demop){        return -ENOMEM;    }    //2. init cdev obj    cdev_init(demop, &fops);    ret = alloc_chrdev_region(&devnum, minor, count, DEVNAME);    if(ret){        goto ERR_STEP;    }    major = MAJOR(devnum);    //3. register cdev obj    ret = cdev_add(demop, devnum, count);    if(ret){        goto ERR_STEP1;    }    cls = class_create(THIS_MODULE, DEVNAME);    if(IS_ERR(cls)){        ret = PTR_ERR(cls);        goto ERR_STEP1;    }    for(i = minor; i < (count+minor); i++){        devp = device_create(cls, NULL, MKDEV(major, i), NULL, "%s%d", DEVNAME, i);        if(IS_ERR(devp)){            ret = PTR_ERR(devp);            goto ERR_STEP2;        }    }    // init atomic_t    atomic_set(&tv, 1);     //原子变量初始化    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d - ok.\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    return 0;ERR_STEP2:    for(--i; i >= minor; i--){        device_destroy(cls, MKDEV(major, i));    }    class_destroy(cls);ERR_STEP1:    unregister_chrdev_region(devnum, count);ERR_STEP:    cdev_del(demop);    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d - fail.\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    return ret;}static void __exit demo_exit(void){    int i;    //get command and pid    printk(KERN_INFO "(%s:pid=%d), %s : %s : %d - leave.\n",        current->comm, current->pid, __FILE__, __func__, __LINE__);    for(i=minor; i < (count+minor); i++){        device_destroy(cls, MKDEV(major, i));    }    class_destroy(cls);    unregister_chrdev_region(MKDEV(major, minor), count);    cdev_del(demop);}module_init(demo_init);module_exit(demo_exit);MODULE_LICENSE("GPL");MODULE_AUTHOR("Farsight");MODULE_DESCRIPTION("Demo for kernel module");