Linux设备驱动之并发与竞态（二）

处理并发的常用技术是加锁或者互斥，既确保在任何时间只有一个执行单元可以操作共享资源。在Linux内核中主要通过semaphore机制和spin_lock机制实现。信号量：定义与初始化信号量的工作可以由如下宏一步完成：DECLARE_MUTEX(name)获取信号量int down_interruptible（struct semaphore *sem）获取信号量，如果信号量不可以用，进程将被置为TASK_INTERRUPTIBLE类型的睡眠状态；down_killable(struct semaphore *sem)获取信号量，如果信号量不可以用，进程将被置为TASK_KILLABLE类型的睡眠状态； 自旋锁定义自旋锁spinlock_t lock初始化自旋锁spin_lock_init(lock)获取自旋锁spin_lock(lock)/spin_trylock（lock）选择信号量与自旋锁的方式：信号量适用于保持时间较长的情况，自旋锁相反；测试程序头文件：#ifndef _MEMDEV_H_#define _MEMDEV_H_#ifndef MEMDEV_MAJOR#define MEMDEV_MAJOR 0  /*预设的mem的主设备号*/#endif#ifndef MEMDEV_NR_DEVS#define MEMDEV_NR_DEVS 2    /*设备数*/#endif#ifndef MEMDEV_SIZE#define MEMDEV_SIZE 4096#endif/*mem设备描述结构体*/struct mem_dev                                     {                                                          char *data;                        unsigned long size;      struct semaphore sem;     /* 定义信号量 */   };#endif /* _MEMDEV_H_ */主体程序：#include #include #include #include #include #include #include #include #include #include #include #include "memdev.h"static int mem_major = MEMDEV_MAJOR;module_param(mem_major, int, S_IRUGO);struct mem_dev *mem_devp; /*设备结构体指针*/struct cdev cdev; /*文件打开函数*/int mem_open(struct inode *inode, struct file *filp){    struct mem_dev *dev;        /*获取次设备号*/    int num = MINOR(inode->i_rdev);    if (num >= MEMDEV_NR_DEVS)             return -ENODEV;    dev = &mem_devp[num];        /*将设备描述结构指针赋值给文件私有数据指针*/    filp->private_data = dev;        return 0; }/*文件释放函数*/int mem_release(struct inode *inode, struct file *filp){  return 0;}/*读函数*/static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos){  unsigned long p =  *ppos;  unsigned int count = size;  int ret = 0;  struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/    /* 获取信号量 */  if (down_interruptible(&dev->sem))    return -ERESTARTSYS;              /*判断读位置是否有效*/  if (p >= MEMDEV_SIZE)    return 0;  if (count > MEMDEV_SIZE - p)    count = MEMDEV_SIZE - p;  /*读数据到用户空间*/  if (copy_to_user(buf, (void*)(dev->data + p), count))  {    ret =  - EFAULT;    goto out;  }  else  {    *ppos += count;    ret = count;        printk(KERN_INFO "read %d bytes(s) from %d\n", count, p);  }out:    /* 释放信号量 */    up(&dev->sem);  return ret;}/*写函数*/static ssize_t mem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos){  unsigned long p =  *ppos;  unsigned int count = size;  int ret = 0;  struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/  /* 获取信号量 */if (down_interruptible(&dev->sem))   return -ERESTARTSYS;   /*分析和获取有效的写长度*/  if (p >= MEMDEV_SIZE)    return 0;  if (count > MEMDEV_SIZE - p)    count = MEMDEV_SIZE - p;      /*从用户空间写入数据*/  if (copy_from_user(dev->data + p, buf, count)){    ret =  - EFAULT;    goto out;  }  else  {    *ppos += count;    ret = count;        printk(KERN_INFO "written %d bytes(s) from %d\n", count, p);  }out:/* 释放信号量 */  up(&dev->sem);  return ret;}/* seek文件定位函数 */static loff_t mem_llseek(struct file *filp, loff_t offset, int whence){     loff_t newpos;    switch(whence) {      case 0: /* SEEK_SET */        newpos = offset;        break;      case 1: /* SEEK_CUR */        newpos = filp->f_pos + offset;        break;      case 2: /* SEEK_END */        newpos = MEMDEV_SIZE -1 + offset;        break;      default: /* can't happen */        return -EINVAL;    }    if ((newposMEMDEV_SIZE))    return -EINVAL;        filp->f_pos = newpos;    return newpos;}/*文件操作结构体*/static const struct file_operations mem_fops ={  .owner = THIS_MODULE,  .llseek = mem_llseek,  .read = mem_read,  .write = mem_write,  .open = mem_open,  .release = mem_release,};/*设备驱动模块加载函数*/static int memdev_init(void){  int result;  int i;  dev_t devno = MKDEV(mem_major, 0);  /* 静态申请设备号*/  if (mem_major)    result = register_chrdev_region(devno, 2, "memdev");  else  /* 动态分配设备号 */  {    result = alloc_chrdev_region(&devno, 0, 2, "memdev");    mem_major = MAJOR(devno);  }      if (result     return result;  /*初始化cdev结构*/  cdev_init(&cdev, &mem_fops);  cdev.owner = THIS_MODULE;  cdev.ops = &mem_fops;    /* 注册字符设备 */  cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);     /* 为设备描述结构分配内存*/  mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);  if (!mem_devp)    /*申请失败*/  {    result =  - ENOMEM;    goto fail_malloc;  }  memset(mem_devp, 0, sizeof(struct mem_dev));    /*为设备分配内存*/  for (i=0; i   {        mem_devp[i].size = MEMDEV_SIZE;        mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);        memset(mem_devp[i].data, 0, MEMDEV_SIZE);                /* 初始化信号量 */        sema_init(&mem_devp[i].sem, 1);  }      return 0;  fail_malloc:   unregister_chrdev_region(devno, 1);    return result;}/*模块卸载函数*/static void memdev_exit(void){  cdev_del(&cdev);   /*注销设备*/  kfree(mem_devp);     /*释放设备结构体内存*/  unregister_chrdev_region(MKDEV(mem_major, 0), 2); /*释放设备号*/}MODULE_AUTHOR("David Xie");MODULE_LICENSE("GPL");module_init(memdev_init);module_exit(memdev_exit);测试程序#include int main(){FILE *fp0 = NULL;char Buf[4096];/*初始化Buf*/strcpy(Buf,"Mem is char dev!");printf("BUF: %s\n",Buf);/*打开设备文件*/fp0 = fopen("/dev/memdev0","r+");if (fp0 == NULL){printf("Open Memdev0 Error!\n");return -1;}/*写入设备*/fwrite(Buf, sizeof(Buf), 1, fp0);/*重新定位文件位置（思考没有该指令，会有何后果)*/fseek(fp0,0,SEEK_SET);/*清除Buf*/strcpy(Buf,"Buf is NULL!");printf("BUF: %s\n",Buf);/*读出设备*/fread(Buf, sizeof(Buf), 1, fp0);/*检测结果*/printf("BUF: %s\n",Buf);return 0;}