同步、互斥、阻塞的概念：
同步：在并发程序设计中，各进程对公共变量的访问必须加以制约，这种制约称为同步。
互斥机制：访问共享资源的代码区叫做临界区，这里的共享资源可能被多个线程需要，但这些共享资源又不能被同时访问，因此临界区需要以某种互斥机制加以保护，以确保共享资源被互斥访问。
阻塞与非阻塞：阻塞调用是指调用结果返回之前，当前线程会被挂起，调用线程只有在得到结果之后才会返回。非阻塞调用指在不能立刻得到结果之前，该调用不会阻塞当前线程，而是直接返回。
在按键驱动的例子中，如果有多个应用程序调用按键驱动的设备文件，这时候就要利用同步与互斥的概念对这个种情况进行处理：
1、利用原子变量标志来判断设备文件是否被打开，原子变量在操作的时候不能被打断，它是利用关闭中断的方式实现的，一旦关闭了中断，内核将不能对进程进行调度，这就保证了原子性。
直接修改驱动代码，先定义一个原子变量
static atomic_t open_flag = ATOMIC_INIT(1); //定义原子变量open_flag 并初始化为1
接着修改打开文件的函数与关闭文件的函数，初始化时open_flag 为1，一旦打开函数被调用则会减1变为0。关闭函数被调用后会加1又变成1。
a、在sixth_drv_open 中利用atomic_dec_and_test函数判断是否已经被调用，如果返回值为0，说明已经被调用。调用atomic_inc函数，并且返回。
b、在sixth_drv_close中第调用atomic_inc。
static int sixth_drv_open (struct inode * inode, struct file * file) { int ret; if(atomic_dec_and_test(&open_flag)==0)//自检后是否为0,不为0说明已经被人调用 { atomic_inc(&open_flag);//原子变量+1 return -EBUSY; } ret = request_irq(IRQ_EINT0, buttons_irq, IRQT_BOTHEDGE, "s1", (void * )&pins_desc[0]); if(ret) { printk("open failed 1 "); return -1; } ret = request_irq(IRQ_EINT2, buttons_irq, IRQT_BOTHEDGE, "s2", (void * )& pins_desc[1]); if(ret) { printk("open failed 2 "); return -1; } ret = request_irq(IRQ_EINT11, buttons_irq, IRQT_BOTHEDGE, "s3", (void * )&pins_desc[2]); if(ret) { printk("open failed 3 "); return -1; } ret = request_irq(IRQ_EINT19, buttons_irq, IRQT_BOTHEDGE, "s4", (void * )&pins_desc[3]); if(ret) { printk("open failed 4 "); return -1; } return 0; } static int sixth_drv_close(struct inode * inode, struct file * file) { atomic_inc(&open_flag);//原子变量+1 free_irq(IRQ_EINT0 ,(void * )&pins_desc[0]); free_irq(IRQ_EINT2 ,(void * )& pins_desc[1]); free_irq(IRQ_EINT11 ,(void * )&pins_desc[2]); free_irq(IRQ_EINT19 ,(void * )&pins_desc[3]); return 0; }
2、利用信号量对打开的文件进行保护：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。
直接修改驱动代码，先定义一个互斥锁
static DECLARE_MUTEX(button_lock); //定义互斥锁
接着更改按键驱动中打开文件的函数与关闭文件的函数：
a、在sixth_drv_open函数中如果文件打开方式非阻塞的，那么调用down_trylock函数获取信号量，此函数如果获取不到信号量，直接返回；如果打开文件的方式是阻塞的，那么调用down函数，如果获取不到信号量，则将进程休眠直到获取信号量为止。
b、在sixth_drv_close函数利用up函数直接释放掉信号量。
static int sixth_drv_open (struct inode * inode, struct file * file) { int ret; if(file->f_flags & O_NONBLOCK)//非阻塞方式 { if(down_trylock(&button_lock))//获取信号量失败则返回 return -EBUSY; } else down(&button_lock);//获得信号量 ret = request_irq(IRQ_EINT0, buttons_irq, IRQT_BOTHEDGE, "s1", (void * )&pins_desc[0]); if(ret) { printk("open failed 1 "); return -1; } ret = request_irq(IRQ_EINT2, buttons_irq, IRQT_BOTHEDGE, "s2", (void * )& pins_desc[1]); if(ret) { printk("open failed 2 "); return -1; } ret = request_irq(IRQ_EINT11, buttons_irq, IRQT_BOTHEDGE, "s3", (void * )&pins_desc[2]); if(ret) { printk("open failed 3 "); return -1; } ret = request_irq(IRQ_EINT19, buttons_irq, IRQT_BOTHEDGE, "s4", (void * )&pins_desc[3]); if(ret) { printk("open failed 4 "); return -1; } return 0; } static int sixth_drv_close(struct inode * inode, struct file * file) { up(&button_lock);//释放信号量 free_irq(IRQ_EINT0 ,(void * )&pins_desc[0]); free_irq(IRQ_EINT2 ,(void * )& pins_desc[1]); free_irq(IRQ_EINT11 ,(void * )&pins_desc[2]); free_irq(IRQ_EINT19 ,(void * )&pins_desc[3]); return 0; }
将完整的按键驱动的源代码贴出
#include <linux/module.h> #include <linux/kernel.h> #include <linux/fs.h> #include <linux/init.h> #include <asm/io.h> //含有iomap函数iounmap函数 #include <asm/uaccess.h>//含有copy_from_user函数 #include <linux/device.h>//含有类相关的处理函数 #include <asm/arch/regs-gpio.h>//含有S3C2410_GPF0等相关的 #include <linux/irq.h> //含有IRQ_HANDLEDIRQ_TYPE_EDGE_RISING #include <asm-arm/irq.h> //含有IRQT_BOTHEDGE触发类型 #include <linux/interrupt.h> //含有request_irq、free_irq函数 #include <linux/poll.h> #include <asm-generic/errno-base.h> //含有各种错误返回值 //#include <asm-armarch-s3c2410irqs.h> static struct class *sixth_drv_class;//类 static struct class_device *sixth_drv_class_dev;//类下面的设备 static int sixthmajor; static unsigned long *gpfcon = NULL; static unsigned long *gpfdat = NULL; static unsigned long *gpgcon = NULL; static unsigned long *gpgdat = NULL; struct fasync_struct *sixth_fasync; static unsigned int key_val; struct pin_desc { unsigned int pin; unsigned int key_val; }; static struct pin_desc pins_desc[4] = { {S3C2410_GPF0,0x01}, {S3C2410_GPF2,0x02}, {S3C2410_GPG3,0x03}, {S3C2410_GPG11,0x04} }; static unsigned int ev_press; static DECLARE_WAIT_QUEUE_HEAD(button_waitq);//注册一个等待队列button_waitq static atomic_t open_flag = ATOMIC_INIT(1); //定义原子变量open_flag 并初始化为1 static DECLARE_MUTEX(button_lock); //定义互斥锁 /* *0x01、0x02、0x03、0x04表示按键被按下 */ /* *0x81、0x82、0x83、0x84表示按键被松开 */ /* *利用dev_id的值为pins_desc来判断是哪一个按键被按下或松开 */ static irqreturn_t buttons_irq(int irq, void *dev_id) { unsigned int pin_val; struct pin_desc * pin_desc = (struct pin_desc *)dev_id;//取得哪个按键被按下的状态 pin_val = s3c2410_gpio_getpin(pin_desc->pin); if(pin_val) //按键松开 key_val = 0x80 | pin_desc->key_val; else key_val = pin_desc->key_val; wake_up_interruptible(&button_waitq); /* 唤醒休眠的进程 */ ev_press = 1; kill_fasync(&sixth_fasync, SIGIO, POLL_IN);//发生信号给进程 return IRQ_HANDLED; } static int sixth_drv_open (struct inode * inode, struct file * file) { int ret; // if(atomic_dec_and_test(&open_flag)==0)//自检后是否为0,不为0说明已经被人调用 // { // atomic_inc(&open_flag);//原子变量+1 // return -EBUSY; // } if(file->f_flags & O_NONBLOCK)//非阻塞方式 { if(down_trylock(&button_lock))//获取信号量失败则返回 return -EBUSY; } else down(&button_lock);//获得信号量 ret = request_irq(IRQ_EINT0, buttons_irq, IRQT_BOTHEDGE, "s1", (void * )&pins_desc[0]); if(ret) { printk("open failed 1 "); return -1; } ret = request_irq(IRQ_EINT2, buttons_irq, IRQT_BOTHEDGE, "s2", (void * )& pins_desc[1]); if(ret) { printk("open failed 2 "); return -1; } ret = request_irq(IRQ_EINT11, buttons_irq, IRQT_BOTHEDGE, "s3", (void * )&pins_desc[2]); if(ret) { printk("open failed 3 "); return -1; } ret = request_irq(IRQ_EINT19, buttons_irq, IRQT_BOTHEDGE, "s4", (void * )&pins_desc[3]); if(ret) { printk("open failed 4 "); return -1; } return 0; } static int sixth_drv_close(struct inode * inode, struct file * file) { // atomic_inc(&open_flag);//原子变量+1 up(&button_lock);//释放信号量 free_irq(IRQ_EINT0 ,(void * )&pins_desc[0]); free_irq(IRQ_EINT2 ,(void * )& pins_desc[1]); free_irq(IRQ_EINT11 ,(void * )&pins_desc[2]); free_irq(IRQ_EINT19 ,(void * )&pins_desc[3]); return 0; } static ssize_t sixth_drv_read(struct file * file, char __user * userbuf, size_t count, loff_t * off) { int ret; if(count != 1) { printk("read error "); return -1; } if(file->f_flags & O_NONBLOCK)//非阻塞方式 { if(!ev_press)//判断是否有按键按下，如果没有直接返回 { key_val = 0; copy_to_user(userbuf, &key_val, 1); return -EBUSY; } } else//如果没有按键动作，直接进入休眠 wait_event_interruptible(button_waitq, ev_press);//将当前进程放入等待队列button_waitq中 ret = copy_to_user(userbuf, &key_val, 1); ev_press = 0;//按键已经处理可以继续睡眠 if(ret) { printk("copy error "); return -1; } return 1; } static unsigned int sixth_drv_poll(struct file *file, poll_table *wait) { unsigned int ret = 0; poll_wait(file, &button_waitq, wait);//将当前进程放到button_waitq列表 if(ev_press) ret |=POLLIN;//说明有数据被取到了 return ret; } static int sixth_drv_fasync(int fd, struct file * file, int on) { int err; printk("fansync_helper "); err = fasync_helper(fd, file, on, &sixth_fasync);//初始化sixth_fasync if (err < 0) return err; return 0; } static struct file_operations sixth_drv_ops = { .owner = THIS_MODULE, .open = sixth_drv_open, .read = sixth_drv_read, .release = sixth_drv_close, .poll = sixth_drv_poll, .fasync = sixth_drv_fasync, }; static int sixth_drv_init(void) { sixthmajor = register_chrdev(0, "buttons", &sixth_drv_ops);//注册驱动程序 if(sixthmajor < 0) printk("failes 1 buttons_drv register "); sixth_drv_class = class_create(THIS_MODULE, "buttons");//创建类 if(sixth_drv_class < 0) printk("failes 2 buttons_drv register "); sixth_drv_class_dev = class_device_create(sixth_drv_class, NULL, MKDEV(sixthmajor,0), NULL,"buttons");//创建设备节点 if(sixth_drv_class_dev < 0) printk("failes 3 buttons_drv register "); gpfcon = ioremap(0x56000050, 16);//重映射 gpfdat = gpfcon + 1; gpgcon = ioremap(0x56000060, 16);//重映射 gpgdat = gpgcon + 1; printk("register buttons_drv "); return 0; } static void sixth_drv_exit(void) { unregister_chrdev(sixthmajor,"buttons"); class_device_unregister(sixth_drv_class_dev); class_destroy(sixth_drv_class); iounmap(gpfcon); iounmap(gpgcon); printk("unregister buttons_drv "); } module_init(sixth_drv_init); module_exit(sixth_drv_exit); MODULE_LICENSE("GPL");
接着改写测试程序，测试加入阻塞方式打开文件，在fd = open(filename, O_RDWR|O_NONBLOCK)函数中加入O_NONBLOCK即可以按阻塞方式打开。
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <stdio.h> #include <poll.h> #include <signal.h> static int fd; //static void fifth_testsignal(int signum) //{ // unsigned char key_val; // // printf("signal = %d ",signum); // read(fd, &key_val, 1); // printf("signumkey_val: 0x%x ",key_val); //} /* *usage ./buttonstest */ int main(int argc, char **argv) { char* filename="dev/buttons"; int oflags,ret; unsigned char key_val; fd = open(filename, O_RDWR|O_NONBLOCK);//打开dev/firstdrv设备文件,非阻塞方式打开 if (fd < 0)//小于0说明没有成功 { printf("error, can't open %s ", filename); return 0; } if(argc !=1) { printf("Usage : %s ",argv[0]); return 0; } // signal(SIGIO, fifth_testsignal);//注册一个信号，函数为fifth_testsignal // // fcntl(fd, F_SETOWN, getpid()); // 告诉内核，发给谁 // // oflags = fcntl(fd, F_GETFL); //取得当前的状态 // // 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 while(1) { ret = read(fd, &key_val, 1); printf("ret = %d,key_val: 0x%x ",ret,key_val); sleep(5); } return 0; }
将驱动程序与测试程序编译后运行。发现以阻塞方式运行的测试程序如果再次运行会处于睡眠状态；如果以非阻塞方式再次运行程序，会导致第二个程序退出。
以上只是记录了怎么调用内核函数来实现互斥、阻塞机制，具体原理还未分析，后面再分析。