問題：

1：在多執行緒(多個應用)中，如果同時訪問某一個變數，如果執行緒A剛剛開始準備對變數賦值的時候，賦值在彙編裡會有幾個動作執行，這時候如果B執行緒又進來了，對變數又賦值，這時候執行緒A的那個賦值就會被中斷，先執行執行緒B的賦值。這時候到A執行緒繼續執行那個變數的時候，那個變數值就已經被B執行緒改變了。

原子量賦值，在A執行緒賦值的過程當中，不會被別線程中斷。

#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 class *sixthdrv_class;
static struct class_device*sixthdrv_class_dev;
volatile unsigned long *gpfcon;
volatile unsigned long *gpfdat;
volatile unsigned long *gpgcon;
volatile unsigned long *gpgdat;
static DECLARE_WAIT_QUEUE_HEAD(button_waitq);
/* 中斷事件標誌, 中斷服務程式將它置1，sixth_drv_read將它清0 */
static volatile int ev_press = 0;
static struct fasync_struct *button_async;
struct pin_desc{
unsigned int pin;
unsigned int key_val;
};

/* 鍵值: 按下時, 0x01, 0x02, 0x03, 0x04 */
/* 鍵值: 鬆開時, 0x81, 0x82, 0x83, 0x84 */
static unsigned char key_val;
struct pin_desc pins_desc[4] = {
{S3C2410_GPF0, 0x01},
{S3C2410_GPF2, 0x02},
{S3C2410_GPG3, 0x03},
{S3C2410_GPG11, 0x04},
};
static atomic_t canopen = ATOMIC_INIT(1);     //定義原子變數並初始化為1
/*
  * 確定按鍵值
  */
static 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)
{
/* 鬆開 */
key_val = 0x80 | pindesc->key_val;
}
else
{
/* 按下 */
key_val = pindesc->key_val;
}


    ev_press = 1;                  /* 表示中斷髮生了 */
    wake_up_interruptible(&button_waitq);   /* 喚醒休眠的程序 */

kill_fasync (&button_async, SIGIO, POLL_IN);

return IRQ_RETVAL(IRQ_HANDLED);
}


static int sixth_drv_open(struct inode *inode, struct file *file)

{

      //這個open函式，對應應用層的open函式，如果有多個執行緒（應用）同時open，下面的原子量函式不會被中斷。

       if (!atomic_dec_and_test(&canopen)) 
{
atomic_inc(&canopen);
return -EBUSY;
}

     /* 配置GPF0,2為輸入引腳 */
/* 配置GPG3,11為輸入引腳 */
request_irq(IRQ_EINT0,  buttons_irq, IRQT_BOTHEDGE, "S2", &pins_desc[0]);
request_irq(IRQ_EINT2,  buttons_irq, IRQT_BOTHEDGE, "S3", &pins_desc[1]);
request_irq(IRQ_EINT11, buttons_irq, IRQT_BOTHEDGE, "S4", &pins_desc[2]);
request_irq(IRQ_EINT19, buttons_irq, IRQT_BOTHEDGE, "S5", &pins_desc[3]);


return 0;
}


ssize_t sixth_drv_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
{
if (size != 1)
return -EINVAL;


if (file->f_flags & O_NONBLOCK)
{
if (!ev_press)
return -EAGAIN;
}
else
{
/* 如果沒有按鍵動作, 休眠 */
wait_event_interruptible(button_waitq, ev_press);
}


/* 如果有按鍵動作, 返回鍵值 */
copy_to_user(buf, &key_val, 1);
ev_press = 0;

return 1;
}




int sixth_drv_close(struct inode *inode, struct file *file)
{
atomic_inc(&canopen);
free_irq(IRQ_EINT0, &pins_desc[0]);
free_irq(IRQ_EINT2, &pins_desc[1]);
free_irq(IRQ_EINT11, &pins_desc[2]);
free_irq(IRQ_EINT19, &pins_desc[3]);
return 0;
}


static unsigned sixth_drv_poll(struct file *file, poll_table *wait)
{
unsigned int mask = 0;
poll_wait(file, &button_waitq, wait); // 不會立即休眠


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 sencod_drv_fops = {
    .owner   =  THIS_MODULE,    /* 這是一個巨集，推向編譯模組時自動建立的__this_module變數 */
    .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", &sencod_drv_fops);


sixthdrv_class = class_create(THIS_MODULE, "sixth_drv");


sixthdrv_class_dev = class_device_create(sixthdrv_class, NULL, MKDEV(major, 0), NULL, "buttons"); /* /dev/buttons */


gpfcon = (volatile unsigned long *)ioremap(0x56000050, 16);
gpfdat = gpfcon + 1;


gpgcon = (volatile unsigned long *)ioremap(0x56000060, 16);
gpgdat = gpgcon + 1;


return 0;
}


static void 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");