#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/spinlock.h>
#include <asm/uaccess.h>
#include <linux/seqlock.h>

static int major;
static char *name = "foo";

static unsigned volatile short count[2] = {0, 0};
static unsigned volatile int prev_sum = 0, curr_sum = 0;

seqlock_t my_lock = SEQLOCK_UNLOCKED;

#define N 0xfffffff

int 
foo_open(struct inode* inode, struct file *filp)
{
	return 0;
}
	
ssize_t 
foo_read(struct file *filp, char *buf, 
	size_t n, loff_t *offp)
{
	unsigned int i;
	unsigned long seq;

	for(i = 0; i < N; i++) {
		do {
			seq = read_seqbegin(&my_lock);
			curr_sum = count[1] + (count[0]<<16);
		} while (read_seqretry(&my_lock, seq));
			
		if(curr_sum < prev_sum) {
			printk("error:curr=%x, prev=%x\n", curr_sum, prev_sum);
			break;
		}
		prev_sum = curr_sum;
	}
	return 0;
}

ssize_t 
foo_write(struct file *filp, const char *buf, 
	size_t n, loff_t *offp)
{
	int i;
	for(i = 0; i < N; i++) {
		write_seqlock(&my_lock);
		if (count[1] == 0xffff) {
			count[1] = 0;
			count[0]++;

		} else  count[1]++;
		write_sequnlock(&my_lock);
	}
	
	return n;	
}

struct file_operations fops = {
		.open = foo_open, 
		.read = foo_read, 
		.write = foo_write
};

static int hello_init(void)
{
	printk("loading..\n");
	major = register_chrdev(0,name, &fops);
	printk("got major = %d\n", major);
	if (major < 0) return major;
	return 0;
}

static void hello_exit(void)
{
	unregister_chrdev(major, name);
	printk("Hello cleaning up...\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");