自旋鎖（擷取自旋鎖->臨界區->釋放自旋鎖）
自旋鎖的名稱源于它的工作原理：嘗試擷取一個自旋鎖，如果鎖空閑就擷取該自旋鎖并繼續向下執行；如果鎖已被占用就循環檢測該鎖是否被釋放（原地打轉直到鎖被釋放）
隻有在占用鎖的時間極短的情況下使用；不能遞歸使用一個自旋鎖（形成死鎖）；占用鎖時不能使用可能引起程序排程的函數，如copy_xx_user()、kmalloc()、msleep()…
自旋鎖主要針對SMP或單CPU搶占核心的情況，而對于單CPU非搶占核心自旋鎖退化為空操作
自旋鎖不能阻止中斷對臨界區的通路，但可以使用帶關中斷功能的變體解決這一問題
自旋鎖的使用流程：
自旋鎖接口函數：
#include <linux/spinlock.h> //自旋鎖頭檔案
定義自旋鎖變量
struct spinlock my_spinlock; 或
spinlock_t my_spinlock;
spin_lock_init(&my_spinlock); //自旋鎖初始化
獲得自旋鎖（可自旋等待，可被軟、硬體中斷）
void spin_lock(spinlock_t *my_spinlock);
獲得自旋鎖(可自旋等待，儲存中斷狀态并 關閉軟、硬體中斷)
void spin_lock_irqsave(spinlock_t *my_spinlock,unsigned long flags);
釋放自旋鎖，退出臨界區
void spin_unlock(spinlock_t *lock)
void spin_unlock_irqrestore(spinlock_t *lock,unsigned long flags)
嘗試獲得自旋鎖（不自旋等待，成功傳回1、失敗則傳回0）
int spin_trylock(spinlock_t *lock)
總結：
執行個體代碼：
驅動端：
#include <linux/device.h>
#include <linux/interrupt.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/irq.h>
#include <asm/uaccess.h>
#include <asm/irq.h>
#include <asm/io.h>
#include <mach/gpio.h>
#include <linux/delay.h>
#include <mach/regs-gpio.h> /*S5PV210_GPH3_BASE*/
#define EINT_DEVICE_ID 1
#define DRIVER_NAME "key_eint_race"
#define err(msg) printk(KERN_ERR "%s: " msg "\n", DRIVER_NAME)
#define __debug(fmt, arg...) printk(KERN_DEBUG fmt, ##arg)
#define GPH3CON (unsigned long)(S5PV210_GPH3_BASE+ 0x00)
#define GPH3DAT (unsigned long)(S5PV210_GPH3_BASE + 0x04)
#define GPH2UP (unsigned long)(S5PV210_GPH2_BASE + 0x08)
static int major = 0; /* Driver Major Number */
static int minor = 0; /* Driver Minor Number */
struct class *key_class;
static struct device *key_device;
static unsigned int key;
static spinlock_t my_spin_lock; //
static unsigned int deal_key_value(unsigned int data)
{
key = data;
udelay(1000);
return key;
}
static unsigned int deal_key_value_excl(unsigned int data)
{
unsigned int value;
unsigned long flag;
/*
* 自旋鎖解決了SMP多處理的并發問題，對于核心的搶占也可以起到關閉的作用，
* 可仍沒有解決中斷産生的并發問題，但是不用擔心，我們可以采用spin_lock
* 的變體spin_lock_irqsave函數完成同時關中斷的功能
*
* 注意：此處用自旋鎖并不合理，因為我們的臨界區udelay了1ms（時間很長），
* 但我們本例子的目的是驗證自旋鎖的作用，在實際程式設計中要特别注意
*/
spin_lock_irqsave(&my_spin_lock,flag); //擷取自旋鎖
value =deal_key_value(data);
spin_unlock_irqrestore(&my_spin_lock,flag); //釋放自旋鎖
return value;
}
irqreturn_t buttons_interrupt(int irq, void *dev_id)
{
deal_key_value_excl((unsigned int)dev_id);
//__debug("in eint function...\n");
return IRQ_HANDLED;
}
static void key_io_port_init(void)
{
unsigned long reg_val;
reg_val = readl(GPH3CON);
reg_val &= ~((0x0f<<0) | (0x0f<<4));
reg_val |= ((0x01<<0) | (0x01<<4));
writel(reg_val, GPH3CON);
reg_val = readl(GPH3DAT);
reg_val &= ~((0x01<<0) | (0x01<<1));
writel(reg_val, GPH3DAT);
reg_val = readl(GPH2UP);
reg_val &= ~(0x03<<8);
reg_val |= 0x02<<8;
writel(reg_val, GPH2UP);
}
static ssize_t key_read(struct file *filp, char *buf, size_t count, loff_t *f_pos)
{
int key_num;
int cpy_len;
int retval;
key_num = deal_key_value_excl(current->pid);
cpy_len = min(sizeof(key_num), count);
retval = copy_to_user(buf, &key_num, cpy_len);
return (cpy_len - retval);
}
/* Driver Operation structure */
static struct file_operations key_fops = {
.owner = THIS_MODULE,
.read = key_read,
};
static int __init key_eint_init(void)
{
int retval;
key_io_port_init();
spin_lock_init(&my_spin_lock);
//__debug("in key_eint_init\n");
retval = set_irq_type(IRQ_EINT(20),IRQ_TYPE_EDGE_FALLING);
if(retval){
err("IRQ_EINT20 set irq type failed");
goto error;
}
retval = request_irq(IRQ_EINT(20), buttons_interrupt, IRQF_DISABLED,
"KEY1", (void *)EINT_DEVICE_ID);
if(retval){
err("request eint20 failed");
goto error;
}
/* Driver register */
major = register_chrdev(major, DRIVER_NAME, &key_fops);
if(major < 0){
err("register char device fail");
retval = major;
goto error_register;
}
key_class=class_create(THIS_MODULE,DRIVER_NAME);
if(IS_ERR(key_class)){
err("class create failed!");
retval = PTR_ERR(key_class);
goto error_class;
}
key_device=device_create(key_class,NULL, MKDEV(major, minor), NULL,DRIVER_NAME);
if(IS_ERR(key_device)){
err("device create failed!");
retval = PTR_ERR(key_device);
goto error_device;
}
__debug("register myDriver OK! Major = %d\n", major);
return 0;
error_device:
class_destroy(key_class);
error_class:
unregister_chrdev(major, DRIVER_NAME);
error_register:
free_irq(IRQ_EINT(20), (void *)EINT_DEVICE_ID);
error:
return retval;
}
static void __exit key_eint_exit(void)
{
//__debug("in key_eint_exit\n");
free_irq(IRQ_EINT(20), (void *)EINT_DEVICE_ID);
unregister_chrdev(major, DRIVER_NAME);
device_destroy(key_class,MKDEV(major, minor));
class_destroy(key_class);
return;
}
module_init(key_eint_init);
module_exit(key_eint_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Eric");
應用層：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
/*Linux核心是搶占式核心，在一個系統調用未結束前，另一個系統調用也可以進入程序上下文，通路同一緩沖區*/
int main(void)
{
int status;
pid_t pid;
//打開檔案raceStation
int fd_driver;
if((fd_driver = open("/dev/key_eint_race", O_RDWR)) < 0){
printf("file open error\n");
exit(1);
}
//建立子程序
if((pid = fork()) < 0){
perror("fork:");
exit(1);
}
else if(pid == 0){ //判斷如果是子程序
int num;
while(1){
read(fd_driver,&num,sizeof(num));
printf("the num value is <son-%d>: %d\n",getpid(), num);
// usleep(50*1000);
}
close(fd_driver);
}else{ //判斷如果是父程序
int num;
while(1){
read(fd_driver,&num,sizeof(num));
printf("the num value is <father-%d>: %d\n",getpid(), num);
// usleep(50*1000);
}
pid = wait(&status);
close(fd_driver);
}
} ————————————————
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