linux驱动程序中的并发控制-3（读写自旋锁）-45

Posted 2021-08-11 杨斌并

读写自旋锁

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• 自旋锁不管读写，都只允许同时只有一个执行单元可以获取自旋锁，即便有多个单元同时读取临界区资源也会被锁住。
• 读写自旋锁，将临界区的读写操作分开，多个执行单元可以同时获取一个读自旋锁，但是只能获取一个写自旋锁
• 如果某个执行单元已经获取了一个读自旋锁，那么在获取写自旋锁时就是自旋等待读自旋锁的释放，读写不能同时进行

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读写自旋锁的使用

1. 定义读写自旋锁

rwlock_t lock;

• rwlock_t 结构体 在头文件 #include<linux/rwlock.h>中

typedef struct {
	arch_rwlock_t raw_lock;
#ifdef CONFIG_GENERIC_LOCKBREAK
	unsigned int break_lock;
#endif
#ifdef CONFIG_DEBUG_SPINLOCK
	unsigned int magic, owner_cpu;
	void *owner;
#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
	struct lockdep_map dep_map;
#endif
} rwlock_t;

2. 初始化读写自旋锁变量

• 通过rwlock_init 宏初始化读写自旋锁变量

rwlock_init(&lock);

3. 获取读写自旋

• 获取读自旋锁

read_lock(&lock);

• 获取写自旋锁

write_lock(&lock)

如果使用read lock 和write. lock 宏成功获取读写自旋锁，read_lock 和write_lock宏会立即返回。如果未获取读写自旋锁，read_lock 和write_lock 宏会被阻塞(在那自旋)，直到可以获取读写自旋锁才返回。如果想不管是否成功获取读写自旋锁都立刻返回，可以使用read_ trylock 和write_ trylock 宏。

4. 释放读写自旋锁

• 释放读自旋锁

read_unlock(&lock);

• 释放写自旋锁

write_unlock(&lock);

5. 读写相关的api

函数	描述
DEFINE RWLOCK(lock)	定义和初始化读写自旋锁变量
void rwlock_init(rwlock_t *lock)	初始化读写自旋锁变量
void read_lock(rwlock_t *lock)	获取读自旋锁。如果无法获取读自旋锁，则不断自旋(循环)来检测读自旋锁是否可用
int read_trylock(rwlock_t *lock)	获取读自旋锁。如果成功获取读自旋锁，则立刻返回非0 值，如果无法获取读自旋锁，则立刻返回0 (并不进行自旋，也就是说该宏不会被阻塞)
void read_unlock(rwlock_t *lock)	释放读自旋锁
void write_lock(rwlock_t *lock)	获取写自旋锁。如果无法获写读自旋锁，则不断自旋(循环)来检测写自旋锁是否可用
int write_trylock(rwlock_t *lock)	获取写自旋锁。如果成功获取写自旋锁，则立刻返回非0 值，如果无法获取写自旋锁，则立刻返回0 (并不进行自旋，也就是说该宏不会被阻塞)
void write_unlock(rwlock_t *lock)	释放写自旋锁
void read_lock_irq(rwlock_t *lock)	获取读自旋锁，并禁止中断。相当于read. lock+ local irq disable
void read_unlock_irq(rwlock_t *lock)	释放读自旋锁，并允许中断。相当于read. _unlock + local_irq_enable
void writc_lock_irq(rwlock_t *lock)	获取写自旋锁，并禁止中断。相当于write_ lock + local_ irq disable
void write_unlock_irq(rwlock_t *lock)	释放写自旋锁，并允许中断。相当于write_ unlock + local irq_ enable
void read_lock_bh(rwlock_t *lock)	释放读自旋锁，并禁止底半部。相当于read_lock + local_bh_disable
void read_unlock_bh(rwlock_t *lock)	释放读自旋锁，并允许底半部。相当于read_ lock + local bh_ enable
void write_lock_bh(rwlock_t *lock)	释放写自旋锁，并禁止底半部。相当于write_lock + local_bh_disable
void write_ _unlock _bh(rwlock t *lock)	释放写自旋锁，并允许底半部。相当于write_lock + local_bh_enable

实例代码

• read_write_lock.c

#include<linux/module.h>
#include<linux/init.h>
#include<linux/kernel.h>
#include<linux/uaccess.h>
#include<linux/fs.h>
#include<linux/miscdevice.h>
#include<linux/rwlock.h>
#include<linux/spinlock.h>
#include<linux/delay.h>

#define DEVICE_NAME "rw_lock"

static  char  *data = "read\\n";
//定义读写锁
static  DEFINE_RWLOCK(rwlock);
//定义自旋锁
static  DEFINE_SPINLOCK(lock);
//rw  代表 读写锁  spin 代表
static char *lock_type = "rw";


static int spin_lock_open(struct inode *node,struct file *file){

    printk("spin_lock_open \\n");


    return 0;

}

static int spin_lock_release(struct inode *node,struct file *file){
    printk("spin_lock_release \\n");

    return 0;
}


static int spin_lock_read(struct file * file, char __user * buf, size_t size, loff_t * ppos){

    int data_size = strlen(data);
    struct timeval tv;

    if (copy_to_user(buf, (void *)data, data_size))
    {
        return -EINVAL;
    }
    //获取单前的毫秒值
    do_gettimeofday(&tv);
    printk("read:start sec: %ld\\n", tv.tv_sec);

    if (strcmp(lock_type, "rw")==0)
    {
        read_lock(&rwlock);
    }else if (strcmp(lock_type, "spin") == 0)
    {
        spin_lock(&lock);
    }

    do_gettimeofday(&tv);
    printk("read: end sec: %ld\\n", tv.tv_sec);

    //模拟写临界代码模拟代码运行过程，所以延迟5秒
    mdelay(10000);

    if (strcmp(lock_type, "rw")==0)
    {
        read_unlock(&rwlock);
        
    }else if (strcmp(lock_type, "spin")==0)
    {
        spin_unlock(&lock);
    }
    return data_size;
}

static int spin_lock_write(struct file *file, const char __user *buf, size_t size, loff_t * ppos){
    struct timeval tv;
    do_gettimeofday(&tv);
    printk("write: start sec:%d \\n", tv.tv_sec);

    if (strcmp(lock_type, "rw") == 0)
    {
        write_lock(&rwlock);
    }else if (strcmp(lock_type, "spin")==0)
    {
        spin_lock(&lock);
    }

    do_gettimeofday(&tv);
    printk("write: end sec: %ld \\n", tv.tv_sec);

    //模拟写临界代码模拟代码运行过程，所以延迟5秒
    mdelay(10000);
    if (strcmp(lock_type, "rw") == 0)
    {
        write_unlock(&rwlock);
    }else if (strcmp(lock_type, "spin") == 0)
    {
        spin_unlock(&lock);
    }

    return size;
}

static struct file_operations dev_fops={
    .owner = THIS_MODULE,
    .open = spin_lock_open,
    .release = spin_lock_release,
    .read = spin_lock_read,
    .write = spin_lock_write
};

static struct miscdevice misc={
    .minor=MISC_DYNAMIC_MINOR,
    .name=DEVICE_NAME,
    .fops=&dev_fops
};


static int demo_init(void){

    int ret=misc_register(&misc);
    if(ret < 0 ){
        printk("atomic_init is error\\n");
        return -1;
    }
    printk("demo_init_success\\n");
    return ret;
}

static void demo_exit(void){
    printk("ademo_exit_success\\n");
    misc_deregister(&misc);
}
module_init(demo_init);
module_exit(demo_exit);
module_param(lock_type,charp ,S_IRUGO|S_IWUSR);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("binbing.Yang");

• app.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

int main(int argc, char *argv[]){
    int handler=open("/dev/rw_lock",0);
    char buf[5];
    printf("handler : %d\\n",handler);
    if (handler > 0)
    {

        read(handler,buf,4);
        buf[4] = '\\0';
        printf("%s \\n", buf);
        
    }

    return 0;
}

• build.sh 测试脚本
• app 可执行文件在/data/local/tmp/ 目录

#!/system/bin/sh
cd /data/local/tmp/
pwd
./app &
./app &
./app &
echo abdc > /dev/rw_lock &
echo abdc > /dev/rw_lock &
echo abdc > /dev/rw_lock &

• 多个可执行单元可以同时执行读的操作
• 写的操作，必须一个执行完，释放掉锁在执行下一个获取到锁的执行单元