Linux源码阅读笔记12-RCU案例分析

在之前的文章中我们已经了解了RCU机制的原理和Linux的内核源码，这里我们要根据RCU机制写一个demo来展示他应该如何使用。

RCU机制的原理

RCU（全称为Read-Copy-Update）,它记录所有指向共享数据的指针的使用者，当要修改构想数据时，首先创建一个副本，并在副本中修改，所哟访问线程都离开读临界区后，使用者的指针指向修改后的副本，并且删除旧数据。
他是一种在共享数据结构中实现高效读取和低延迟写入操作的技术。在Linux内核中，RCU是一种基于时间窗口的锁机制，通过充分利用多核处理器和内存系统的特性，在保证并发性的同时提供高性能。

代码示例

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/spinlock.h>
#include <linux/kthread.h>
#include <linux/delay.h>
#include <linux/slab.h>
#include <linux/init.h>struct RCUStruct {int a;struct rcu_head rcu;
};static struct RCUStruct* Global_pointer;static struct task_struct* RCURDThread1, *RCURDThread2, *RCUWTThread;static int RCURDThreadFunc1(void* argc) {struct RCUStruct* pointer = NULL;while(1) {msleep(5);rcu_read_lock();mdelay(10);pointer = rcu_dereference(Global_pointer);if(pointer)printk("%s : read a = %d\n", __func__, pointer->a);rcu_read_unlock();}return 0;
}static int RCURDThreadFunc2(void* argc) {struct RCUStruct* pointer = NULL;while(1) {msleep(5);rcu_read_lock();mdelay(10);pointer = rcu_dereference(Global_pointer);if(pointer)printk("%s : read a = %d\n", __func__, pointer->a);rcu_read_unlock();}return 0;
}static void MyRCUDel(struct rcu_head* rcuh) {struct RCUStruct* p = container_of(rcuh, struct RCUStruct, rcu);printk("%s : a = %d\n", __func__, p->a);kfree(p);
}static int RCUWTThreadFunc(void* argc) {struct RCUStruct* old_pointer;struct RCUStruct* new_pointer;int value = (unsigned long)argc;while(1) {msleep(10);new_pointer = kmalloc(sizeof(struct RCUStruct), GFP_KERNEL);old_pointer = Global_pointer;*new_pointer = *old_pointer;new_pointer->a = value;rcu_assign_pointer(Global_pointer, new_pointer);call_rcu(&old_pointer->rcu, MyRCUDel);printk("%s : write to new %d\n", __func__, value);value++;}return 0;
}static int __init RCUFuncInit(void) {int value = 2;printk("Prompt:Successfully initialized the kernel module.\n");Global_pointer = kzalloc(sizeof(struct RCUStruct), GFP_KERNEL);RCURDThread1 = kthread_run(RCURDThreadFunc1, NULL, "RCURD1");RCURDThread2 = kthread_run(RCURDThreadFunc2, NULL, "RCURD2");RCUWTThread = kthread_run(RCUWTThreadFunc, (void*)(unsigned long)value, "RCUWT");return 0;
}static void __exit RCUFuncExit(void) {printk("Prompt:Successfully uninstalled kernel module!\n");kthread_stop(RCURDThread1);kthread_stop(RCURDThread2);kthread_stop(RCUWTThread);if(Global_pointer)kfree(Global_pointer);
}module_init(RCUFuncInit);
module_exit(RCUFuncExit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("lenn louis");

Makefile

obj-m:=rcu.o	CURRENT_PAHT:=$(shell pwd) 
LINUX_KERNEL:=$(shell uname -r)   LINUX_KERNEL_PATH:=/usr/src/linux-headers-$(LINUX_KERNEL)
all:make -C $(LINUX_KERNEL_PATH) M=$(CURRENT_PAHT) modulesclean:make -C $(LINUX_KERNEL_PATH) M=$(CURRENT_PAHT) cleals