无锁编程:c++11基于atomic实现共享读写锁(写优先)

在多线程状态下，对一个对象的读写需要加锁，基于CAS指令的原子语句可以实现高效的线程间协调。关于CAS的概念参见下面的文章：

无锁编程以及CAS

在c++11中CAS指令已经被封装成了 非常方便使用的atomic模板类, 详情参见:

atomic参考

以下代码利用atomic实现了一个读写资源锁,并且可以根据需要通过构造函数参数设置成写优先(write_first)(代码在gcc5和vs2015下编译通过):

readLock/Unlock 实现共享的读取加/解锁，线程数不限，有读取线程工作时，所有的申请写入线程都会等待
writeLock/Unlock 实现独占的写入加/解锁,同时只允许一个线程写入，当有线程在读取时，写入线程等待，当写入线程执行时，所有的读取线程都被等待。

locck/unlock语句允许嵌套
比如

lock.readLock();lock.readLock();...lock.readUnlock();lock.readUnlock();

也允许在写入状态下嵌套读取,比如

lock.writeLock();lock.writeLock();lock.readLock();...lock.readUnlock();lock.writeUnlock();lock.writeUnlock();

RWLock.h

#include <cstdlib>#include <cassert>#include <atomic>#include <thread>#include "raii.h"/* * atomic实现读写资源锁,独占写,共享读,禁止复制构造函数和'='赋值操作符 * WRITE_FIRST为true时为写优先模式,如果有线程等待读取(m_writeWaitCount>0)则等待,优先让写线程先获取锁 * 允许嵌套加锁 * readLock/Unlock 实现共享的读取加/解锁，线程数不限 * writeLock/Unlock 实现独占的写入加/解锁,同时只允许一个线程写入， * 当有线程在读取时，写入线程阻塞，当写入线程执行时，所有的读取线程都被阻塞。 */class RWLock {#define WRITE_LOCK_STATUS -1#define FREE_STATUS 0private:    /* 初始为0的线程id */    static const  std::thread::id NULL_THEAD;    const bool WRITE_FIRST;    /* 用于判断当前是否是写线程 */    thread::id m_write_thread_id;    /* 资源锁计数器,类型为int的原子成员变量,-1为写状态，0为自由状态,>0为共享读取状态 */    atomic_int m_lockCount;    /* 等待写线程计数器,类型为unsigned int的原子成员变量*/    atomic_uint m_writeWaitCount;public:    // 禁止复制构造函数    RWLock(const RWLock&) = delete;    // 禁止对象赋值操作符    RWLock& operator=(const RWLock&) = delete;    //RWLock& operator=(const RWLock&) volatile = delete;    RWLock(bool writeFirst=false);;//默认为读优先模式    virtual ~RWLock()=default;    int readLock();    int readUnlock();    int writeLock();    int writeUnlock();    // 将读取锁的申请和释放动作封装为raii对象，自动完成加锁和解锁管理    raii read_guard()const noexcept{        return make_raii(*this,&RWLock::readUnlock,&RWLock::readLock);    }    // 将写入锁的申请和释放动作封装为raii对象，自动完成加锁和解锁管理    raii write_guard()noexcept{        return make_raii(*this,&RWLock::writeUnlock,&RWLock::writeLock);    }};

RWLock.cpp

RWLock::RWLock(bool writeFirst):    WRITE_FIRST(writeFirst),    m_write_thread_id(),    m_lockCount(0),    m_writeWaitCount(0){}int RWLock::readLock() {    // ==时为独占写状态,不需要加锁    if (this_thread::get_id() != this->m_write_thread_id) {        int count;        if (WRITE_FIRST)//写优先模式下,要检测等待写的线程数为0(m_writeWaitCount==0)            do {                while ((count = m_lockCount) == WRITE_LOCK_STATUS || m_writeWaitCount > 0);//写锁定时等待            } while (!m_lockCount.compare_exchange_weak(count, count + 1));        else            do {                while ((count = m_lockCount) == WRITE_LOCK_STATUS); //写锁定时等待            } while (!m_lockCount.compare_exchange_weak(count, count + 1));    }    return m_lockCount;}int RWLock::readUnlock() {    // ==时为独占写状态,不需要加锁    if (this_thread::get_id() != this->m_write_thread_id)            --m_lockCount;    return m_lockCount;}int RWLock::writeLock(){    // ==时为独占写状态,避免重复加锁    if (this_thread::get_id() != this->m_write_thread_id){        ++m_writeWaitCount;//写等待计数器加1        // 没有线程读取时(加锁计数器为0)，置为-1加写入锁，否则等待        for(int zero=FREE_STATUS;!this->m_lockCount.compare_exchange_weak(zero,WRITE_LOCK_STATUS);zero=FREE_STATUS);        --m_writeWaitCount;//获取锁后,计数器减1        m_write_thread_id=this_thread::get_id();    }    return m_lockCount;}int RWLock::writeUnlock(){    if(this_thread::get_id() != this->m_write_thread_id){        throw runtime_error("writeLock/Unlock mismatch");    }    assert(WRITE_LOCK_STATUS==m_lockCount);    m_write_thread_id=NULL_THEAD;    m_lockCount.store(FREE_STATUS);    return m_lockCount;}const std::thread::id RWLock::NULL_THEAD;

说明1

atomic_int,atomic_uint都是从atomic类模板中派生出来的类,对应不同的数据类型

atomic是c++11标准,在gcc编译的时候必须加入std=c++11选项才能正确编译,,vs编译至少要用vs2012,因为visual studio 2012以上才支持atomic模板

说明2

如果按照默认的类定义方法，提供复制构造函数和赋值操作符=,那么可以想见，在应用中可能会产生不可预知的问题，所以参照atomic模板的写法，加入了禁止复制构造函数和对象复制操作符=的代码,

    //禁止复制构造函数    RWLock(const RWLock&) = delete;    //禁止对象赋值操作符    RWLock& operator=(const RWLock&) = delete;    RWLock& operator=(const RWLock&) volatile = delete;

说明3

这个代码还有欠缺的地方就是没有实现超时异常中止。

说明4

read_guard,write_guard函数返回的raii类参见我的另一篇博客《C++11实现模板化(通用化)RAII机制》