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);
}
};
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
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;
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
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》