避免在增加计数器时出现争用条件

Avoid race condition when incrementing a counter

本文关键字：争用条件 计数器 增加      更新时间：2023-10-16

我以前发过关于这个话题的文章，但到目前为止我还没有太多的运气。我把它归结为我的一个糟糕的问题。这次我做了一个简短的可编译示例，显示了我试图避免的不良行为。我希望这得到赞赏。

问题是两个（或多个）线程被设置为运行相同的进程，它们的"id"决定了它们对变量数据的哪一部分进行操作。目前，两个线程都将更新计数器。

电流输出如下所示，

tid = 0, var[tid] = 0
tid = 0, var[tid] = 1
tid = 0, var[tid] = 2
tid = 0, var[tid] = 3
tid = 0, var[tid] = 4
tid = 0, var[tid] = 5
tid = 0, var[tid] = 6
tid = 0, var[tid] = 7
tid = 0, var[tid] = 8
tid = 0, var[tid] = 9
tid = 1, var[tid] = 0
Press any key to continue . . .

所需的输出应该是这样的...

tid = 0, var[tid] = 0
tid = 1, var[tid] = 0
tid = 0, var[tid] = 1
tid = 1, var[tid] = 1
tid = 0, var[tid] = 2
tid = 1, var[tid] = 2
tid = 0, var[tid] = 3
tid = 1, var[tid] = 3 etc.

此处的任何指导将不胜感激。

编辑：我用按预期工作的代码更新了答案。

[注意效率在这里很重要，我想尽快完成这个过程]

#include <iostream>  
#include <boost/thread.hpp>
int var[2];
int mT;
int mTotalSamples;
boost::mutex mCountMutex;
boost::thread *threadMap[2];
using namespace std;
void process()
{
    int tid = 1;
    // sleep for 1 seconds - just to make sure threadMap 
    // has been assigned (only ncessary for this demo).
    boost::this_thread::sleep(boost::posix_time::seconds(1));
    if (threadMap[0]->get_id() == boost::this_thread::get_id()){ tid = 0;}
    while ( mT < mTotalSamples ) 
    {
        // perform processing
        var[tid] = mT; 
        // processing complete
        mCountMutex.lock(); // (a thread waits to aquire mutex)
        cout << "tid = " << tid << ", var[tid] = " << var[tid] << endl;
        mT++;           // How to stop both threads incrementing this?      
        mCountMutex.unlock();       
    }   
}
int main()
{
    boost::thread_group threads;
    mT = 0;
    mTotalSamples = 10;
    threadMap[0] = threads.create_thread( boost::bind(&process) );
    threadMap[1] = threads.create_thread( boost::bind(&process) );
    threads.join_all();
    return 0;
}

从预期的输出来看，您希望线程在每次更新后同步。boost库提供了 boost：：barrier，如果你在 process 中 while-循环的开头或结尾为它放一个wait，应该可以解决问题。

#include <iostream>  
#include <boost/thread.hpp>
int var[2];
int mT;
int mTotalSamples;
boost::mutex mCountMutex;
boost::thread *threadMap[2];
boost::barrier bar(2);
using namespace std;
void process()
{
    int tid = 1;        
    // sleep for 2 seconds - just to make sure threadMap 
    // has been assigned (only ncessary for this demo).
    boost::this_thread::sleep(boost::posix_time::seconds(2));
    if (threadMap[0]->get_id() == boost::this_thread::get_id()){ tid = 0;}
    while ( mT < mTotalSamples ) 
    {
        // perform processing
        var[tid] = mT; 
        // processing complete

        bar.wait();
        if (threadMap[0]->get_id() == boost::this_thread::get_id())
        {
            mT++;               
            cout << "var[0] = " << var[0] << endl;
            cout << "var[1] = " << var[1] << endl;                      
        }           
        bar.wait();
    }   
}
int main()
{
    boost::thread_group threads;
    mT = 0;
    mTotalSamples = 10;
    threadMap[0] = threads.create_thread( boost::bind(&process) );
    threadMap[1] = threads.create_thread( boost::bind(&process) );
    threads.join_all();
    return 0;
}

在 process（） 中将此int mT;设为本地 - 而不是全局。或者你需要一个int mT[2];那么你就不需要互斥锁了。

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