c++ - c++11 中 3 个线程和 2 个共享资源的同步问题

Question

我有 3 个线程和 2 个共享资源，需要一些锁定...我试图用 2 个缓冲区来说明资源...- 线程 1 只能访问资源 1- 线程 2 可以访问资源 1 和 2- 线程 3 可以访问资源 1 和 2

有人能告诉我为什么以下锁定失败吗？因为 thread2 和 thread3 将访问资源 1 和 2...我想我可以使用 try_lock？ ...似乎问题突然出现，当 thread2 和 thread3 一次只能锁定 1 个互斥锁时...有什么想法吗？

#include <iostream> 
#include <thread> 
#include <mutex> 
#include <condition_variable> 
#include <vector> 
#include <algorithm> 
#include <cassert> 
using namespace std; 

class SynchronizationTest { 
private: 
    mutex lock_r1; 
    mutex lock_r2; 
    vector<pair<string, int>> buffer_r1; 
    vector<pair<string, int>> buffer_r2; 
    unsigned int buffer_r1_max_size; 
    unsigned int buffer_r2_max_size; 

    bool buffer_r1_inc_element(const string &thread_id) {       
        if (buffer_r1.size() == buffer_r1_max_size) { 
            return true; 
        } 

        if (buffer_r1.size() == 0) { 
            buffer_r1.push_back(make_pair(thread_id, 0)); 
        } 
        else { 
            int last_val = buffer_r1.back().second; 
            buffer_r1.push_back(make_pair(thread_id, ++last_val)); 
        } 

        return false; 
    } 

    bool buffer_r2_inc_element(const string &thread_id) {   
        if (buffer_r2.size() == buffer_r2_max_size) { 
            return true; 
        } 

        if (buffer_r2.size() == 0) { 
            buffer_r2.push_back(make_pair(thread_id, 0)); 
        } 
        else { 
            int last_val = buffer_r2.back().second; 
            buffer_r2.push_back(make_pair(thread_id, ++last_val)); 
        } 

        return false; 
    } 

public: 
    SynchronizationTest(int buff_r1_size, int buff_r2_size) : buffer_r1_max_size(buff_r1_size), 
                                                              buffer_r2_max_size(buff_r2_size) {} 

    void thread1() { 
        bool buffer_r1_full = false; 

        while (!buffer_r1_full) { 
            { 
                unique_lock<mutex> l(lock_r1, std::defer_lock); 
                if (l.try_lock()) { 
                    buffer_r1_full = buffer_r1_inc_element("thread1"); 
                } 
            } 

            std::this_thread::sleep_for(std::chrono::milliseconds(10)); 
        } 
    } 

    void thread2() { 
        bool buffer_r1_full = false; 
        bool buffer_r2_full = false; 

        while (!buffer_r1_full || !buffer_r2_full) { 
            { 
                unique_lock<mutex> lock1(lock_r1, defer_lock); 
                unique_lock<mutex> lock2(lock_r2, defer_lock); 

                int result = try_lock(lock1, lock2); 
                if(result == -1) { 
                    buffer_r1_full = buffer_r1_inc_element("thread2"); 
                    buffer_r2_full = buffer_r2_inc_element("thread2"); 
                } 
                else if(result != 0) { 
                    buffer_r1_full = buffer_r1_inc_element("thread2"); 
                } 
                else if(result != 1) { 
                    buffer_r2_full = buffer_r2_inc_element("thread2"); 
                } 
            } 

            std::this_thread::sleep_for(std::chrono::milliseconds(10)); 
        } 
    } 

    void thread3() { 
        bool buffer_r1_full = false; 
        bool buffer_r2_full = false; 

        while (!buffer_r1_full || !buffer_r2_full) { 
            { 
                unique_lock<mutex> lock1(lock_r1, defer_lock); 
                unique_lock<mutex> lock2(lock_r2, defer_lock); 

                int result = try_lock(lock1, lock2); 
                if(result == -1) { 
                    buffer_r1_full = buffer_r1_inc_element("thread3"); 
                    buffer_r2_full = buffer_r2_inc_element("thread3"); 
                } 
                else if(result != 0) { 
                    buffer_r1_full = buffer_r1_inc_element("thread3"); 
                } 
                else if(result != 1) { 
                    buffer_r2_full = buffer_r2_inc_element("thread3"); 
                } 
            } 

            std::this_thread::sleep_for(std::chrono::milliseconds(10)); 
        } 
    } 

    void print_buffer() { 
        for_each(buffer_r1.begin(), buffer_r1.end(), [](pair<string, int> p) { cout << p.first.c_str() << " " << p.second << endl; }); 
        cout << '\n'; 
        for_each(buffer_r2.begin(), buffer_r2.end(), [](pair<string, int> p) { cout << p.first.c_str() << " " << p.second << endl; }); 
    } 
}; 

int main() { 
    // your code goes here 
    SynchronizationTest st(20, 20); 

    thread t1(&SynchronizationTest::thread1, &st); 
    thread t2(&SynchronizationTest::thread2, &st); 
    thread t3(&SynchronizationTest::thread3, &st); 

    t1.join(); 
    t2.join(); 
    t3.join(); 

    st.print_buffer(); 

    return 0; 
}

Answer 1

std::try_lock那样不行。如果它返回 -1，则所有锁都被持有。如果它返回一个非负整数，则没有持有锁。返回值表明哪个锁失败了，但是任何成功锁定的锁都会在 try_lock 返回之前释放。