我应该如何使用条件变量解决哲学家就餐问题？

Question

我用condition_variables写了一个模拟哲学家就餐的简单程序，问题是有些哲学家从来没有机会吃饭。

我尝试在 _condVar.Wait lambda 函数中放置断点，以确保 putdownForks 方法通知其他等待线程，我确实这样做了，但我仍然不明白为什么其他线程永远不会吃东西。

#include <condition_variable>
#include <iostream>
#include <thread>
#include <unistd.h>

#define thread_num 5
#define HUNGRY 0
#define EATING 1
#define THINKING 2

using namespace std;

thread _threads[thread_num];
mutex _mutex;
condition_variable _condVar;
int _states[thread_num];
bool _isCancelRequested;

void think(int id){
    usleep(rand() % 1000000 + 2000000);
}
void eat(int id){
    usleep(rand() % 1000000 + 2000000);
}

void putdownForks(int id){   
    {
        unique_lock<mutex> lck(_mutex);          
        _states[id] = THINKING;
    }
    _condVar.notify_all();    
}

void pickupForks(int id){
    int leftNeighbourId = (id -1 )%thread_num;
    if(leftNeighbourId<0)leftNeighbourId+=thread_num;
    int rightNeighbourId= (id+1)%thread_num;
    {
        unique_lock<mutex> lck(_mutex);
        _states[id] = HUNGRY;
        bool isLeftNeighbourEating = _states[leftNeighbourId] == EATING;
        bool isRightNeighbourEating = _states[rightNeighbourId] == EATING;
        _condVar.wait(lck, 
            [isLeftNeighbourEating, isRightNeighbourEating]
                {
                    return !(isLeftNeighbourEating || isRightNeighbourEating);
                });    
        _states[id] = EATING;
    }  
}

void philosopher(int id){
    while(!_isCancelRequested){
        think(id);
        pickupForks(id);
        eat(id);
        putdownForks(id);
    }
}

void print_info()
{
    {
        unique_lock<mutex> lck(_mutex);
        for(int i=0;i<thread_num;i++)
        {
            cout << "Id: "<< i<< " State: "<< _states[i]<<endl;
        }
        cout << endl;
    }
}

void info(){
    while (!_isCancelRequested)
    {
        usleep(1000000);
        print_info();
    }    
}

int main(){
    for(int i=0; i<thread_num;i++){
        _threads[i]= thread(philosopher,i);
    }
    thread info_thread(info);

    for(int i=0; i<thread_num;i++){
        _threads[i].join();
    }  
    info_thread.join();
}

这是状态随时间变化的输出

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 1
Id: 4 State: 0

Id: 0 State: 1
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

Id: 0 State: 2
Id: 1 State: 0
Id: 2 State: 0
Id: 3 State: 2
Id: 4 State: 0

如您所见，id 为 1、2 和 4 的哲学家从不吃东西。

Answer 1

哇，这次 stackoverflow 效果对我来说很难。 通知后，等待条件需要重新评估，因此每次通知条件变量时我都需要“更新”邻居的状态。 PickupForks 方法应该是这样的：

void pickupForks(int id){
    int leftNeighbourId = (id -1 )%thread_num;
    if(leftNeighbourId<0)leftNeighbourId+=thread_num;
    int rightNeighbourId= (id+1)%thread_num;
    {
        unique_lock<mutex> lck(_mutex);
        _states[id] = HUNGRY;
        _condVar.wait(lck, 
            [leftNeighbourId, rightNeighbourId]
                {
                    bool isLeftNeighbourEating = _states[leftNeighbourId] == EATING;
                    bool isRightNeighbourEating = _states[rightNeighbourId] == EATING;
                    return !(isLeftNeighbourEating || isRightNeighbourEating);
                });    
        _states[id] = EATING;
    }  
}