pthreads_cond_broadcast不会唤醒所有正在等待的线程

问题描述

我正在创建一个多线程文件服务器。我创建了一个线程池来处理来自客户端的请求。这是我的线程池代码：

void *worker_call(void *thread_id){
    printf("Initialized thread #%ld \n",(long)thread_id);  

    //pull in initialized global configs
    extern pthread_mutex_t queue_m;
    extern pthread_cond_t worker_c;
    extern steque_t queue; 


    while(1){
        if (pthread_mutex_lock(&queue_m) != 0){
            fprintf(stderr,"An error occured while locking mutex in #%ld \n",(long)thread_id);
        }
            while(steque_isempty(&queue) == 1){
                printf("thread #%ld - Going to sleep...\n",(long) thread_id);
                pthread_cond_wait(&worker_c,&queue_m);
                printf("thread #%ld - I'm waking up...\n",(long) thread_id);
            }
            
            int *work = steque_pop(&queue);
            

        if (pthread_mutex_unlock(&queue_m) != 0){
            fprintf(stderr,"An error occured while unlocking mutex in #%ld \n",(long)thread_id);
        }

        pthread_cond_broadcast(&worker_c);

        sleep(1); //added to make sure that the other threads have a chance to wake up

        printf("thread #%ld - what is the value of work: %d\n",(long) thread_id,*work);
        // process_request(&(work->ctx),work->path,work->arg,(long)thread_id);
        free(work);
    }

    return NULL;
}

要测试我的线程池是否正确启动并且所有线程都在正常工作，我创建了以下测试：

int main(){
    int nthreads = 6;

    pthread_t threads[nthreads];
    long thread_ids[nthreads];

    pthread_attr_t thread_attr;
    pthread_attr_init(&thread_attr);
    pthread_attr_setdetachstate(&thread_attr,PTHREAD_CREATE_JOINABLE);
    pthread_attr_setscope(&thread_attr,PTHREAD_ScopE_SYstem);


    for(int t=0;t<nthreads;t++){
        thread_ids[t] = t;
        if( pthread_create(&threads[t],NULL,worker_call,(void *)thread_ids[t]) != 0 ) {
            printf("An error occured while creating thread: %d\n",t);
        }
    }

    pthread_attr_destroy(&thread_attr);

    //init multi-threading configs
    extern pthread_mutex_t queue_m;

    pthread_mutexattr_t m_attr;
    pthread_mutexattr_init(&m_attr);
    pthread_mutexattr_settype(&m_attr,PTHREAD_MUTEX_ERRORCHECK);

    pthread_mutex_init(&queue_m,NULL);

    extern pthread_cond_t worker_c;
    pthread_cond_init(&worker_c,NULL);

    extern steque_t queue; 

    steque_init(&queue);
    //create a simple queue with each item being an int. 
    //the goal is to simply remove the items from the queue.
    for(int i = 0; i < 5; i++){
        
        int *work = malloc(sizeof(int));
        *work = i;
        steque_enqueue(&queue,work);
    }
    

    printf("queue size after creating it: %d\n",steque_size(&queue));

    pthread_cond_broadcast(&worker_c);

    for(int t=0;t<nthreads;t++){
        pthread_join(threads[t],NULL);
    }

    printf("All the threads finished processing\n");

    return 0;
}

现在此代码已成功运行，但是，只有一个线程被唤醒，它可以完成所有工作，如下面的输出所示：

Initialized thread #0 
thread #0 - Going to sleep...
Initialized thread #1 
thread #1 - Going to sleep...
Initialized thread #2 
thread #2 - Going to sleep...
Initialized thread #3 
thread #3 - Going to sleep...
Initialized thread #4 
thread #4 - Going to sleep...
Initialized thread #5 
thread #5 - Going to sleep...
queue size after creating it: 5
thread #5 - I'm waking up...
thread #5 - what is the value of work: 0
thread #5 - what is the value of work: 1
thread #5 - what is the value of work: 2
thread #5 - what is the value of work: 3
thread #5 - what is the value of work: 4
thread #5 - Going to sleep...

我的问题是，为什么其他线程不醒来并从队列中抓取项目？我已经尝试在发出pthread_cond_broadcast之后添加一秒的延迟，以给其他线程足够的时间来锁定互斥锁，但是该方法没有成功。有人看到我在做什么错吗？

解决方法

我无法发布简单的评论，因为我没有+50代表。因此，我必须将其发布为答案。

当您说“成功运行”时，您对成功的定义是什么？这里有几个比赛条件令人震惊，我很惊讶这一切都奏效。例如，worker_call（）在实际初始化之前访问队列，互斥量和cond变量。

此外，当您最初将项目推入main（）中的队列时，您应该将该代码包装在互斥锁中，因为您的线程也在尝试访问该变量。

我的建议是先解决这些比赛条件，然后再更新结果。

编辑1：

另外，在worker_call中，worker在调用cond_wait之前正在检查队列是否为空，但在唤醒后不检查是否为空。由于项目数量可能少于工作人员，因此该功能将需要再次检查队列是否为空。