pthreads_cond_broadcast 不会唤醒所有正在等待的线程

Question

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;
}

为了测试我的线程池是否正确启动并且所有线程都在工作，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;
}

现在这段代码成功运行，但是，只有一个线程被唤醒，并且它完成了所有工作，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，为什么其他线程没有醒来并从队列中获取项目？我尝试在发出 pthread_cond_broadcast 后添加一秒的延迟，以便为其他线程提供足够的时间来锁定互斥体，但该方法没有成功。有人看到我做错了什么吗？

Answer 1

当你说“成功跑步”时，你对成功的定义是什么？这里有几个令人担忧的竞争条件，我很惊讶这竟然有效。例如，worker_call() 在实际初始化之前访问队列、互斥锁和条件变量。

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

我的建议是修复这些竞争条件，然后更新你的结果。

此外，在worker_call中，worker在调用cond_wait之前检查队列是否为空，但在唤醒后不会检查队列是否为空。由于项目的数量可能少于工作人员，因此该函数将需要再次检查队列是否为空。