Question

我有一个简单的线程程序，它使用条件变量和rwlock。我一直在盯着它看几个小时尝试不同的方法。问题是一段时间后，一个或多个线程在rwlock处停止，尽管它没有被锁定以进行写入。也许我会想念这些锁是如何工作的或它们是如何实现的。

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#include <unistd.h>

//global variables
pthread_mutex_t mutex;
pthread_cond_t cond;
pthread_rwlock_t rwlock;
int counter;
int listLength = 1;

void* worker(void* arg){
   do {
      usleep(200);
      printf("Before rwlock\n");
      pthread_rwlock_rdlock(&rwlock);
      printf("Before mutex\n");
      pthread_mutex_lock(&mutex);
      printf("Afer mutex\n");
      counter++;
      //signal the main
      if (counter == 5 ||
               (listLength < 5 && counter == listLength)){
          printf("Signal main\n");
          pthread_cond_signal(&cond);
          counter = 0;
      }
      pthread_mutex_unlock(&mutex);
      pthread_rwlock_unlock(&rwlock);
   } while(listLength != 0);

   return NULL;
}


int main(int argc, char* argv[]){
    if (argc != 2){
        perror("Invalid number of args");
        exit(1);
    }
    //get arguments
    int workers = atoi(argv[1]);

    //initialize sync vars
    pthread_rwlockattr_t attr;
    pthread_rwlockattr_setkind_np(&attr,
            PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init(&cond, NULL);
    pthread_rwlock_init(&rwlock, &attr);
    counter = 0;

    //create threads
    pthread_t threadArray[workers];
    int threadOrder[workers];
    for (int i = 0; i < workers; i++){
        threadOrder[i] = i;
        if (pthread_create(&threadArray[i], NULL,
                    worker, &threadOrder[i]) != 0){
            perror("Cannot create thread");
            exit(1);
        }
    }

    while(listLength != 0) {
        //wait for signal and lock the list
        pthread_mutex_lock(&mutex);
        while (pthread_cond_wait(&cond, &mutex) != 0);
        pthread_rwlock_wrlock(&rwlock);
        printf("In write lock\n");

        pthread_mutex_unlock(&mutex);
        pthread_rwlock_unlock(&rwlock);
        printf("release wrlock\n");
    }

    //join the threads
    for (int i = 0; i < workers; i++){
        if (pthread_join(threadArray[i], NULL) !=0){
            perror("Cannot join thread");
           exit(1);
        }
    }

    //release resources
    pthread_mutex_destroy(&mutex);
    pthread_cond_destroy(&cond);
    pthread_rwlock_destroy(&rwlock);

    return 0;
}

Answer 1

看起来这段代码中有几处不一致。

您将mutex与rwlock一起使用，这意味着此类所有线程始终处于锁定状态。如果您删除 rwlock代码 - 它不会改变行为。
我看不到pthread_rwlock_init()来电，并假设您已在其他地方拨打电话。无论如何要注意你做调用它而你不要用相同的行锁对象调用它两次或更多次。

同样适用于pthread_rwlockattr_destroy()
我看不出pthread_rwlock_rdlock（）在没有写锁定的情况下会阻塞的原因。一定要不要这样做。或者你可以互相锁定mutex

条件变量和rwlock死锁

1 个答案: