c++ - 读者作家 - 作家线程总是卡在多个读者线程中

Question

新来的。我一直在研究读者/作者问题的解决方案。它适用于 1 位读者和 1 位作者。但是当我将 reader 修改为 2 时；作家线程总是饿死。帮帮我!

似乎 Writer 线程卡在某处等待 wrt mutex。

#include <stdio.h>
#include <conio.h>
#include <windows.h>

HANDLE mutex, wrt;
int g_ReadCount = 0;
int g_GlobalData=0;

const int max = 2;
HANDLE reader[max], writer[max];
CRITICAL_SECTION rSect, wSect;
bool bTerminate = true;

DWORD Readers(LPVOID lpdwThreadParam ) 
{
  while(bTerminate)
  {
    WaitForSingleObject(mutex, INFINITE);
    g_ReadCount++;
    if(g_ReadCount == 1)
    {      
      WaitForSingleObject(wrt, INFINITE);
    }
    ReleaseMutex(mutex);

    EnterCriticalSection(&wSect);
    printf("ThreadId : %d --> Read data : %d  ReaderCount %d\n", GetCurrentThreadId(), g_GlobalData, g_ReadCount);
    LeaveCriticalSection(&wSect);

    WaitForSingleObject(mutex, INFINITE);
    g_ReadCount--;
    if(g_ReadCount == 0)
    {
        ReleaseMutex(wrt);
        printf("ThreadId : %d Realesed Mutex wrt\n", GetCurrentThreadId());
    }
    printf("ThreadId : %d ReaderCount %d\n", GetCurrentThreadId(), g_ReadCount);
    ReleaseMutex(mutex);
    printf("Reader ThreadId : %d Realesed Mutex mutex\n", g_ReadCount);
    Sleep(0);
   }
  return 0;
}


DWORD Writers(LPVOID lpdwThreadParam ) 
{
  int n = GetCurrentThreadId();
  int temp = 1;
  while(bTerminate)
  {  
    printf("ThreadId : %d Waiting for WRT\n", GetCurrentThreadId());
    WaitForSingleObject(wrt, INFINITE);
    printf("WRITER ThreadId : %d ***Got  WRT\n", GetCurrentThreadId());
    ++n;
    temp++;

    if(temp == 100)
    {
      //bTerminate = false;
    }

    EnterCriticalSection(&wSect);
    printf("Write by ThreadId : %d Data : %d   Temp %d\n", GetCurrentThreadId(), n, temp);
    g_GlobalData = n;
    LeaveCriticalSection(&wSect);
    ReleaseMutex(wrt);
  }

  printf("***VVV***Exiting Writer Thread\n");
  return 0;
}

void main()
{
  mutex = CreateMutex(NULL, false, "Writer");
  wrt = CreateMutex(NULL, false, "wrt");

  InitializeCriticalSection(&rSect);
  InitializeCriticalSection(&wSect);
  DWORD dwThreadId = 0;



  for(int i=0; i < max; i++)
  {
    reader[i] = CreateThread(NULL, //Choose default security
      0, //Default stack size
      (LPTHREAD_START_ROUTINE)&Readers,
      //Routine to execute
      (LPVOID) 0, //Thread parameter
      0, //Immediately run the thread
      &dwThreadId //Thread Id
      );
  }

  for(int i=0; i < 1; i++)
  {
    writer[i] = CreateThread(NULL, //Choose default security
      0, //Default stack size
      (LPTHREAD_START_ROUTINE)&Writers,
      //Routine to execute
      (LPVOID) 0, //Thread parameter
      0, //Immediately run the thread
      &dwThreadId //Thread Id
      );
  }


  getchar();


}

Answer 1

如果读取器线程超过 1 个，g_ReadCount 很可能永远不会变为零，因此永远不会释放 wrt 互斥量(从而使写入器挨饿) .您可能需要某种指示器来指示编写器线程正在等待。然后，读者线程需要在某个时刻优先于作者。

例如，在我编写的一个实现中(并不是说这是一种好方法，但它确实有效)我使用了一个标志，该标志通过原子递增/递减操作设置/清除，指示编写器线程是否正在等待锁.如果是这样，读者会推迟。当然，在那种情况下，您还需要注意相反的情况，即编写线程(如果有多个线程)可能会使读者饿死。读/写锁很棘手。