工作线程挂起/恢复实现

Question

在尝试向我的 Worker [线程] class 添加暂停/恢复功能时，我遇到了一个我无法解释的问题。 (C++1y / VS2015)

这个问题看起来像是一个死锁，但是一旦附加了调试器并且在某个点之前设置了断点（请参阅#1），我似乎无法重现它 - 所以它看起来像是一个时间问题。

我能找到的修复程序 (#2) 对我来说意义不大，因为它需要更长时间地保留互斥锁，并且客户端代码可能会尝试获取其他互斥锁，我理解这一点实际上增加了死锁的机会。

但它确实解决了这个问题。

Worker 循环：

Job* job;
while (true)
{
  {
    std::unique_lock<std::mutex>  lock(m_jobsMutex);
    m_workSemaphore.Wait(lock);

    if (m_jobs.empty() && m_finishing)
    {
      break;
    }

    // Take the next job
    ASSERT(!m_jobs.empty());
    job = m_jobs.front();
    m_jobs.pop_front();
  }

  bool done = false;
  bool wasSuspended = false;
  do
  {
    // #2
    { // Removing this extra scoping seemingly fixes the issue BUT
      // incurs us holding on to m_suspendMutex while the job is Process()ing,
      // which might 1, be lengthy, 2, acquire other locks.
      std::unique_lock<std::mutex> lock(m_suspendMutex);
      if (m_isSuspended && !wasSuspended)
      {
        job->Suspend();
      }
      wasSuspended = m_isSuspended;

      m_suspendCv.wait(lock, [this] {
        return !m_isSuspended;
      });

      if (wasSuspended && !m_isSuspended)
      {
        job->Resume();
      }
      wasSuspended = m_isSuspended;
    }

    done = job->Process();
  }
  while (!done);
}

暂停/恢复只是：

void Worker::Suspend()
{
  std::unique_lock<std::mutex>  lock(m_suspendMutex);
  ASSERT(!m_isSuspended);
  m_isSuspended = true;
}

void Worker::Resume()
{
  {
    std::unique_lock<std::mutex>  lock(m_suspendMutex);
    ASSERT(m_isSuspended);
    m_isSuspended = false;
  }
  m_suspendCv.notify_one(); // notify_all() doesn't work either.
}

(Visual Studio) 测试：

  struct Job: Worker::Job
  {
    int durationMs = 25;
    int chunks = 40;
    int executed = 0;

    bool Process()
    {
      auto now = std::chrono::system_clock::now();
      auto until = now + std::chrono::milliseconds(durationMs);
      while (std::chrono::system_clock::now() < until)
      { /* busy, busy */
      }

      ++executed;
      return executed < chunks;
    }

    void Suspend() { /* nothing here */ }
    void Resume() { /* nothing here */ }
  };

  auto worker = std::make_unique<Worker>();

  Job j;
  worker->Enqueue(j);

  std::this_thread::sleep_for(std::chrono::milliseconds(j.durationMs));  // Wait at least one chunk.

  worker->Suspend();

  Assert::IsTrue(j.executed < j.chunks);  // We've suspended before we finished.
  const int testExec = j.executed;

  std::this_thread::sleep_for(std::chrono::milliseconds(j.durationMs * 4));

  Assert::IsTrue(j.executed == testExec); // We haven't moved on.

  // #1
  worker->Resume(); // Breaking before this call means that I won't see the issue.
  worker->Finalize();

  Assert::IsTrue(j.executed == j.chunks); // Now we've finished.

我错过了什么/做错了什么？为什么作业的 Process()ing 必须由 suspend 互斥锁保护？

编辑：Resume()在通知时不应该持有互斥体；已解决 -- 问题仍然存在。

Answer 1

当然，作业的 Process()ing 不必由 suspend 互斥锁保护。

j.executed 的访问 - 对于断言和递增 - 但是确实需要同步（通过使其成为 std::atomic<int> 或通过互斥锁等来保护它。 ).

目前还不清楚为什么问题会以这种方式出现（因为我没有在主线程上写入变量）——可能是 undefined behaviour propagating backwards in time.

的情况

工作线程挂起/恢复实现

Worker thread suspend / resume implementation

concurrency

multithreading

worker-thread

c++14