c++ - std::timed_mutex::try_lock_for 立即失败

Question

我第一次使用 std::timed_mutex，它的行为与我预期的不同。它似乎立即失败，而不是等待互斥锁。我以毫秒为单位提供锁定超时(如此处所示 http://www.cplusplus.com/reference/mutex/timed_mutex/try_lock_for/ )。但是对 try_lock_for() 的调用立即失败。

这是处理锁定和解锁互斥量的类:

  const unsigned int DEFAULT_MUTEX_WAIT_TIME_MS = 5 * 60 * 1000;

  class ScopedTimedMutexLock
  {
  public:
     ScopedTimedMutexLock(std::timed_mutex* sourceMutex, unsigned int numWaitMilliseconds=DEFAULT_MUTEX_WAIT_TIME_MS)
        m_mutex(sourceMutex)
     {
        if( !m_mutex->try_lock_for( std::chrono::milliseconds(numWaitMilliseconds) ) )
        {
           std::string message = "Timeout attempting to acquire mutex lock for ";
           message += Conversion::toString(numWaitMilliseconds);
           message += "ms";
           throw MutexException(message);
        }
     }
     ~ScopedTimedMutexLock()
     {
        m_mutex->unlock();
     }
  private:
     std::timed_mutex* m_mutex;
  };

这就是它被使用的地方:

  void  CommandService::Process( RequestType& request )
  {
     unsigned long callTime =
        std::chrono::duration_cast< std::chrono::milliseconds >(
           std::chrono::system_clock::now().time_since_epoch()
        ).count();

     try
     {
        ScopedTimedMutexLock lock( m_classMutex, request.getLockWaitTimeMs(DEFAULT_MUTEX_WAIT_TIME_MS) );
        // ... command processing code goes here
     }
     catch( MutexException& mutexException )
     {
        unsigned long catchTime =
           std::chrono::duration_cast< std::chrono::milliseconds >(
              std::chrono::system_clock::now().time_since_epoch()
           ).count();
        cout << "The following error occured while attempting to process command"
             << "\n   call  time: " << callTime
             << "\n   catch time: " << catchTime;
        cout << mutexException.description();
     }
  }

这是控制台输出:

The following error occured while attempting to process command
   call  time: 1131268914
   catch time: 1131268914
Timeout attempting to acquire mutex lock for 300000ms

知道哪里出了问题吗？到 std::chrono::milliseconds 的转换是否正确？如何让 try_lock_for() 等待锁？

附加信息:对try_lock_for() 的调用并不总是立即失败。多次调用获得了锁，一切都按预期进行。我看到的失败是断断续续的。有关失败原因的详细信息，请参阅下面的回答。

Answer 1

问题的根本原因在 http://en.cppreference.com/w/cpp/thread/timed_mutex/try_lock_for 的 try_lock_for() 描述中提到.在描述的末尾，它说:

As with try_lock(), this function is allowed to fail spuriously and return false even if the mutex was not locked by any other thread at some point during timeout_duration.

我天真地假设只有两种可能的结果:(1)函数在时间段内获取锁，或者(2)函数在等待时间结束后失败。但还有另一种可能，(3)功能在相对较短的时间后无故失败。 TL; DR，我的错。

我通过重写 ScopedTimedMutexLock 构造函数来循环 try_lock() 直到获得锁或超过等待时间限制来解决这个问题。

 ScopedTimedMutexLock(std::timed_mutex* sourceMutex, unsigned int numWaitMilliseconds=DEFAULT_MUTEX_WAIT_TIME_MS)
    m_mutex(sourceMutex)
 {
    const unsigned SLEEP_TIME_MS = 5;
    bool isLocked = false;
    unsigned long startMS = now();
    while( now() - startMS < numWaitMilliseconds && !isLocked )
    {
       isLocked = m_sourceMutex->try_lock();
       if( !isLocked )
       {
          std::this_thread::sleep_for(
              std::chrono::milliseconds(SLEEP_TIME_MS));
       }
    }
    if( !isLocked )
    {
       std::string message = "Timeout attempting to acquire mutex lock for ";
       message += Conversion::toString(numWaitMilliseconds);
       message += "ms";
       throw MutexException(message);
    }
 }

now() 的定义如下:

  private:
     unsigned long now() {
        return std::chrono::duration_cast< std::chrono::milliseconds >(
            std::chrono::system_clock::now().time_since_epoch() ).count();
     }