c++ - 确保线程不会两次锁定互斥体？

Question

假设我有一个线程运行成员方法，例如下面的示例中的 runController:

class SomeClass {
public:
    SomeClass() { 
         // Start controller thread
         mControllerThread = std::thread(&SomeClass::runController, this) 
    }

    ~SomeClass() {
         // Stop controller thread
         mIsControllerThreadInterrupted = true;
         // wait for thread to die.
         std::unique_lock<std:::mutex> lk(mControllerThreadAlive); 
    }

    // Both controller and external client threads might call this
    void modifyObject() {
         std::unique_lock<std::mutex> lock(mObjectMutex);
         mObject.doSomeModification();
    }
    //...
private:
    std::mutex mObjectMutex;
    Object mObject;

    std::thread mControllerThread;
    std::atomic<bool> mIsControllerInterrupted;
    std::mutex mControllerThreadAlive;

    void runController() {        
        std::unique_lock<std::mutex> aliveLock(mControllerThreadAlive);
        while(!mIsControllerInterruped) {
            // Say I need to synchronize on mObject for all of these calls
            std::unique_lock<std::mutex> lock(mObjectMutex);
            someMethodA();
            modifyObject(); // but calling modifyObject will then lock mutex twice
            someMethodC();
        }
    }
    //...
};

runController 中的部分(或全部)子例程需要修改线程之间共享并由互斥锁保护的数据。其中一些(或全部)也可能被需要修改此共享数据的其他线程调用。

拥有 C++11 的所有荣耀，我如何确保没有线程会两次锁定互斥量？

现在，我将 unique_lock 引用作为参数传递给方法，如下所示。但这看起来很笨重，难以维护，可能是灾难性的，等等......

void modifyObject(std::unique_lock<std::mutex>& objectLock) {

    // We don't even know if this lock manages the right mutex... 
    // so let's waste some time checking that.
    if(objectLock.mutex() != &mObjectMutex)
         throw std::logic_error();

    // Lock mutex if not locked by this thread
    bool wasObjectLockOwned = objectLock.owns_lock();
    if(!wasObjectLockOwned)
        objectLock.lock();

    mObject.doSomeModification();

    // restore previous lock state
    if(!wasObjectLockOwned)
        objectLock.unlock();

}

谢谢!

Answer 1

有几种方法可以避免这种编程错误。我建议在类设计级别上进行:

• 将公共(public)和私有(private)成员函数分开，
• 只有 public 成员函数锁定 mutex，
• 和public成员函数永远不会被其他成员函数调用。

如果一个函数在内部和外部都需要，则创建该函数的两个变体，并将其中一个委托(delegate)给另一个:

public:
    // intended to be used from the outside
    int foobar(int x, int y)
    {
         std::unique_lock<std::mutex> lock(mControllerThreadAlive);
         return _foobar(x, y);
    }
private:
    // intended to be used from other (public or private) member functions
    int _foobar(int x, int y)
    {
        // ... code that requires locking
    }