Saturday, June 21, 2014

C++ User Group in St. Petersburg, 21 June 2014

Today I've given the presentation related to multithreading topic: "Fine-grained locking". The considered approach uses fairly simple idea: to incorporate lock/unlock mutex operations into object access using overloaded operator->:

template<typename T, typename T_mutex>
struct Access : std::unique_lock<T_mutex>
    Access(T* t_, T_mutex& m)
        : std::unique_lock<T_mutex>(m), t(t_) {}

    template<typename T_lockType>
    Access(T* t_, T_mutex& m, T_lockType type)
        : std::unique_lock<T_mutex>(m, type), t(t_) {}

    T* operator->()     { init(); return t; }

    void init()         { if (!this->owns_lock()) this->lock(); }
    T* t;

template<typename T>
struct SmartMutex
    typedef Access<T, Mutex> WAccess;
    typedef Access<const T, Mutex> RAccess;

    RAccess operator->() const { return read(); }
    WAccess operator->()       { return write(); }
    RAccess read() const       { return {get(), mutex()}; }
    RAccess readLazy() const   { return {get(), mutex(), std::defer_lock}; }
    WAccess write()            { return {get(), mutex()}; }
    WAccess writeLazy()        { return {get(), mutex(), std::defer_lock}; }

    T* get() const             { return data.get(); }
    Mutex& mutex() const       { return *mutexData.get(); }

    std::shared_ptr<T> data = std::make_shared<T>();
    std::shared_ptr<Mutex> mutexData = std::make_shared<Mutex>();

This allows to avoid race conditions and to use atomicity on the object data level.

Lazy methods are suitable to avoid another issue of multithreaded applications: deadlock. Here is an example how to use it correctly with std::lock:

SmartMutex<X> x, y;
auto rx = x.readLazy();
auto ry = y.readLazy();
std::lock(rx, ry);
// now rx and ry can be used

The same approach was used to implement SmartSharedMutex allowing to share read access. But instead of usual overloaded ->, new operator was introduced: ---> (long arrow). How was it implemented? See related article: Useful Multithreaded Idioms of C++ (in Russian)

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