问使用shared_ptr实现读-复制-更新？

EN

template <typename T>
class rcu_protected
{
public:
    typedef T                                   type;
    typedef const T                             const_type;
    typedef std::tr1::shared_ptr<type>          rcu_pointer;
    typedef std::tr1::shared_ptr<const_type>    rcu_const_pointer;

    rcu_protected() : m_is_writing(0),
                      m_is_swapping(0),
                      m_data_ptr (new type())
    {}

    rcu_const_pointer get_reading_copy ()
    {
        spin_until_eq (m_is_swapping, 0);

        return m_data_ptr;
    }

    rcu_pointer get_updating_copy ()
    {
        spin_until_eq (m_is_swapping, 0);

        while (!CAS (m_is_writing, 0, 1))
        {/* do sleep for back-off when exceeding maximum retry times */}

        rcu_pointer new_data_ptr(new type(*m_data_ptr));

        // as spin_until_eq does not have memory barrier protection,
        // we need to place a read barrier to protect the loading of
        // new_data_ptr not to be re-ordered before its construction
        _ReadBarrier();

        return new_data_ptr;
    }

    void update (rcu_pointer new_data_ptr)
    {
        while (!CAS (m_is_swapping, 0, 1))
        {}

        m_data_ptr.swap (new_data_ptr);

        // as spin_until_eq does not have memory barrier protection,
        // we need to place a write barrier to protect the assignments of
        // m_is_writing/m_is_swapping be re-ordered bofore the swapping
        _WriteBarrier();

        m_is_writing = 0;
        m_is_swapping = 0;
    }

private:
    volatile long m_is_writing;
    volatile long m_is_swapping;
    rcu_pointer m_data_ptr;
};