graphics-d3d/meshdata.cpp (excerpt)

/*
    Threads:
        (g): graphics thread
        (l): logic thread
    Stable states:
        (1): valid front, valid back
        (2):  zero front, valid back; front held at (g)
        (3): valid front,  zero back; back held at (l) or at (g)
        (4):  zero front,  zero back; both held at (g) or at (g) and (l)
*/

    MeshBuffer* MeshData::trylockfront()
    {
        // (1) -> (2), returns front
        // (2) -> (2), returns 0
        // (3) -> (4), returns front
        // (4) -> (4), returns 0
        MeshBuffer* mb = m_frontbuffer.exchange( 0, std::memory_order_acquire );
        return mb;
    }

    void MeshData::unlockfront( MeshBuffer* mb )
    {
        // (1) invalid
        // (2) -> (1), returns 0
        // (3) -> (3), returns front (which means mb is now back)
        // (4) -> (3), returns 0
        MeshBuffer* oldmb = m_frontbuffer.exchange( mb, std::memory_order_relaxed );
        if( oldmb )
        {
            // (3) -> (1)
            m_frontbuffer.store( oldmb, std::memory_order_relaxed );
            m_backbuffer.store( mb, std::memory_order_release ); // so (l) won't see front == back
        }
    }

    MeshBuffer* MeshData::trylockback()
    {
        // (1) -> (3), returns back
        // (2) -> (4), returns back
        // (3) -> (3), returns 0
        // (4) -> (4), returns 0
        MeshBuffer* mb = m_backbuffer.exchange( 0, std::memory_order_relaxed );
        return mb;
    }

    void MeshData::unlockback( MeshBuffer* mb )
    {
        // (1) invalid
        // (2) invalid
        // (3) -> (3), returns front
        // (4) -> (3), returns 0
        MeshBuffer* oldmb = m_frontbuffer.exchange( mb, std::memory_order_release );
        if( oldmb )
        {
            // (3) -> (1)
            m_backbuffer.store( oldmb, std::memory_order_relaxed );
        }
        m_dirty.store( true, std::memory_order_relaxed );
    }