What is this code an example of?

    private static object objA;
    private static object objB;
    private static void performTaskA()
    {
        lock (objB)
        {
            Thread.Sleep(1000);
            lock (objA) { }
        }
    }
    private static void PerformTaskB()
    {
        lock (objA)
        {
            lock (objB) { }
        }
    }