Как правильно работать с callback функциями в многопоточных программах?

#include <iostream>
#include <thread>
#include <mutex>
#include <chrono>

int my_callback(int& val, std::recursive_mutex& m)
{
  std::lock_guard<std::recursive_mutex> lock(m);
  for(int i = 0; i < 10; ++i)
  {
    val++;
    std::cout << val << '\t'
              << std::this_thread::get_id() << '\n';
  }
  return val;
}

int my_callback2(int& val, std::recursive_mutex& m)
{
  std::lock_guard<std::recursive_mutex> lock(m);
  for(int i = 0; i < 10; ++i)
  {
    val--;
    std::cout << val << '\t'
              << std::this_thread::get_id() << '\n';
  }
  return val;
}

class LibClass
{
  using callback = int(*)(int&, std::recursive_mutex&);
  callback cbFn[2];
  int idx;
public:
  LibClass() : cbFn{nullptr, nullptr}, idx{}{}

  void register_callback(callback cb)
  {
    cbFn[idx] = cb;
    ++idx;
  }

  void run(int& val, std::recursive_mutex& m)
  {
    std::thread t(std::ref(cbFn[0]), std::ref(val), std::ref(m));
    std::thread t2(std::ref(cbFn[1]), std::ref(val), std::ref(m));
    t.detach();
    t2.detach();
  }
};

class MyClass
{
  int val;
  LibClass libclass;
  std::recursive_mutex m;
public:
  MyClass() : val{}, libclass{}
  {
    libclass.register_callback(&my_callback);
    libclass.register_callback(&my_callback2);
    libclass.run(val, m);
  }
  void printVal()
  {
    m.lock();
    std::cout << this << " val == " << val << '\n';
    m.unlock();
  }
};

int main()
{
  MyClass m;
  std::this_thread::sleep_for(std::chrono::milliseconds(100)); // Ждем
  m.printVal();
}