How do I make memory stores in one thread "promptly" visible in other threads?

Suppose I wanted to copy the contents of a device register into a variable that would be read by multiple threads. Is there a good general way of doing this? Here are examples of two possible methods of doing this:

#include <atomic>

volatile int * const Device_reg_ptr = reinterpret_cast<int *>(0x666);

// This variable is read by multiple threads.
std::atomic<int> device_reg_copy;

// ...

// Method 1
const_cast<volatile std::atomic<int> &>(device_reg_copy)
  .store(*Device_reg_ptr, std::memory_order_relaxed);

// Method 2
device_reg_copy.store(*Device_reg_ptr, std::memory_order_relaxed);
std::atomic_thread_fence(std::memory_order_release);

More generally, in the face of possible whole program optimization, how does one correctly control the latency of memory writes in one thread being visible in other threads?