> C++ atomics are no good here, because they are not guaranteed to be lock free or address free.

That's not right; you can still use std::memory_order to get the memory barriers generated that are required. These are going to obviously be lock free, they deal with memory ordering—what you tried to deal with volatile, but in general case.

See: https://en.cppreference.com/w/cpp/atomic/atomic/store

Effectively std::atomic stores and loads generate volatile accesses plus the required memory barriers to get the desired behavior.