Synchronising with mutex and relaxed memory order atomic

Question

I have a shared data structure that is already internally synchronised with a mutex. Can I use an atomic with memory order relaxed to signal changes. A very simplified view of what I mean in code

Thread 1

shared_conf.set("somekey","someval");
is_reconfigured.store(true, std::memory_order_relaxed);

Thread 2

if (is_reconfigured.load(std::memory_order_relaxed)) {
  inspect_shared_conf();
}

Is it guaranteed that I'll see updates in the shared_map?; the shared map itself internally synchronises every write/read call with a mutex

Answer 1

Relaxed order means that ordering of atomics and external operations only happens with regard to operations on the specific atomic object (and even then, the compiler is free to re-order them outside of the program-defined order). Thus, a relaxed store has no relationship to any state in external objects. So a relaxed load will not synchronize with your other mutexes.

The whole point of acquire/release semantics is to allow an atomic to control visibility of other memory. If you want an atomic load to mean that something is available, it must be an acquire and the value it acquired must have been released.