> The atomic_() functions are (on most architectures, x86 included) implemented using GCC/Clang's __atomic_* functions, with various __ATOMIC_* orderings chosen as appropriate.
There is no one-size-fits-all choice here, so "as appropriate" is almost definitionally a mistake. It turns out that "as appropriate" for Haiku means Sequential Consistency except on simple load and store, which have Acquire-release semantics instead for some (unexplained in my brief exploration) reason.
Still that does answer the question of why these structures seem to work for Haiku despite the lack of what you'd ordinarily expect in terms of ordering guarantees, it's eating a Fence for each Entry and a Fence for each mutex operation. It's a steeplechase!
There is no one-size-fits-all choice here, so "as appropriate" is almost definitionally a mistake. It turns out that "as appropriate" for Haiku means Sequential Consistency except on simple load and store, which have Acquire-release semantics instead for some (unexplained in my brief exploration) reason.
Still that does answer the question of why these structures seem to work for Haiku despite the lack of what you'd ordinarily expect in terms of ordering guarantees, it's eating a Fence for each Entry and a Fence for each mutex operation. It's a steeplechase!