FWIW your response comes across to me as quite rude and patronizing. I assume you didn’t mean this but the fact that you’ve decided to capitalize terms as if you have some sort of true definition for them, plus you dragging this conversation towards the specific complaint you had in another comment (Haiku might use sequential consistency when it doesn’t need to…although it’s not even clear if you’ve done the appropriate work to verify this before popping off). Maybe consider this for next time you comment? If you need an example that demonstrates curiosity rather than smugness look at the first comment in this thread.
The capitalization reflects terms of art in the C++ 11 memory model. So, not my "true definition" but the one provided by the language used, in this case C++. This matters because Haiku is written in languages which use this model (or in some cases languages which don't specify any model but in practice conform to the C++ 11 model)
If you're Linus Torvalds you can insist compiler vendors adjust things as you prefer to some extent, thus the Linux memory model isn't quite the C++ 11 memory model despite the fact that GCC is used to compile Linux and GCC notionally confirms to C11 (and thus has the C++ 11 memory model), much of what Linux does is not conforming to the ISO document. Haiku can't expect the same benefit of the doubt.
The question of whether Haiku needs sequential consistency where it has it is vexed. Hyrum's law applies. The least scary approach might be to follow C++ and provide sequential consistency by default with an opt-out, then introduce use of the opt-out carefully.
I'll respond to your comment anyways but I do want to remind you that I think it's more a reflection of you talking about what you want to talk about rather than being particularly relevant to this thread.
Operating systems are often written in C or C++. Both of these share a formal memory model to provide a set of useful semantics for well-formed programs. For most code the choice to adhere to this model is the right one. In fact, in many cases it can be appropriate to pick more "heavyweight" constructs despite the fact that they can be a bit slower because they are easier to reason about. LLVM's libc used sequential consistency for its shared_ptr implementation for quite a while until it was updated to use a more efficient set of primitives.
On the flip side, sometimes it is not appropriate to use this memory model. Linux has its own because it predates the C(++)11 memory model. Other good reasons to use your own model can be if the standard one doesn't efficiently map to what you are trying to do on the hardware you're on, or if the operations you need to perform are not encoded in the standard. These kinds of things are actually quite common in operating systems, which is why most of them do not strictly conform: C11 has no concept if "this region of code runs with interrupts disabled" or "I need a full serializing barrier for device memory". Haiku chooses to do its own thing, which may or may not be appropriate for its use case. Coming in and claiming immediately that whatever it's doing must be bad is inappropriate and lacks context.
