[waddlesplash]

> Your "no possible way" is assuming Sequential Consistency,

I am assuming events cannot finish before they are started, yes! I am pretty sure that even the (in)famous DEC Alpha could not possibly have time-traveling results.

Once again: there is a distinction between the API itself, and the API's implementation. Once the "Add" function returns, the "Entry" is now waiting on the "Variable". How the "Add" function ensures that is entirely abstracted away from the consumers of the API.

The implementation of "Add", at least, is synchronized using a very standard spinlock, just like similar operations are for mutexes, semaphores, etc. not just on Haiku, but on all the BSDs and Linux. We don't even need to think about CPU operations ordering for that, the spinlock takes care of it for us.

I am pretty sure Linux (and every other operating system, ever) would be in all kinds of trouble if you could read stale values from memory protected by spinlocks, so I don't know why you are casting doubt on these things.

> It's possible that the vaguely named "atomic" operations in Haiku provide you with adequate ordering

Hey, you already wrote a comment elsewhere in this thread about this point, and I replied to your comment with a bunch of information proving definitively: they do, in fact, provide that ordering. But in the cases I described in the parent comment here, it does not matter, because here we are talking about API consumption, not implementation.

[tialaramex]

> I am assuming events cannot finish before they are started, yes! I am pretty sure that even the (in)famous DEC Alpha could not possibly have time-traveling results.

You seem to think this is a joke, but it isn't. Obviously from the CPU's point of view there is no "time travel" but that's cold comfort for users. The Alpha doesn't promise that there's any coherent ordering at all unless you've imposed one, which the sequentially consistent atomics Haiku is apparently asking for do.

To get Sequential Consistency you're paying a very high price on such weakly ordered architectures. On x86 (and x86-64) the relative price is smaller, because everybody is paying for Acquire-release basically all the time, but it's still substantial on larger modern systems.

This price isn't unique to Haiku, but the choice to pay it (almost) everywhere is, at least in terms of operating systems people would be using today.

[waddlesplash]

> The Alpha doesn't promise that there's any coherent ordering at all unless you've imposed one

Yes. But why did you bring this up in this thread about API usage? It's the implementation's problem to make this work out. "Add()" should be the equivalent of a full memory barrier (at least for the condition variable's memory) no matter how that happens internally.

> This price isn't unique to Haiku, but the choice to pay it (almost) everywhere is, at least in terms of operating systems people would be using today.

Haiku is, in many ways, poorly optimized when compared on such details with Linux or FreeBSD, all the developers know this fact, and we make no secret of it. If this was your entire point in the first place, why not just say so?

By the way, as far as I can tell, OpenBSD's kernel atomics (sys/atomic.h) do not have different versions for different memory orderings; in fact they use the older-style GCC builtins and not the C++11-style ones, so they are also using sequential consistency everywhere they use atomics. Is OpenBSD not a "modern operating system people would be using today"?