C++: Strongly Happens Before?

I mean, dining philosophers is very simple as well. Dijkstra’s shortest path is simple.

Simple can still be difficult to understand.

The data-race-free memory model was an observation back in the early 90's that a correctly-synchronized program that has no data races will, even on a weak memory model multiprocessor, be indistinguishable from a fully sequentially-consistent memory model. This was adapted into the Java 5 memory model, with the happens-before relation becoming the definition of correctly-synchronized, and then C++11 explicitly borrowed that model and extended it to include weaker atomics, and pretty much everybody else borrows directly or indirectly from that C++ memory model. However, C++ had to go back and patch the definition because their original definition didn't work, and it took C++ standardizing a model to get the academic community to a state where we could finally formalize weak memory models.

In Java, happens-before is composed essentially of the union of two relations: program order (i.e., the order imposed within a single thread by imperative programming model) and synchronizes-with (i.e., the cross-thread synchronization constructs). C++ started out doing the same. However, this is why it broke: in the presence of weak atomics, you can construct a sequence of atomic accesses and program order relations across multiple threads to suggest that something should have a happens-before relation that actually doesn't in the hardware memory model. To describe the necessary relations, you need to add several more kinds of dependencies, and I'm not off-hand sure which dependencies ended up with which labels.

Note that, for a user, all of this stuff generally doesn't matter. You can continue to think of happens-before as a basic program order unioned with a cross-thread synchronizes-with and your code will all work, you just end up with a weaker (fewer things allowed) version of synchronizes-with. The basic motto I use is, to have a value be written on thread A and read on thread B, A needs to write the value then do a release-store on some atomic, and B then needs to load-acquire on the same atomic and only then can it read the value.

Sure, that’s an entirely valid execution. But this is about what exact pairs of values (x, y) are observable by each thread. Some are allowed, others are not, by the semantics of atomic loads and stores guaranteed by the CPU. The starting or joining order of the threads doesn’t matter, except insofar that thread starting and joining both synchronize-with the parent thread.

In general, in the presence of hardware parallelism (ie. always since 2007 or so) the very real corner cases are much more involved than "what if there’s an interrupt" and thinking in terms of single-threaded concurrency is not very fruitful in the presence of memory orderings less strict than seq_cst. It’s not about what order things can happen in (because there isn’t an order), it’s principally about how writes are propagated from the cache of one core to that of another.

x86 processors have sort of lulled many programmers of concurrent code into a false sense of safety because almost everything is either unordered or sequentially consistent. But the other now-common architectures aren’t as forgiving.

It doesn't it matter for this article whether there exist possible executions other than the one the author inquires about.

The point of weak memory models is to formally define the set of all possible legal executions of a concurrent program. This gets very complicated in a hurry because of the need to accommodate 1) hardware properties such as cache coherence and 2) desired compiler optimization opportunities that will want to reason over what's possible / what's guaranteed.

In this case, there was a conflict between a behavior of Power processors and an older C++ standard that meant that a correct compiler would have to introduce extra synchronization to prevent the forbidden behavior, thus impacting performance. The solution was to weaken the memory model of the standard in a very small way.

The article walks us through how exactly the newer standard permits the funny unintuitive execution of the example program.

The exercise is academic, sure. A lot of hard academic research has gone into this field to get it right. But it has to be that precise because the problems it solves are that subtle.

The comments show the actual values observed in one particular execution. The author asks if this is compatible with the C++ memory model.

In other words, the author considers this execution to be surprising under the C++ memory model, and then goes on to explain it.

> sure, use an extra atomic to synchronize threads

What? That would make the situation worse. The execution has a weird unintuitive quirk where the actions of thread 3 seem to precede the actions of thread 1, which seem to precede the actions of thread 2, yet thread 2 observes an action of thread 3. Stronger synchronization would forbid such a thing.

The main question of the article is "Is the memory model _weak enough_ to permit the proposed execution?"