Comment by staticassertion | Hacker News Reader

Comment by staticassertion | Hacker News Reader

staticassertion

> There are a couple of interesting implications from this outcome, should it hold. The first of those is that, as Rust code reaches more deeply into the core kernel, its code for concurrent access to shared data will look significantly different from the equivalent C code, even though the code on both sides may be working with the same data. Understanding lockless data access is challenging enough when dealing with one API; developers may now have to understand two APIs, which will not make the task easier.

The thing is, it'll be far less challenging for the Rust code, which will actually define the ordering semantics explicitly. That's the point of rejecting the READ_ONCE/WRITE_ONCE approach - it's unclear what the goal is when using those, what guarantee you actually want.

I suspect that if Rust continues forward with this approach it will basically end up as the code where someone goes to read the actual semantics to determine what the C code should do.

In my experience, in practice, it usually isn't that hard to figure out what people meant by a READ/WRITE_ONCE().

Most common cases I see are:

1. I'm sharing data between concurrent contexts but they are all on the same CPU (classic is sharing a percpu variable between IRQ and task).

2. I'm reading some isolated piece of data that I know can change any time, but it doesn't form part of a data structure or anything, it can't be "in an inconsistent state" as long as I can avoid load-tearing (classic case: a performance knob that gets mutated via sysfs). I just wanna READ it ONCE into a local variable, so I can do two things with it and know they both operate with the same value.

I actually don't think C++ or Rust have existing semantics that satisfy this kinda thing? So will be interesting to see what they come up with.

It sounds like std::atomic_ref<T>::{load,store}(relaxed) to me. 1. ad-hoc atomicity with ref 2. no ordering

https://godbolt.org/z/4h893P7hG

I believe that lets the compiler reorder the accesses. So this would be fine for my second example but it would be broken for my first example.

({READ,WRITE}_ONCE() only lets the compiler reorder the accesses if they happen in the same C statement).

I think C++ doesn't have an equivalent because it just doesn't make sense as a primitive in very many environments.

> I suspect that if Rust continues forward with this approach it will basically end up as the code where someone goes to read the actual semantics to determine what the C code should do.

That will also put it on the unfortunate position of being the place that breaks every time somebody adds a bug to the C code.

Anyway, given the cultures involved, it's probably inevitable.

> That will also put it on the unfortunate position of being the place that breaks every time somebody adds a bug to the C code.

Can someone explain charitably what the poster is getting at? To me, the above makes zero sense. If the Rust code is what is implemented correctly, and has the well-defined semantics, then, when the C code breaks, it's obviously the C code's problem?

I think a charitable interpretation is that given that the Rust code will be less forgiving, it will "break" C code and patterns that "used to work", albeit with latent UB or other nonobvious correctness issues. Now, obviously this is ultimately a good thing, and no developer worth their salt would seriously argue that latent bugs should stay latent, but as we've already seen, people have egos and aren't always exceedingly rational.

Well, thanks.

It's not exactly that the Rust implementation will be less forgiving. But developers will have a clear picture of the operations semantics, while C developers will have a harder time understanding the operations, and consequently avoid them.

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