Re: [WIP 0/3] Memory model and atomic API in Rust

Dr. David Alan Gilbert
2024-03-26T14:35:25Z

* Linus Torvalds (torvalds@linux-foundation.org) wrote:
> On Mon, 25 Mar 2024 at 17:05, Dr. David Alan Gilbert <dave@treblig.org> wrote:
> >
> > Isn't one of the aims of the Rust/C++ idea that you can't forget to access
> > a shared piece of data atomically?
>
> If that is an aim, it's a really *bad* one.
>
> Really.
>
> It very much should never have been an aim, and I hope it wasn't. I
> think, and hope, that the source of the C++ and Rust bad decisions is
> cluelessness, not active malice.

Oh that hit a nerve :-)

> Take Rust - one big point of Rust is the whole "safe" thing, but it's
> very much not a straightjacket like Pascal was. There's a "safe" part
> to Rust, but equally importantly, there's also the "unsafe" part to
> Rust.
>
> The safe part is the one that most programmers are supposed to use.
> It's the one that allows you to not have to worry too much about
> things. It's the part that makes it much harder to screw up.
>
> But the *unsafe* part is what makes Rust powerful. It's the part that
> works behind the curtain. It's the part that may be needed to make the
> safe parts *work*.
>
> And yes, an application programmer might never actually need to use
> it, and in fact in many projects the rule might be that unsafe Rust is
> simply never even an option - but that doesn't mean that the unsafe
> parts don't exist.
>
> Because those unsafe parts are needed to make it all work in reality.
>
> And you should never EVER base your whole design around the "safe"
> part. Then you get a language that is a straight-jacket.
>
> So I'd very strongly argue that the core atomics should be done the
> "unsafe" way - allow people to specify exactly when they want exactly
> what access. Allow people to mix and match and have overlapping
> partial aliases, because if you implement things like locking, you
> *need* those partially aliasing accesses, and you need to make
> overlapping atomics where sometimes you access only one part of the
> field.
>
> And yes, that will be unsafe, and it might even be unportable, but
> it's exactly the kind of thing you need in order to avoid having to
> use assembly language to do your locking.
>
> And by all means, you should relegate that to the "unsafe corner" of
> the language. And maybe don't talk about the unsafe sharp edges in the
> first chapter of the book about the language.
>
> But you should _start_ the design of your language memory model around
> the unsafe "raw atomic access operations" model.
>
> Then you can use those strictly more powerful operations, and you
> create an object model *around* it.
>
> So you create safe objects like just an atomic counter. In *that*
> corner of the language, you have the "safe atomics" - they aren't the
> fundamental implementation, but they are the safe wrappers *around*
> the more powerful (but unsafe) core.
>
> With that "atomic counter" you cannot forget to do atomic accesses,
> because that safe corner of the world doesn't _have_ anything but the
> safe atomic accesses for every time you use the object.
>
> See? Having the capability to do powerful and maybe unsafe things does
> not force people to expose and use all that power. You can - and
> should - wrap the powerful model with safer and simpler interfaces.

I'd agree it's important to get the primitives right; but
I'd argue that from a design point of view it's probably better to keep
both in mind from early on; you need to create a safe interface which
people can actually use most of the time, otherwise you're not getting
any benefit; so yes get the bases right, but just keep a feel for how
they'll get encapsulated so most of the more boring code can be safe.

> This isn't something specific to atomics. Not even remotely. This is
> quite fundamental. You often literally _cannot_ do interesting things
> using only safe interfaces. You want safe memory allocations - but to
> actually write the allocator itself, you want to have all those unsafe
> escape methods - all those raw pointers with arbitrary arithmetic etc.
>
> And if you don't have unsafe escapes, you end up doing what so many
> languages did: the libraries are written in something more powerful
> like C, because C literally can do things that other languages
> *cannot* do.

Yeh that's fine, I'm not at all arguing against that; but it doesn't
mean you shouldn't keep an eye on how the safe side should look; even in the
kernel.
Get it right and those unsafe escapes shouldn't be needed too commonly;
get it wrong and you'll either have painful abstractions or end up with
unsafes shotgunned all over the place.

> Don't let people fool you with talk about Turing machines and similar
> smoke-and-mirror garbage. It's a bedtime story for first-year CS
> students. It's not true.

My infinitely long tape is still on back order.

Dave

> things. If your language doesn't have those unsafe escapes, your
> language is inherently weaker, and inherently worse for it.
>
> Linus
>
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