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SubjectRe: [PATCH v9 12/27] rust: add `kernel` crate
On Tue, 20 Sep 2022 10:55:27 -0500
"Eric W. Biederman" <ebiederm@xmission.com> wrote:

> Linus Torvalds <torvalds@linux-foundation.org> writes:
>
> > On Mon, Sep 19, 2022 at 4:58 PM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> >>
> >> This is not some kind of "a few special things".
> >>
> >> This is things like absolutely _anything_ that allocates memory, or
> >> takes a lock, or does a number of other things.
> >
> > Examples of "number of other things" ends up being things like
> > "accessing user memory", which depending on what you are doing may
> > be very common too.
> >
> > And btw, it's not only about the (multiple kinds of) atomic regions.
> >
> > We have other context rules in the kernel too, like "does floating
> > point or vector unit calculations". Which you can actually do, but
> > only in a kernel_fpu_begin/kernel_fpu_end region.
> >
> > Now, the floating point thing is rare enough that it's probably fine
> > to just say "no floating point at all in Rust code". It tends to be
> > very special code, so you'd write it in C or inline assembly,
> > because you're doing special things like using the vector unit for
> > crypto hashes using special CPU instructions.
>
> I just want to point out that there are ways of representing things
> like the context you are running in during compile time. I won't
> argue they are necessarily practical, but there are ways and by
> exploring those ways some practical solutions may result.
>
> Instead of saying:
> spin_lock(&lock);
> do_something();
> spin_unlock(&lock);
>
> It is possible to say:
> with_spin_lock(&lock, do_something());
>
> This can be taken a step farther and with_spin_lock can pass a
> ``token'' say a structure with no members that disappears at compile
> time that let's the code know it has the spinlock held.
>
> In C I would do:
> struct have_spin_lock_x {
> // nothing
> };
>
> do_something(struct have_spin_lock_x context_guarantee)
> {
> ...;
> }
>
> I think most of the special contexts in the kernel can be represented
> in a similar manner. A special parameter that can be passed and will
> compile out.
>
> I don't recall seeing anything like that tried in the kernel so I
> don't know if it makes sense or if it would get too wordy to live,
> but it is possible. If passing a free context parameter is not too
> wordy it would catch silly errors, and would hopefully leave more
> mental space for developers to pay attention to the other details of
> the problems they are solving.
>
> *Shrug* I don't know if rust allows for free parameters like that and
> if it does I don't know if it would be cheap enough to live.

I believe this was mentioned by Wedson in one of his previous emails.
This pattern is quite common in Rust code. It looks like this:

#[derive(Clone, Copy)]
pub struct Token<'a>(PhantomData<&'a ()>);

pub fn with_token<T>(f: impl for<'a> FnOnce(Token<'a>) -> T) ->
T { f(Token(PhantomData))
}

Any function that requires something can just take token by value, e.g.
with `token: Token<'_>`. Since Token is a zero-sized type (ZST), this
parameter will be omitted during code generation, so it won't affect
the ABI and this has no runtime cost.

Example on godbolt: https://godbolt.org/z/9n954cG4d, showing that the
token is actually all optimised out.

It should be noted however, atomic context is not something that a
token can represent. You can only use tokens to restrict what you *can*
do, but not what you *can't* do. There is no negative reasoning with
tokens, you can't create a function that can only be called when you
don't have token.

You can use tokens to represent non-atomic contexts, but that'll be
really painful because this requires carrying a token in almost all
functions. This kind of API also works well for FPU contexts.

Best,
Gary

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