linux-kernel - Re: [PATCH v3 07/16] rust: add `Revocable` type

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Message-ID: <CAH5fLgiG31JLL9ce_Wwaz_okg4roxAhmOniDEyw0hQG8vSTtig@mail.gmail.com>
Date: Tue, 3 Dec 2024 11:58:52 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com, 
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, 
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me, 
	tmgross@...ch.edu, a.hindborg@...sung.com, airlied@...il.com, 
	fujita.tomonori@...il.com, lina@...hilina.net, pstanner@...hat.com, 
	ajanulgu@...hat.com, lyude@...hat.com, robh@...nel.org, 
	daniel.almeida@...labora.com, saravanak@...gle.com, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-pci@...r.kernel.org, devicetree@...r.kernel.org, 
	Wedson Almeida Filho <wedsonaf@...il.com>
Subject: Re: [PATCH v3 07/16] rust: add `Revocable` type

On Tue, Dec 3, 2024 at 10:36 AM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On 12/3/24 10:24 AM, Alice Ryhl wrote:
> > On Tue, Dec 3, 2024 at 10:21 AM Danilo Krummrich <dakr@...nel.org> wrote:
> >>
> >> On 10/29/24 2:26 PM, Alice Ryhl wrote:
> >>> On Tue, Oct 22, 2024 at 11:33 PM Danilo Krummrich <dakr@...nel.org> wrote:
> >>>> +/// A guard that allows access to a revocable object and keeps it alive.
> >>>> +///
> >>>> +/// CPUs may not sleep while holding on to [`RevocableGuard`] because it's in atomic context
> >>>> +/// holding the RCU read-side lock.
> >>>> +///
> >>>> +/// # Invariants
> >>>> +///
> >>>> +/// The RCU read-side lock is held while the guard is alive.
> >>>> +pub struct RevocableGuard<'a, T> {
> >>>> +    data_ref: *const T,
> >>>> +    _rcu_guard: rcu::Guard,
> >>>> +    _p: PhantomData<&'a ()>,
> >>>> +}
> >>>
> >>> Is this needed? Can't all users just use `try_access_with_guard`?
> >>
> >> Without this guard, how to we access `T` with just the `rcu::Guard`?
> >
> > I don't think `try_access_with_guard` provides any access that you
> > can't get by doing `try_access_with_guard`.
> >
> > That said, I guess this guard functions as a convenience accessors, so
> > I don't mind it.
>
> What I mean is, how does the following work without `RevocableGuard`?
>
> ```
> struct Foo;
>
> impl Foo {
>     pub fn bar() { ... }
> }
>
> let data: Revocable<Foo> = ...;
> let guard = data.try_access()?;
>
> guard.bar();
> ```

Is there a reason you can do this?

let guard = rcu::Guard::new();
let value = data.try_access_with_guard(&guard);
value.bar();

Alice