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

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Message-ID: <5f34564a-618e-457c-868b-0a3901b6d69b@kernel.org>
Date: Tue, 3 Dec 2024 10:35:59 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
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 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();
```

> 
> Alice
>