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

[Danilo Krummrich <dakr@kernel.org>]

On 12/3/24 10:24 AM, Alice Ryhl wrote:
> On Tue, Dec 3, 2024 at 10:21 AM Danilo Krummrich <dakr@kernel.org> wrote:
>>
>> On 10/29/24 2:26 PM, Alice Ryhl wrote:
>>> On Tue, Oct 22, 2024 at 11:33 PM Danilo Krummrich <dakr@kernel.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
>