Re: [PATCH 1/3] rust: revocable: implement Revocable::access()

[Benno Lossin]

On Sat Apr 26, 2025 at 3:30 PM CEST, Danilo Krummrich wrote:
> Implement an unsafe direct accessor for the data stored within the
> Revocable.
>
> This is useful for cases where we can proof that the data stored within
> the Revocable is not and cannot be revoked for the duration of the
> lifetime of the returned reference.
>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> ---
> The explicit lifetimes in access() probably don't serve a practical
> purpose, but I found them to be useful for documentation purposes.
> ---
>  rust/kernel/revocable.rs | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
> index 971d0dc38d83..33535de141ce 100644
> --- a/rust/kernel/revocable.rs
> +++ b/rust/kernel/revocable.rs
> @@ -139,6 +139,18 @@ pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> {
>          self.try_access().map(|t| f(&*t))
>      }
>  
> +    /// Directly access the revocable wrapped object.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked
> +    /// for the duration of `'a`.

Ah I missed this in my other email, in case you want to directly refer
to the lifetime, you should keep it defined. I would still remove the
`'s` lifetime though.
> +    pub unsafe fn access<'a, 's: 'a>(&'s self) -> &'a T {
> +        // SAFETY: By the safety requirement of this function it is guaranteed that
> +        // `self.data.get()` is a valid pointer to an instance of `T`.

I don't see how the "not-being revoked" state makes the `data` ptr be
valid. Is that an invariant of `Revocable`? (it's not documented to have
any invariants)

---
Cheers,
Benno

> +        unsafe { &*self.data.get() }
> +    }
> +
>      /// # Safety
>      ///
>      /// Callers must ensure that there are no more concurrent users of the revocable object.