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

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Message-ID: <aa747122-fc78-45db-a410-ceb53b4df65e@gmail.com>
Date: Sat, 26 Apr 2025 18:44:03 +0200
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Danilo Krummrich <dakr@...nel.org>, gregkh@...uxfoundation.org,
 rafael@...nel.org, bhelgaas@...gle.com, kwilczynski@...nel.org,
 zhiw@...dia.com, cjia@...dia.com, jhubbard@...dia.com, bskeggs@...dia.com,
 acurrid@...dia.com, joelagnelf@...dia.com, ttabi@...dia.com,
 acourbot@...dia.com, ojeda@...nel.org, alex.gaynor@...il.com,
 boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
 benno.lossin@...ton.me, a.hindborg@...nel.org, aliceryhl@...gle.com,
 tmgross@...ch.edu
Cc: linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] rust: revocable: implement Revocable::access()

On 26.04.25 3:30 PM, 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@...nel.org>
> ---
> 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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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`.
> +    pub unsafe fn access<'a, 's: 'a>(&'s self) -> &'a T {
I'm not sure if the `'s` lifetime really carries much meaning here.
I find just (explicit) `'a` on both parameter and return value is clearer to me,
but I'm not sure what others (particularly those not very familiar with rust)
think of this.

Either way:

Reviewed-by: Christian Schrefl <chrisi.schrefl@...il.com>

> +        // SAFETY: By the safety requirement of this function it is guaranteed that
> +        // `self.data.get()` is a valid pointer to an instance of `T`.
> +        unsafe { &*self.data.get() }
> +    }
> +
>      /// # Safety
>      ///
>      /// Callers must ensure that there are no more concurrent users of the revocable object.