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

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Message-ID: <D9GUNZ0PMDA4.AZXA0FWQUSB0@proton.me>
Date: Sat, 26 Apr 2025 20:24:14 +0000
From: Benno Lossin <benno.lossin@...ton.me>
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, 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 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@...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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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`.

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.