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

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Message-ID: <aA0P4lr0A2s--5bI@Mac.home>
Date: Sat, 26 Apr 2025 09:54:58 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Christian Schrefl <chrisi.schrefl@...il.com>
Cc: 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, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me, a.hindborg@...nel.org, aliceryhl@...gle.com,
	tmgross@...ch.edu, 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 06:44:03PM +0200, Christian Schrefl wrote:
> 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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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.

Yeah, I don't think we need two lifetimes here, the following version
should be fine (with implicit lifetime):

	pub unsafe fn access(&self) -> &T { ... }

, because if you do:

	let revocable: &'1 Revocable = ...;
	...
	let t: &'2 T = unsafe { revocable.access() };

'1 should already outlive '2 (i.e. '1: '2).

Regards,
Boqun

> 
> 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.
>