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

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Message-ID: <aA0RgOL09bBa0M19@pollux>
Date: Sat, 26 Apr 2025 19:01:52 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Christian Schrefl <chrisi.schrefl@...il.com>,
	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 09:54:58AM -0700, Boqun Feng wrote:
> 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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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).

Yes, this is indeed sufficient, that's why I wrote

	"The explicit lifetimes in access() probably don't serve a practical
	purpose, but I found them to be useful for documentation purposes."

below the commit message. :)

Any opinions in terms of documentation purposes?

> > 
> > 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.
> >