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

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Message-ID: <aA1Ns2GELPVbEsWV@pollux>
Date: Sat, 26 Apr 2025 23:18:43 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: 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, 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 08:24:14PM +0000, Benno Lossin wrote:
> 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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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)

What else makes it valid?

AFAICS, try_access() and try_access_with_guard() argue the exact same way,
except that the reason for not being revoked is the atomic check and the RCU
read lock.