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

[Danilo Krummrich <dakr@kernel.org>]

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@kernel.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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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.