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

Danilo Krummrich
2025-04-27T10:13:16Z

On Sun, Apr 27, 2025 at 08:37:00AM +0000, Benno Lossin wrote:
> On Sat Apr 26, 2025 at 11:18 PM CEST, Danilo Krummrich wrote:
> > 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 + /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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?
>
> IMO an `# Invariants` section with the corresponding invariant that
> `data` is valid when `is_available` is true.

Yeah, I agree that the # Invariants section is indeed missing and should be
fixed.

> > 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.
>
> Just because other code is doing the same mistake doesn't make it
> correct. If I had reviewed the patch at that time I'm sure I would have
> pointed this out.

I would say that try_access() and try_access_with_guard() are wrong, they rely
on the correct thing, we just missed documenting the corresponding invariant.

> I opened an issue about this:
>
> https://github.com/Rust-for-Linux/linux/issues/1160

Thanks for creating the issue!

What do you suggest for this patch?