Re: [PATCH v6 3/3] rust: revocable: Document RevocableGuard invariants and refine Deref safety

[Marcelo Moreira <marcelomoreira1905@gmail.com>]

Em ter., 8 de jul. de 2025 às 12:04, Benno Lossin <lossin@kernel.org> escreveu:
>
> On Tue Jul 8, 2025 at 2:33 AM CEST, Marcelo Moreira wrote:
> > Improves the `RevocableGuard` documentation by explicitly stating that
> > its `data_ref` member is a valid pointer as an invariant. Additionally,
> > the `Deref` implementation's `SAFETY` comment is refined to justify
> > the `unsafe` dereference based on this new invariant and the
> > `_rcu_guard` ensuring data accessibility.
> >
> > These changes address feedback regarding the clarity and completeness
> > of `RevocableGuard`'s safety guarantees.
> >
> > Signed-off-by: Marcelo Moreira <marcelomoreira1905@gmail.com>
> > ---
> >  rust/kernel/revocable.rs | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
> > index 6d8e9237dbdf..64fe44b4f5a3 100644
> > --- a/rust/kernel/revocable.rs
> > +++ b/rust/kernel/revocable.rs
> > @@ -233,7 +233,8 @@ fn drop(self: Pin<&mut Self>) {
> >  ///
> >  /// # Invariants
> >  ///
> > -/// The RCU read-side lock is held while the guard is alive.
> > +/// - `data_ref` is a valid pointer to a `T` object for the entire lifetime of this guard.
>
> This should be:
>
>     /// - `data_ref` is a valid pointer for as long as the RCU read-side lock is held.
>
> > +/// - The RCU read-side lock is held while the guard is alive.
>
> And this invariant is unnecessary.
>
> >  pub struct RevocableGuard<'a, T> {
> >      // This can't use the `&'a T` type because references that appear in function arguments must
> >      // not become dangling during the execution of the function, which can happen if the
> > @@ -258,8 +259,8 @@ impl<T> Deref for RevocableGuard<'_, T> {
> >      type Target = T;
> >
> >      fn deref(&self) -> &Self::Target {
> > -        // SAFETY: By the type invariants, we hold the rcu read-side lock, so the object is
> > -        // guaranteed to remain valid.
> > +        // SAFETY: `self.data_ref` is valid for writes because of `Self`'s type invariants,
> > +        // and `_rcu_guard` ensures the data's accessibility for the lifetime of this guard.
>
> This also needs to be adjusted.
>
> Also this patch should fix any invariant comments needed to construct
> `RevocableGuard`.

Regarding this point, "...fix any invariant comments needed to
construct `RevocableGuard`."

I looked at the function that constructs `RevocableGuard`
(try_access->RevocableGuard::new) and I think it's correct.

In `try_access`, the comment justifies the validity of self.data.get()
by mentioning that `self.is_available` is true and that the RCU read
lock prevents data from being dropped. This aligns well with what we
already have in try_access_with_guard, for example (from patch 1/3).

Since there are no other code snippets that construct
`RevocableGuard`, I thought I'd submit the patch with just the
adjustments I made previously.

--
Cheers,
Marcelo Moreira