Re: [PATCH v2] rust: doc: Clarify safety invariants for Revocable type

[Danilo Krummrich <dakr@kernel.org>]

On Fri, May 09, 2025 at 12:10:08PM +0200, Benno Lossin wrote:
> On Sat May 3, 2025 at 4:53 PM CEST, Marcelo Moreira wrote:
> >  /// # Invariants
> >  ///
> > -/// - The wrapped object `data` is valid if and only if `is_available` is `true`.
> > -/// - Access to `data` must occur only while holding the RCU read-side lock (e.g., via
> > -///   [`Revocable::try_access`] or [`Revocable::try_access_with_guard`]).
> > -/// - Once `is_available` is set to `false`, further access to `data` is disallowed,
> > -///   and the object is dropped either after an RCU grace period (in [`revoke`]),
> > -///   or immediately (in [`revoke_nosync`]).
> > +/// - `data` is valid if and only if `is_available` is true.
> > +/// - Access to `data` requires holding the RCU read-side lock.
> 
> I'm not sure what the correct wording here should be. The current
> wording makes the `revoke_internal` function illegal, as it doesn't hold
> the read-side lock, but still accesses `data`.
> 
> Maybe @Danilo can help here, but as I understand it, the value in `data`
> is valid for as long as the rcu read-side lock is held *and* if
> `is_available` was true at some point while holding the lock.

IMHO, the RCU read lock is *not* a requirement, it's (for some methods) the
justification for how it is ensured that the `is_available` atomic cannot be
altered during the usage of `data`. So, it shouldn't be part of the type
invariants.

For instance, we also have the Revocalbe::access() [1], which is an unsafe
direct accessor for `data`. It has the following safety requirement:

	 "The caller must ensure this [`Revocable`] instance hasn't been revoked
	 and won't be revoked as long as the returned `&T` lives."

Which is equal to the caller must ensure that `is_available` is true, and won't
be altered to false as long as the returned reference lives.

One valid way for the caller would be to wrap it into an RCU read side critical
section and check `is_available`. However, depending on the context, there are
also other justifications, e.g. [2].

[1] https://gitlab.freedesktop.org/drm/nova/-/blob/nova-next/rust/kernel/revocable.rs?ref_type=heads#L148
[2] https://gitlab.freedesktop.org/drm/nova/-/blob/nova-next/rust/kernel/devres.rs?ref_type=heads#L221

> > +/// - Once is_available is set to false, further access to data is disallowed,
> > +///   and the object is dropped either after an RCU grace period (in [revoke]),
> > +///   or immediately (in [revoke_nosync]).

Same here, RCU isn't a relevant factor for the type invariant IMHO. It's just
how part of the implementation guarantees to up-hold the invariant.