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

[Marcelo Moreira <marcelomoreira1905@gmail.com>]

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.
+/// - The RCU read-side lock is held while the guard is alive.
 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.
         unsafe { &*self.data_ref }
     }
 }
-- 
2.50.0