[PATCH v4 3/3] rust: revocable: split revoke_internal into revoke and revoke_nosync

[Marcelo Moreira <marcelomoreira1905@gmail.com>]

This commit refactors the revocation mechanism by removing the generic
`revoke_internal` function. Its logic is now directly integrated into
two distinct public functions: `revoke()` and `revoke_nosync()`.

`revoke_nosync()` is an `unsafe` function that requires the caller to
guarantee no concurrent users, thus avoiding an RCU grace period.
`revoke()` is a safe function that internally waits for the RCU grace
period to ensure all concurrent accesses have completed before dropping
the wrapped object.

This change improves API clarity and simplifies associated `SAFETY`
comments by making the synchronization behavior explicit in the function
signatures.

Suggested-by: Benno Lossin <lossin@kernel.org>
Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Marcelo Moreira <marcelomoreira1905@gmail.com>
---
 rust/kernel/revocable.rs | 38 +++++++++++++++-----------------------
 1 file changed, 15 insertions(+), 23 deletions(-)

diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
index 43cc9bdc94f4..daf22e3a7d20 100644
--- a/rust/kernel/revocable.rs
+++ b/rust/kernel/revocable.rs
@@ -126,22 +126,6 @@ pub fn try_access_with_guard<'a>(&'a self, _guard: &'a rcu::Guard) -> Option<&'a
         }
     }
 
-    /// # Safety
-    ///
-    /// Callers must ensure that there are no more concurrent users of the revocable object.
-    unsafe fn revoke_internal<const SYNC: bool>(&self) {
-        if self.is_available.swap(false, Ordering::Relaxed) {
-            if SYNC {
-                // SAFETY: Just an FFI call, there are no further requirements.
-                unsafe { bindings::synchronize_rcu() };
-            }
-
-            // SAFETY: We know `self.data` is valid because only one CPU can succeed the
-            // `compare_exchange` above that takes `is_available` from `true` to `false`.
-            unsafe { drop_in_place(self.data.get()) };
-        }
-    }
-
     /// Revokes access to and drops the wrapped object.
     ///
     /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`],
@@ -151,10 +135,12 @@ unsafe fn revoke_internal<const SYNC: bool>(&self) {
     ///
     /// Callers must ensure that there are no more concurrent users of the revocable object.
     pub unsafe fn revoke_nosync(&self) {
-        // SAFETY: By the safety requirement of this function, the caller ensures that nobody is
-        // accessing the data anymore and hence we don't have to wait for the grace period to
-        // finish.
-        unsafe { self.revoke_internal::<false>() }
+        if self.is_available.swap(false, Ordering::Relaxed) {
+            // SAFETY: `Self::data` is valid for writes because of `Self`'s type invariants,
+            // as `Self::is_available` is false due to the atomic swap, and by the safety
+            // requirements of this function, no thread is accessing `data` anymore.
+            unsafe { drop_in_place(self.data.get()) };
+        }
     }
 
     /// Revokes access to and drops the wrapped object.
@@ -165,9 +151,15 @@ pub unsafe fn revoke_nosync(&self) {
     /// [`Revocable::try_access`] beforehand and still haven't dropped the returned guard), this
     /// function waits for the concurrent access to complete before dropping the wrapped object.
     pub fn revoke(&self) {
-        // SAFETY: By passing `true` we ask `revoke_internal` to wait for the grace period to
-        // finish.
-        unsafe { self.revoke_internal::<true>() }
+        if self.is_available.swap(false, Ordering::Relaxed) {
+            // SAFETY: Just an FFI call, there are no further requirements.
+            unsafe { bindings::synchronize_rcu() };
+
+            // SAFETY: `Self::data` is valid for writes because of `Self`'s type invariants,
+            // as `Self::is_available` is false due to the atomic swap, and `synchronize_rcu`
+            // ensures all prior RCU read-side critical sections have completed.
+            unsafe { drop_in_place(self.data.get()) };
+        }
     }
 }
 
-- 
2.49.0