rust: revocable: indicate whether data has been revoked already

Return a boolean from Revocable::revoke() and Revocable::revoke_nosync()
to indicate whether the data has been revoked already.

Return true if the data hasn't been revoked yet (i.e. this call revoked
the data), false otherwise.

This is required by Devres in order to synchronize the completion of the
revoke process.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250612121817.1621-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

rust/kernel/revocable.rs

@@ -154,8 +154,10 @@ impl<T> Revocable<T> {
     /// # 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) {
+    unsafe fn revoke_internal<const SYNC: bool>(&self) -> bool {
+        let revoke = self.is_available.swap(false, Ordering::Relaxed);
+
+        if revoke {
             if SYNC {
                 // SAFETY: Just an FFI call, there are no further requirements.
                 unsafe { bindings::synchronize_rcu() };
@@ -165,17 +167,22 @@ impl<T> Revocable<T> {
             // `compare_exchange` above that takes `is_available` from `true` to `false`.
             unsafe { drop_in_place(self.data.get()) };
         }
+
+        revoke
     }
 
     /// Revokes access to and drops the wrapped object.
     ///
     /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`],
     /// expecting that there are no concurrent users of the object.
     ///
+    /// Returns `true` if `&self` has been revoked with this call, `false` if it was revoked
+    /// already.
+    ///
     /// # Safety
     ///
     /// Callers must ensure that there are no more concurrent users of the revocable object.
-    pub unsafe fn revoke_nosync(&self) {
+    pub unsafe fn revoke_nosync(&self) -> bool {
         // 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.
@@ -189,7 +196,10 @@ impl<T> Revocable<T> {
     /// If there are concurrent users of the object (i.e., ones that called
     /// [`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) {
+    ///
+    /// Returns `true` if `&self` has been revoked with this call, `false` if it was revoked
+    /// already.
+    pub fn revoke(&self) -> bool {
         // SAFETY: By passing `true` we ask `revoke_internal` to wait for the grace period to
         // finish.
         unsafe { self.revoke_internal::<true>() }