rust: devres: do not dereference to the internal Revocable

commit 20c96ed upstream.

We can't expose direct access to the internal Revocable, since this
allows users to directly revoke the internal Revocable without Devres
having the chance to synchronize with the devres callback -- we have to
guarantee that the internal Revocable has been fully revoked before
the device is fully unbound.

Hence, remove the corresponding Deref implementation and, instead,
provide indirect accessors for the internal Revocable.

Note that we can still support Devres::revoke() by implementing the
required synchronization (which would be almost identical to the
synchronization in Devres::drop()).

Fixes: 76c01de ("rust: add devres abstraction")
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250611174827.380555-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Danilo Krummrich

rust/kernel/devres.rs

@@ -12,13 +12,11 @@ use crate::{
     error::{Error, Result},
     ffi::c_void,
     prelude::*,
-    revocable::Revocable,
-    sync::{Arc, Completion},
+    revocable::{Revocable, RevocableGuard},
+    sync::{rcu, Arc, Completion},
     types::ARef,
 };
 
-use core::ops::Deref;
-
 #[pin_data]
 struct DevresInner<T> {
     dev: ARef<Device>,
@@ -196,21 +194,23 @@ impl<T> Devres<T> {
 
         Ok(())
     }
-}
 
-impl<T> Deref for Devres<T> {
-    type Target = Revocable<T>;
+    /// [`Devres`] accessor for [`Revocable::try_access`].
+    pub fn try_access(&self) -> Option<RevocableGuard<'_, T>> {
+        self.0.data.try_access()
+    }
 
-    fn deref(&self) -> &Self::Target {
-        &self.0.data
+    /// [`Devres`] accessor for [`Revocable::try_access_with_guard`].
+    pub fn try_access_with_guard<'a>(&'a self, guard: &'a rcu::Guard) -> Option<&'a T> {
+        self.0.data.try_access_with_guard(guard)
     }
 }
 
 impl<T> Drop for Devres<T> {
     fn drop(&mut self) {
         // SAFETY: When `drop` runs, it is guaranteed that nobody is accessing the revocable data
         // anymore, hence it is safe not to wait for the grace period to finish.
-        if unsafe { self.revoke_nosync() } {
+        if unsafe { self.0.data.revoke_nosync() } {
             // We revoked `self.0.data` before the devres action did, hence try to remove it.
             if !DevresInner::remove_action(&self.0) {
                 // We could not remove the devres action, which means that it now runs concurrently,