[RFC 3/5] rust: sync: Arc: Introduces Arc::get_inner() helper

[Boqun Feng]

Getting a `&ArcInner<T>` should always be safe from a `Arc<T>`,
therefore add this helper function to avoid duplicate unsafe
`ptr.as_ref()`.

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 rust/kernel/sync/arc.rs | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index fbfceaa3096e..4d8de20c996f 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -201,6 +201,13 @@ impl<T: ?Sized> Arc<T> {
         // reference can be created.
         unsafe { ArcBorrow::new(self.ptr) }
     }
+
+    /// Returns a reference to the internal [`ArcInner`].
+    fn get_inner(&self) -> &ArcInner<T> {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        unsafe { self.ptr.as_ref() }
+    }
 }
 
 impl<T: 'static> ForeignOwnable for Arc<T> {
@@ -233,9 +240,7 @@ impl<T: ?Sized> Deref for Arc<T> {
     type Target = T;
 
     fn deref(&self) -> &Self::Target {
-        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
-        // safe to dereference it.
-        unsafe { &self.ptr.as_ref().data }
+        &self.get_inner().data
     }
 }
 
@@ -244,7 +249,7 @@ impl<T: ?Sized> Clone for Arc<T> {
         // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
         // safe to increment the refcount.
-        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+        unsafe { bindings::refcount_inc(self.get_inner().refcount.get()) };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }
@@ -253,11 +258,7 @@ impl<T: ?Sized> Clone for Arc<T> {
 
 impl<T: ?Sized> Drop for Arc<T> {
     fn drop(&mut self) {
-        // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
-        // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
-        // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
-        // freed/invalid memory as long as it is never dereferenced.
-        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+        let refcount = self.get_inner().refcount.get();
 
         // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
         // this instance is being dropped, so the broken invariant is not observable.
-- 
2.39.1