rust: arc: split unsafe block, add missing comment

The new SAFETY comment style is taken from existing comments in `deref`
and `drop.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20241120-borrow-mut-v6-3-80dbadd00951@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 378925e553ecfb8594cb071ee18d970603419577..93eac650146abb8f9b2420f80dbadf81d1f95437 100644 (file)
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -389,10 +389,14 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
 
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+
         // 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(refcount) };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }