rust: sync: add Arc::into_unique_or_drop

Decrement the refcount of an `Arc`, but handle the case where it hits
zero by taking ownership of the now-unique `Arc`, instead of destroying
and deallocating it.

This is a dependency of the linked list that Rust Binder uses. The
linked list uses this method as part of its `ListArc` abstraction.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240311-arc-for-list-v3-2-cba1883c62eb@google.com
[boqun: Fix the rustfmt issue reported by Miguel]

Author: Darksonn

rust/kernel/sync/arc.rs

@@ -291,6 +291,38 @@ impl<T: ?Sized> Arc<T> {
     pub fn ptr_eq(this: &Self, other: &Self) -> bool {
         core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_ptr())
     }
+
+    /// Converts this [`Arc`] into a [`UniqueArc`], or destroys it if it is not unique.
+    ///
+    /// When this destroys the `Arc`, it does so while properly avoiding races. This means that
+    /// this method will never call the destructor of the value.
+    pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
+        // We will manually manage the refcount in this method, so we disable the destructor.
+        let me = ManuallyDrop::new(self);
+        // SAFETY: We own a refcount, so the pointer is still valid.
+        let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
+
+        // If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
+        // return without further touching the `Arc`. If the refcount reaches zero, then there are
+        // no other arcs, and we can create a `UniqueArc`.
+        //
+        // SAFETY: We own a refcount, so the pointer is not dangling.
+        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
+        if is_zero {
+            // SAFETY: We have exclusive access to the arc, so we can perform unsynchronized
+            // accesses to the refcount.
+            unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) };
+
+            // INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We
+            // must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin
+            // their values.
+            Some(Pin::from(UniqueArc {
+                inner: ManuallyDrop::into_inner(me),
+            }))
+        } else {
+            None
+        }
+    }
 }
 
 impl<T: 'static> ForeignOwnable for Arc<T> {