[PATCH 2/2] rust: sync: add `Arc::into_unique_or_drop`

[Alice Ryhl <aliceryhl@google.com>]

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>
---
 rust/kernel/sync/arc.rs | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index a5314df409e7..a7e7f7ccace4 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -257,6 +257,37 @@ pub fn as_arc_borrow(&self) -> ArcBorrow<'_, 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();
+
+        // SAFETY: 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`.
+        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)) };
+
+            // SAFETY: We own one refcount, so we can create a `UniqueArc`. It needs to be pinned,
+            // since an `Arc` is pinned.
+            unsafe {
+                Some(Pin::new_unchecked(UniqueArc {
+                    inner: Arc::from_inner(me.ptr),
+                }))
+            }
+        } else {
+            None
+        }
+    }
 }
 
 /// Converts a pointer to the contents of an [`Arc`] into a pointer to the [`ArcInner`].

-- 
2.44.0.rc0.258.g7320e95886-goog