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 [1].
Boqun Feng has authored the examples.
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Boqun Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected] [1]
Co-developed-by: Boqun Feng <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
---
rust/kernel/sync/arc.rs | 62 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 62 insertions(+)
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 53addb8876c2..30777241d8cf 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -290,6 +290,68 @@ 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.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::{Arc, UniqueArc};
+ ///
+ /// let arc = Arc::try_new(42)?;
+ /// let unique_arc = arc.into_unique_or_drop();
+ ///
+ /// // The above conversion should succeed since refcount of `arc` is 1.
+ /// assert!(unique_arc.is_some());
+ ///
+ /// assert_eq!(*(unique_arc.unwrap()), 42);
+ ///
+ /// # Ok::<(), Error>(())
+ /// ```
+ ///
+ /// ```
+ /// use kernel::sync::{Arc, UniqueArc};
+ ///
+ /// let arc = Arc::try_new(42)?;
+ /// let another = arc.clone();
+ ///
+ /// let unique_arc = arc.into_unique_or_drop();
+ ///
+ /// // The above conversion should fail since refcount of `arc` is >1.
+ /// assert!(unique_arc.is_none());
+ ///
+ /// # Ok::<(), Error>(())
+ /// ```
+ 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> {
--
2.44.0.478.gd926399ef9-goog