Re: [PATCH v3 2/2] rust: sync: add `Arc::into_unique_or_drop`

[Boqun Feng]

On Mon, Mar 11, 2024 at 04:08:19PM +0000, Alice Ryhl wrote:
> 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@xxxxxxxxxx>
> ---
>  rust/kernel/sync/arc.rs | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 53addb8876c2..ef8b520f65a8 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -290,6 +290,36 @@ 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();
> +
> +        // 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> {
> 

The code looks good, and FWIW, I added the Examples section for the
function since I at least need something to run when queuing it to
rust-dev branch ;-) And I think it's overall good to have examples for
every `pub` function. If there is a next version (which is unlikely),
feel free to fold it. I can also send it alone once Miguel put your
patches into the rust-next branch.

Regards,
Boqun

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