Re: [PATCH v2 04/14] rust: sync: add `Arc::clone_from_raw`

[Benno Lossin]

On 18.09.24 00:27, Andreas Hindborg wrote:
> Add a method to clone an arc from a pointer to the data managed by the
> `Arc`.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> ---
>  rust/kernel/sync/arc.rs | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a57ea3e2b44c..2c95712d12a2 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -282,6 +282,26 @@ pub unsafe fn from_raw(ptr: *const T) -> Self {
>          unsafe { Self::from_inner(ptr) }
>      }
> 
> +    /// Clones an [`Arc`] instance from a pointer to the contained data.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must point to an allocation that is contained within a live [`Arc<T>`].
> +    pub unsafe fn clone_from_raw(ptr: *const T) -> Self {
> +        // SAFETY: The caller promises that this pointer points to data
> +        // contained in an `Arc` that is still valid.
> +        let inner = unsafe { ArcInner::container_of(ptr).as_ref() };
> +
> +        // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot
> +        // overflow to zero. SAFETY: By the function safety requirement, there
> +        // is necessarily a reference to the object, so it is safe to increment
> +        // the refcount.
> +        unsafe { bindings::refcount_inc(inner.refcount.get()) };
> +
> +        // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
> +        unsafe { Self::from_inner(inner.into()) }

The implementation of this function looks a bit strange to me, how about
this?:

    // SAFETY: this function has the same safety requirements as `from_raw`.
    let arc = unsafe { Self::from_raw(ptr) };
    let clone = arc.clone();
    // Prevent decrementing the refcount.
    mem::forget(arc);
    clone

(of course you would need to change the safety requirements of
`clone_from_raw` to point to `from_raw`)

---
Cheers,
Benno

> +    }
> +
>      /// Returns an [`ArcBorrow`] from the given [`Arc`].
>      ///
>      /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
> --
> 2.46.0
> 
>