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

[Gary Guo <gary@garyguo.net>]

On Wed, 18 Sep 2024 18:19:20 +0000
Benno Lossin <benno.lossin@proton.me> wrote:

> 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@kernel.org>
> > ---
> >  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`)

Wouldn't this function simply be

	// SAFETY: ...
	let borrow = unsafe { ArcBorrow::from_raw(ptr) }
  	borrow.into()

?

Maybe this function doesn't even need to exist...

Best,
Gary