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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Gary Guo" <gary@garyguo.net> writes:

> 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:
[...]
>> > +    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...

Maybe that could work. But my use case does not satisfy the safety
requirements on `ArcBorrow::from_raw`. The`Arc::into_raw` was not
called. Perhaps we can update the requirements for that function?

Best regards,
Andreas