Re: [PATCH v2 2/2] rust: sync: add `Arc::into_unique_or_drop`

[Benno Lossin <benno.lossin@proton.me>]

On 3/11/24 10:03, Alice Ryhl wrote:
> On Sat, Mar 9, 2024 at 2:02 PM Benno Lossin <benno.lossin@proton.me> wrote:
>>
>> On 2/28/24 14:00, Alice Ryhl wrote:
>>> +        // SAFETY: 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`.
>>
>> This comment is not explaining why it is safe to call
>> `refcount_dec_and_test` on `refcount`.
>> It dose however explain what you are going to do, so please keep it, but
>> not as a SAFETY comment.
> 
> I'll reword.
> 
>>> +        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)) };
>>> +
>>> +            // SAFETY: We own one refcount, so we can create a `UniqueArc`. It needs to be pinned,
>>> +            // since an `Arc` is pinned.
>>
>> The `unsafe` block is only needed due to the `new_unchecked` call, which
>> you could avoid by using `.into()`. The `SAFETY` should also be an
>> `INVARIANT` comment instead.
>>
>>> +            unsafe {
>>> +                Some(Pin::new_unchecked(UniqueArc {
>>> +                    inner: Arc::from_inner(me.ptr),
>>> +                }))
>>> +            }
> 
> The from_inner method is also unsafe.

Ah I missed that, might be a good reason to split the block.
It confused me that the SAFETY comment did not mention why calling
`new_unchecked` is sound.

> I think that using new_unchecked here makes more sense. That method is
> usually used in the case where something is already pinned, whereas
> into() is usually used to pin something that was not previously
> pinned.

I get your argument, but doing it this way avoids an unsafe function
call. I think it would be fine to use `.into()` in this case.
Splitting the unsafe block would also be fine with me.

-- 
Cheers,
Benno