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Date:   Mon, 25 Sep 2023 16:11:51 +0000
To:     Boqun Feng <boqun.feng@gmail.com>, Alice Ryhl <alice@ryhl.io>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Alice Ryhl <aliceryhl@google.com>, a.hindborg@samsung.com,
        alex.gaynor@gmail.com, bjorn3_gh@protonmail.com, gary@garyguo.net,
        linux-kernel@vger.kernel.org, ojeda@kernel.org,
        rust-for-linux@vger.kernel.org, walmeida@microsoft.com,
        wedsonaf@gmail.com
Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`
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On 25.09.23 18:02, Boqun Feng wrote:
> On Mon, Sep 25, 2023 at 05:30:05PM +0200, Alice Ryhl wrote:
>> On 9/25/23 17:17, Boqun Feng wrote:
>>> On Mon, Sep 25, 2023 at 03:00:47PM +0000, Alice Ryhl wrote:
>>>>>>> I'm concerned about this change, because an `&WithRef<T>` only has
>>>>>>> immutable permissions for the allocation. No pointer derived from i=
t
>>>>>>> may be used to modify the value in the Arc, however, the drop
>>>>>>> implementation of Arc will do exactly that.
>>>>>>
>>>>>> That is indeed a problem. We could put the value in an `UnsafeCell`,=
 but
>>>>>> that would lose us niche optimizations and probably also other optim=
izations.
>>>>>>
>>>>>
>>>>> Not sure I understand the problem here, why do we allow modifying the
>>>>> value in the Arc if you only have a shared ownership?
>>>>
>>>> Well, usually it's when you have exclusive access even though the valu=
e
>>>> is in an `Arc`.
>>>>
>>>> The main example of this is the destructor of the `Arc`. When the last
>>>> refcount drops to zero, this gives you exclusive access. This lets you
>>>> run the destructor. The destructor requires mutable access.
>>>>
>>>> Another example would be converting the `Arc` back into an `UniqueArc`
>>>> by checking that the refcount is 1. Once you have a `UniqueArc`, you c=
an
>>>> use it to mutate the inner value.
>>>>
>>>> Finally, there are methods like `Arc::get_mut_unchecked`, where you
>>>> unsafely assert that nobody else is using the value while you are
>>>> modifying it. We don't have that in our version of `Arc` right now, bu=
t
>>>> we might want to add it later.
>>>>
>>>
>>> Hmm.. but the only way to get an `Arc` from `&WithRef` is
>>>
>>> =09impl From<&WithRef<T>> for Arc<T> {
>>> =09    ...
>>> =09}
>>>
>>> , and we clone `Arc` in the that function (i.e. copying the raw
>>> pointer), so we are still good?
>>>
>>
>> No, the raw pointer in the Arc was created from the immutable reference,=
 so
>> the raw pointer has the same restrictions as the immutable reference did=
.
>>
>=20
> I see, this was the part I was missing. Thanks!
>=20
> Looks like the only fix is replacing `&WithRef<T>` with
> `&UnsafeCell<WithRef<T>>`? But that's a bit wordy and I'm not sure
> whether it's better than `ArcBorrow<'_, T>`...

It should be sufficient to change only the type of the field
within `WithRef`, so no visible API change needed.

--
Cheers,
Benno