linux-kernel - Re: [PATCH] rust: arc: remove unused PhantomData

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Message-ID: <CAJ-ks9=xW_WWZXB0CbDvU-3otkYs-TY+PSYeiPyidM58QujC3g@mail.gmail.com>
Date: Tue, 5 Nov 2024 16:13:22 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: arc: remove unused PhantomData

On Tue, Nov 5, 2024 at 8:29 AM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> On Mon, Nov 4, 2024 at 11:42 PM Boqun Feng <boqun.feng@...il.com> wrote:
> >
> > I believe we need this `PhantomData` to inform drop chec [1] that the
> > drop of `Arc` may result into the drop of an `ArcInner<T>`. Rust std
> > `Arc` has the similar definition [2], you can find more information
> > about PhantomData usage on drop checking at [3].
>
> Hmm... But they use `may_dangle` in their `Drop` and we don't (and we
> have a `Drop` unlike something like `Unique`), no? Or am I confused?
> i.e. if I understand correctly, that would have been needed in the
> past, but not anymore.

Doing a bit of archaeology I found the reasoning for the presence of
`PhantomData` in std's `Arc`[0]. The TL;DR is that the presence of a
type parameter `T` implies "owns T", but `Arc` owns `ArcInner<T>`
rather than `T`. Thus in order to get correct dropck behavior it is
necessary to opt out of "owns T" using `may_dangle` and opt into "owns
ArcInner<T>" using `PhantomData`.

Please check my understanding; I couldn't find detailed documentation
of the interaction between `may_dangle` and `PhantomData`. If this
sounds correct, should we add `may_dangle` to our dropck? compile-fail
tests would be useful here.

Cheers,
Tamir

Link: https://github.com/rust-lang/rust/pull/46749#issuecomment-352146569 [0]