linux-kernel - Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into

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Message-ID: <87a5xsetq5.fsf@metaspace.dk>
Date:   Thu, 25 May 2023 09:45:29 +0200
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     alex.gaynor@...il.com, benno.lossin@...ton.me,
        bjorn3_gh@...tonmail.com, boqun.feng@...il.com, gary@...yguo.net,
        jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        ojeda@...nel.org, patches@...ts.linux.dev,
        rust-for-linux@...r.kernel.org, tj@...nel.org, wedsonaf@...il.com
Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}`


Alice Ryhl <aliceryhl@...gle.com> writes:

> Andreas Hindborg <nmi@...aspace.dk> writes:
>> Alice Ryhl <aliceryhl@...gle.com> writes:
>>> +        // This preserves the metadata in the pointer, if any.
>>> +        let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
>> 
>> I can't follow this. How does this work? `ptr` was for field
>> `inner.data: T`, but we are casting to `ArcInner<T>`.
>> 
>>> +        let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
>>> +        let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>> 
>> Metadata was obtained from a pointer pointing to `inner.data`, we then
>> move it back to beginning of `ArcInner<T>` and then reconstruct the
>> potentially fat pointer with metadata from the pointer to `T`? How can
>> this be right?
>
> The metadata of a struct is always the metadata of its last field, so
> both `*mut T` and `*mut ArcInner<T>` have the same metadata. Because of
> that, moving the metadata over from one type to the other is ok.
>
> The reason that I cast to an `ArcInner<T>` pointer before calling
> `metadata` is because I get a type mismatch otherwise for the metadata,
> since the compiler doesn't unify the metadata types when the type is
> generic.

OK, cool. In that case, since this is common knowledge (is it?),
could you maybe include a link to the relevant documentation, or a
comment indicating why this is OK?

BR Andreas