From: Andreas Hindborg <nmi@metaspace.dk>
To: Alice Ryhl <aliceryhl@google.com>
Cc: alex.gaynor@gmail.com, benno.lossin@proton.me,
bjorn3_gh@protonmail.com, boqun.feng@gmail.com, gary@garyguo.net,
jiangshanlai@gmail.com, linux-kernel@vger.kernel.org,
ojeda@kernel.org, patches@lists.linux.dev,
rust-for-linux@vger.kernel.org, tj@kernel.org,
wedsonaf@gmail.com
Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}`
Date: Thu, 25 May 2023 09:45:29 +0200 [thread overview]
Message-ID: <87a5xsetq5.fsf@metaspace.dk> (raw)
In-Reply-To: <20230524111127.2491643-1-aliceryhl@google.com>
Alice Ryhl <aliceryhl@google.com> writes:
> Andreas Hindborg <nmi@metaspace.dk> writes:
>> Alice Ryhl <aliceryhl@google.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
next prev parent reply other threads:[~2023-05-25 7:50 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-17 20:31 [PATCH v1 0/7] Bindings for the workqueue Alice Ryhl
2023-05-17 20:31 ` [PATCH v1 1/7] rust: workqueue: add low-level workqueue bindings Alice Ryhl
2023-05-18 14:51 ` Martin Rodriguez Reboredo
2023-05-19 9:40 ` Alice Ryhl
2023-05-19 12:04 ` Martin Rodriguez Reboredo
2023-05-23 10:03 ` Alice Ryhl
2023-05-30 8:26 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 2/7] rust: add offset_of! macro Alice Ryhl
2023-05-18 14:51 ` Martin Rodriguez Reboredo
2023-05-23 15:48 ` Gary Guo
2023-05-24 12:26 ` Alice Ryhl
2023-05-30 8:40 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` Alice Ryhl
2023-05-18 14:51 ` Martin Rodriguez Reboredo
2023-05-23 15:43 ` Gary Guo
2023-05-24 11:19 ` Alice Ryhl
2023-05-24 10:20 ` Andreas Hindborg
2023-05-24 11:11 ` Alice Ryhl
2023-05-25 7:45 ` Andreas Hindborg [this message]
2023-05-25 16:32 ` Gary Guo
2023-05-30 7:23 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 4/7] rust: workqueue: define built-in queues Alice Ryhl
2023-05-18 14:52 ` Martin Rodriguez Reboredo
2023-05-25 11:40 ` Andreas Hindborg
2023-05-31 14:02 ` Alice Ryhl
2023-06-02 10:23 ` Andreas Hindborg (Samsung)
2023-05-17 20:31 ` [PATCH v1 5/7] rust: workqueue: add helper for defining work_struct fields Alice Ryhl
2023-05-18 23:18 ` Martin Rodriguez Reboredo
2023-05-24 14:50 ` Benno Lossin
2023-05-30 8:44 ` Andreas Hindborg
2023-05-31 9:00 ` Alice Ryhl
2023-05-31 10:18 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 6/7] rust: workqueue: add safe API to workqueue Alice Ryhl
2023-05-19 0:17 ` Martin Rodriguez Reboredo
2023-05-23 11:07 ` Alice Ryhl
2023-05-30 7:19 ` Andreas Hindborg
2023-05-30 13:23 ` Martin Rodriguez Reboredo
2023-05-30 14:13 ` Miguel Ojeda
2023-05-24 14:51 ` Benno Lossin
2023-05-31 9:07 ` Alice Ryhl
2023-05-30 8:51 ` Andreas Hindborg
2023-05-31 14:07 ` Alice Ryhl
2023-05-17 20:31 ` [PATCH v1 7/7] rust: workqueue: add `try_spawn` helper method Alice Ryhl
2023-05-19 0:22 ` Martin Rodriguez Reboredo
2023-05-24 14:52 ` Benno Lossin
2023-05-31 14:03 ` Alice Ryhl
2023-05-17 21:48 ` [PATCH v1 0/7] Bindings for the workqueue Tejun Heo
2023-05-17 22:22 ` Alice Ryhl
2023-05-23 14:08 ` Andreas Hindborg
2023-05-23 14:14 ` Andreas Hindborg
2023-05-24 12:33 ` Alice Ryhl
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