linux-kernel - Re: [PATCH 2/2] rust: workqueue: remove HasWork::OFFSET

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Message-ID: <D8H1FFDMNLR3.STRVYQI7J496@proton.me>
Date: Sat, 15 Mar 2025 18:06:13 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Bjorn Helgaas <bhelgaas@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: workqueue: remove HasWork::OFFSET

On Sat Mar 15, 2025 at 4:37 PM CET, Tamir Duberstein wrote:
> On Sat, Mar 15, 2025 at 5:30 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> On Fri Mar 14, 2025 at 9:44 PM CET, Tamir Duberstein wrote:
>> > On Fri, Mar 14, 2025 at 3:20 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>> >>
>> >> On Fri Mar 7, 2025 at 10:58 PM CET, Tamir Duberstein wrote:
>> >> >      /// Returns a pointer to the struct containing the [`Work<T, ID>`] field.
>> >> >      ///
>> >> >      /// # Safety
>> >> >      ///
>> >> >      /// The pointer must point at a [`Work<T, ID>`] field in a struct of type `Self`.
>> >> > -    #[inline]
>> >> > -    unsafe fn work_container_of(ptr: *mut Work<T, ID>) -> *mut Self
>> >> > -    where
>> >> > -        Self: Sized,
>> >>
>> >> This bound is required in order to allow the usage of `dyn HasWork` (ie
>> >> object safety), so it should stay.
>> >>
>> >> Maybe add a comment explaining why it's there.
>> >
>> > I guess a doctest would be better, but I still don't understand why
>> > the bound is needed. Sorry, can you cite something or explain in more
>> > detail please?
>>
>> Here is a link: https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility
>>
>> But I realized that the trait wasn't object safe to begin with due to
>> the `OFFSET` associated constant. So I'm not sure we need this. Alice,
>> do you need `dyn HasWork`?
>
> I wrote a simple test:

[...]

> so I don't think adding the Sized bound makes sense - we'd end up
> adding it on every item in the trait.

Yeah the `Sized` bound was probably to make the cast work, so let's
remove it.

---
Cheers,
Benno