linux-kernel - Re: [PATCH v4 4/7] rust: workqueue: add helper for defining work

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Date:   Tue, 26 Sep 2023 10:01:30 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     benno.lossin@...ton.me
Cc:     alex.gaynor@...il.com, aliceryhl@...gle.com,
        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 v4 4/7] rust: workqueue: add helper for defining
 work_struct fields

>> +impl<T: ?Sized, const ID: u64> Work<T, ID> {
>> +    /// Creates a new instance of [`Work`].
>> +    #[inline]
>> +    #[allow(clippy::new_ret_no_self)]
>> +    pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self>
>> +    where
>> +        T: WorkItem<ID>,
>> +    {
>> +        // SAFETY: The `WorkItemPointer` implementation promises that `run` can be used as the work
>> +        // item function.
>> +        unsafe {
>> +            kernel::init::pin_init_from_closure(move |slot| {
>> +                let slot = Self::raw_get(slot);
>> +                bindings::init_work_with_key(
>> +                    slot,
>> +                    Some(T::Pointer::run),
>> +                    false,
>> +                    name.as_char_ptr(),
>> +                    key.as_ptr(),
>> +                );
>> +                Ok(())
>> +            })
>> +        }
> 
> I would suggest this instead:
> ```
>         pin_init!(Self {
>             // SAFETY: The `WorkItemPointer` implementation promises that `run` can be used as the
>             // work item function.
>             work <- Opaque::ffi_init(|slot| unsafe {
>                 bindings::init_work_with_key(
>                     slot,
>                     Some(T::Pointer::run),
>                     false,
>                     name.as_char_ptr(),
>                     key.as_ptr(),
>                 )
>             }),
>             _inner: PhantomData,
>         })
> ```

I tried setting up a patch with a few nits fixed, including this one.
However, I ran into an error when adding #[pin_data] to the Work struct:

error: defaults for const parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
   --> rust/kernel/workqueue.rs:192:28
    |
192 | pub struct Work<T: ?Sized, const ID: u64 = 0> {
    |                            ^^^^^^^^^^^^^^^^^

This nit can be fixed when #[pin_data] is updated to support default
values in struct declarations.

Alice