Re: [PATCH v4 4/7] rust: workqueue: add helper for defining work_struct fields

[Andreas Hindborg <a.hindborg@samsung.com>]

Alice Ryhl <aliceryhl@google.com> writes:

> The main challenge with defining `work_struct` fields is making sure
> that the function pointer stored in the `work_struct` is appropriate for
> the work item type it is embedded in. It needs to know the offset of the
> `work_struct` field being used (even if there are several!) so that it
> can do a `container_of`, and it needs to know the type of the work item
> so that it can call into the right user-provided code. All of this needs
> to happen in a way that provides a safe API to the user, so that users
> of the workqueue cannot mix up the function pointers.
>
> There are three important pieces that are relevant when doing this:
>
>  * The pointer type.
>  * The work item struct. This is what the pointer points at.
>  * The `work_struct` field. This is a field of the work item struct.
>
> This patch introduces a separate trait for each piece. The pointer type
> is given a `WorkItemPointer` trait, which pointer types need to
> implement to be usable with the workqueue. This trait will be
> implemented for `Arc` and `Box` in a later patch in this patchset.
> Implementing this trait is unsafe because this is where the
> `container_of` operation happens, but user-code will not need to
> implement it themselves.
>
> The work item struct should then implement the `WorkItem` trait. This
> trait is where user-code specifies what they want to happen when a work
> item is executed. It also specifies what the correct pointer type is.
>
> Finally, to make the work item struct know the offset of its
> `work_struct` field, we use a trait called `HasWork<T, ID>`. If a type
> implements this trait, then the type declares that, at the given offset,
> there is a field of type `Work<T, ID>`. The trait is marked unsafe
> because the OFFSET constant must be correct, but we provide an
> `impl_has_work!` macro that can safely implement `HasWork<T>` on a type.
> The macro expands to something that only compiles if the specified field
> really has the type `Work<T>`. It is used like this:
>
> ```
> struct MyWorkItem {
>     work_field: Work<MyWorkItem, 1>,
> }
>
> impl_has_work! {
>     impl HasWork<MyWorkItem, 1> for MyWorkItem { self.work_field }
> }
> ```
>
> Note that since the `Work` type is annotated with an id, you can have
> several `work_struct` fields by using a different id for each one.
>
> Co-developed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>