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Date:   Sun, 11 Jun 2023 16:01:05 +0000
To:     Alice Ryhl <aliceryhl@google.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Tejun Heo <tj@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH v2 6/8] rust: workqueue: implement `WorkItemPointer` for pointer types
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On 01.06.23 15:49, Alice Ryhl wrote:
> This implements the `WorkItemPointer` trait for the pointer types that
> you are likely to use the workqueue with. The `Arc` type is for
> reference counted objects, and the `Pin<Box<T>>` type is for objects
> where the caller has exclusive ownership of the object.
>=20
> Co-developed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

I have a small nit below.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> ---
>   rust/kernel/workqueue.rs | 97 +++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 96 insertions(+), 1 deletion(-)
>=20
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index dbf0aab29a85..f06a2f036d8b 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -28,8 +28,10 @@
>   //!
>   //! C header: [`include/linux/workqueue.h`](../../../../include/linux/w=
orkqueue.h)
>=20
> -use crate::{bindings, prelude::*, types::Opaque};
> +use crate::{bindings, prelude::*, sync::Arc, types::Opaque};
> +use alloc::boxed::Box;
>   use core::marker::{PhantomData, PhantomPinned};
> +use core::pin::Pin;
>=20
>   /// A kernel work queue.
>   ///
> @@ -323,6 +325,99 @@ unsafe fn raw_get_work(ptr: *mut Self) -> *mut $crat=
e::workqueue::Work<$work_typ
>       )*};
>   }
>=20
> +unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Arc<T>
> +where
> +    T: WorkItem<ID, Pointer =3D Self>,
> +    T: HasWork<T, ID>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct) {
> +        // SAFETY: The `__enqueue` method always uses a `work_struct` st=
ored in a `Work<T, ID>`.
> +        let ptr =3D ptr as *mut Work<T, ID>;
> +        // SAFETY: This computes the pointer that `__enqueue` got from `=
Arc::into_raw`.
> +        let ptr =3D unsafe { T::work_container_of(ptr) };
> +        // SAFETY: This pointer comes from `Arc::into_raw` and we've bee=
n given back ownership.
> +        let arc =3D unsafe { Arc::from_raw(ptr) };
> +
> +        T::run(arc)
> +    }
> +}
> +
> +unsafe impl<T, const ID: u64> RawWorkItem<ID> for Arc<T>
> +where
> +    T: WorkItem<ID, Pointer =3D Self>,
> +    T: HasWork<T, ID>,
> +{
> +    type EnqueueOutput =3D Result<(), Self>;
> +
> +    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutpu=
t
> +    where
> +        F: FnOnce(*mut bindings::work_struct) -> bool,
> +    {
> +        // Casting between const and mut is not a problem as long as the=
 pointer is a raw pointer.
> +        let ptr =3D Arc::into_raw(self) as *mut T;

I personally would prefer `cast_mut()` here.

--=20
Cheers,
Benno

> +
> +        // SAFETY: Pointers into an `Arc` point at a valid value.
> +        let work_ptr =3D unsafe { T::raw_get_work(ptr) };
> +        // SAFETY: `raw_get_work` returns a pointer to a valid value.
> +        let work_ptr =3D unsafe { Work::raw_get(work_ptr) };
> +
> +        if queue_work_on(work_ptr) {
> +            Ok(())
> +        } else {
> +            // SAFETY: The work queue has not taken ownership of the poi=
nter.
> +            Err(unsafe { Arc::from_raw(ptr) })
> +        }
> +    }
> +}
> +
> +unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Pin<Box<T>>
> +where
> +    T: WorkItem<ID, Pointer =3D Self>,
> +    T: HasWork<T, ID>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct) {
> +        // SAFETY: The `__enqueue` method always uses a `work_struct` st=
ored in a `Work<T, ID>`.
> +        let ptr =3D ptr as *mut Work<T, ID>;
> +        // SAFETY: This computes the pointer that `__enqueue` got from `=
Arc::into_raw`.
> +        let ptr =3D unsafe { T::work_container_of(ptr) };
> +        // SAFETY: This pointer comes from `Arc::into_raw` and we've bee=
n given back ownership.
> +        let boxed =3D unsafe { Box::from_raw(ptr) };
> +        // SAFETY: The box was already pinned when it was enqueued.
> +        let pinned =3D unsafe { Pin::new_unchecked(boxed) };
> +
> +        T::run(pinned)
> +    }
> +}
> +
> +unsafe impl<T, const ID: u64> RawWorkItem<ID> for Pin<Box<T>>
> +where
> +    T: WorkItem<ID, Pointer =3D Self>,
> +    T: HasWork<T, ID>,
> +{
> +    type EnqueueOutput =3D ();
> +
> +    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutpu=
t
> +    where
> +        F: FnOnce(*mut bindings::work_struct) -> bool,
> +    {
> +        // SAFETY: We're not going to move `self` or any of its fields, =
so its okay to temporarily
> +        // remove the `Pin` wrapper.
> +        let boxed =3D unsafe { Pin::into_inner_unchecked(self) };
> +        let ptr =3D Box::into_raw(boxed);
> +
> +        // SAFETY: Pointers into a `Box` point at a valid value.
> +        let work_ptr =3D unsafe { T::raw_get_work(ptr) };
> +        // SAFETY: `raw_get_work` returns a pointer to a valid value.
> +        let work_ptr =3D unsafe { Work::raw_get(work_ptr) };
> +
> +        if !queue_work_on(work_ptr) {
> +            // SAFETY: This method requires exclusive ownership of the b=
ox, so it cannot be in a
> +            // workqueue.
> +            unsafe { ::core::hint::unreachable_unchecked() }
> +        }
> +    }
> +}
> +
>   /// Returns the system work queue (`system_wq`).
>   ///
>   /// It is the one used by `schedule[_delayed]_work[_on]()`. Multi-CPU m=
ulti-threaded. There are
> --
> 2.41.0.rc0.172.g3f132b7071-goog
>=20