Re: [PATCH v2 6/8] rust: workqueue: implement `WorkItemPointer` for pointer types

["Andreas Hindborg (Samsung)" <nmi@metaspace.dk>]

Alice Ryhl <aliceryhl@google.com> writes:

> 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.
>
> 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 (Samsung) <nmi@metaspace.dk>

> ---
>  rust/kernel/workqueue.rs | 97 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 96 insertions(+), 1 deletion(-)
>
> 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/workqueue.h)
>  
> -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;
>  
>  /// A kernel work queue.
>  ///
> @@ -323,6 +325,99 @@ unsafe fn raw_get_work(ptr: *mut Self) -> *mut $crate::workqueue::Work<$work_typ
>      )*};
>  }
>  
> +unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Arc<T>
> +where
> +    T: WorkItem<ID, Pointer = Self>,
> +    T: HasWork<T, ID>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct) {
> +        // SAFETY: The `__enqueue` method always uses a `work_struct` stored in a `Work<T, ID>`.
> +        let ptr = ptr as *mut Work<T, ID>;
> +        // SAFETY: This computes the pointer that `__enqueue` got from `Arc::into_raw`.
> +        let ptr = unsafe { T::work_container_of(ptr) };
> +        // SAFETY: This pointer comes from `Arc::into_raw` and we've been given back ownership.
> +        let arc = unsafe { Arc::from_raw(ptr) };
> +
> +        T::run(arc)
> +    }
> +}
> +
> +unsafe impl<T, const ID: u64> RawWorkItem<ID> for Arc<T>
> +where
> +    T: WorkItem<ID, Pointer = Self>,
> +    T: HasWork<T, ID>,
> +{
> +    type EnqueueOutput = Result<(), Self>;
> +
> +    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
> +    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 = Arc::into_raw(self) as *mut T;
> +
> +        // SAFETY: Pointers into an `Arc` point at a valid value.
> +        let work_ptr = unsafe { T::raw_get_work(ptr) };
> +        // SAFETY: `raw_get_work` returns a pointer to a valid value.
> +        let work_ptr = unsafe { Work::raw_get(work_ptr) };
> +
> +        if queue_work_on(work_ptr) {
> +            Ok(())
> +        } else {
> +            // SAFETY: The work queue has not taken ownership of the pointer.
> +            Err(unsafe { Arc::from_raw(ptr) })
> +        }
> +    }
> +}
> +
> +unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Pin<Box<T>>
> +where
> +    T: WorkItem<ID, Pointer = Self>,
> +    T: HasWork<T, ID>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct) {
> +        // SAFETY: The `__enqueue` method always uses a `work_struct` stored in a `Work<T, ID>`.
> +        let ptr = ptr as *mut Work<T, ID>;
> +        // SAFETY: This computes the pointer that `__enqueue` got from `Arc::into_raw`.
> +        let ptr = unsafe { T::work_container_of(ptr) };
> +        // SAFETY: This pointer comes from `Arc::into_raw` and we've been given back ownership.
> +        let boxed = unsafe { Box::from_raw(ptr) };
> +        // SAFETY: The box was already pinned when it was enqueued.
> +        let pinned = 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 = Self>,
> +    T: HasWork<T, ID>,
> +{
> +    type EnqueueOutput = ();
> +
> +    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
> +    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 = unsafe { Pin::into_inner_unchecked(self) };
> +        let ptr = Box::into_raw(boxed);
> +
> +        // SAFETY: Pointers into a `Box` point at a valid value.
> +        let work_ptr = unsafe { T::raw_get_work(ptr) };
> +        // SAFETY: `raw_get_work` returns a pointer to a valid value.
> +        let work_ptr = unsafe { Work::raw_get(work_ptr) };
> +
> +        if !queue_work_on(work_ptr) {
> +            // SAFETY: This method requires exclusive ownership of the box, 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 multi-threaded. There are