Date:   Tue, 30 May 2023 10:51:14 +0200
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH v1 6/7] rust: workqueue: add safe API to workqueue


Alice Ryhl <aliceryhl@...gle.com> writes:

> This commit introduces `ArcWorkItem`, `BoxWorkItem`, and
> `define_work_adapter_newtype!` that make it possible to use the
> workqueue without any unsafe code whatsoever.
>
> The `ArcWorkItem` and `BoxWorkItem` traits are used when a struct has a
> single `work_struct` field.
>
> The `define_work_adapter_newtype!` macro is used when a struct has
> multiple `work_struct` fields. For each `work_struct` field, a newtype
> struct is defined that wraps `Arc<TheStruct>`, and pushing an instance
> of the newtype to a workqueue will enqueue it using the associated
> `work_struct` field. The newtypes are matched with `work_struct` fields
> by having the T in `Work<T>` be the newtype.
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/workqueue.rs | 332 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 331 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index 7509618af252..007005ddcaf0 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -4,8 +4,9 @@
>  //!
>  //! 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 core::marker::{PhantomData, PhantomPinned};
> +use core::result::Result;
>  
>  /// A kernel work queue.
>  ///
> @@ -279,6 +280,335 @@ macro_rules! impl_has_work {
>      )*};
>  }
>  
> +/// Declares that [`Arc<Self>`] should implement [`WorkItem`].
> +///
> +/// # Examples
> +///
> +/// The example below will make [`Arc<MyStruct>`] implement the [`WorkItem`] trait so that you can
> +/// enqueue it in a workqueue.
> +///
> +/// ```
> +/// use kernel::sync::Arc;
> +///
> +/// struct MyStruct {
> +///     work_field: Work<Arc<MyStruct>>,
> +/// }
> +///
> +/// kernel::impl_has_work! {
> +///     impl HasWork<Arc<MyStruct>> for MyStruct { self.work_field }
> +/// }
> +///
> +/// impl ArcWorkItem for MyStruct {
> +///     fn run(self: Arc<Self>) {
> +///         pr_info!("Executing MyStruct on a workqueue.");
> +///     }
> +/// }
> +/// ```
> +///
> +/// [`Arc<Self>`]: crate::sync::Arc
> +/// [`Arc<MyStruct>`]: crate::sync::Arc
> +pub trait ArcWorkItem {
> +    /// Called when this work item is executed.
> +    fn run(self: Arc<Self>);
> +}
> +
> +unsafe impl<T> WorkItem for Arc<T>
> +where
> +    T: ArcWorkItem + HasWork<Self> + ?Sized,
> +{
> +    type EnqueueOutput = Result<(), Self>;
> +
> +    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
> +    where
> +        F: FnOnce(*mut bindings::work_struct) -> bool,
> +    {
> +        let ptr = Arc::into_raw(self);
> +
> +        // Using `get_work_offset` here for object-safety.
> +        //
> +        // SAFETY: The pointer is valid since we just got it from `into_raw`.
> +        let off = unsafe { (&*ptr).get_work_offset() };
> +
> +        // SAFETY: The `HasWork` impl promises that this offset gives us a field of type
> +        // `Work<Self>` in the same allocation.
> +        let work_ptr = unsafe { (ptr as *const u8).add(off) as *const Work<Self> };

We have this functionality in the default impl of
`HasWork<T>::raw_get_work() where Self: Sized`. I am uncertain about the
`Sized` bound. If it is sound to do the offset calculation here where
`T: ?Sized`, it should also be sound in the default implementation of
`HasWork<T>`. Should we not be able to change the bound on
`HasWork<T>::raw_get_work()` to `Self: ?Sized` and call into that from
here?

        let work_ptr = unsafe { <T as HasWork<Self>>::raw_get_work(ptr as _) };

Same for Box.

BR Andreas

> +        // SAFETY: The pointer is not dangling.
> +        let work_ptr = unsafe { Work::raw_get(work_ptr) };
> +
> +        match (queue_work_on)(work_ptr) {
> +            true => Ok(()),
> +            // SAFETY: The work queue has not taken ownership of the pointer.
> +            false => Err(unsafe { Arc::from_raw(ptr) }),
> +        }
> +    }
> +}
> +
> +// Let `Work<Arc<T>>` be usable with types that are `ArcWorkItem`.
> +//
> +// We do not allow unsized types here. The `Work<Arc<T>>` field should always specify the actual
> +// concrete type stored in the `Arc`.
> +//
> +// SAFETY: The `Work<Arc<T>>` field must be initialized with this `run` method because the `Work`
> +// struct prevents you from initializing it in any other way. The `__enqueue` trait uses the
> +// same `Work<Arc<T>>` field because `HasWork` promises to always return the same field.
> +unsafe impl<T> WorkItemAdapter for Arc<T>
> +where
> +    T: ArcWorkItem + HasWork<Self> + Sized,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct) {
> +        // SAFETY: The `__enqueue` method always uses a `work_struct` stored in a `Work<Self>`.
> +        let ptr = ptr as *mut Work<Self>;
> +        // 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) };
> +
> +        arc.run();
> +    }
> +}
> +
> +/// Declares that [`Pin`]`<`[`Box`]`<Self>>` should implement [`WorkItem`].
> +///
> +/// # Examples
> +///
> +/// The example below will make [`Pin`]`<`[`Box`]`<MyStruct>>` implement the [`WorkItem`] trait so
> +/// that you can enqueue it in a workqueue.
> +///
> +/// ```
> +/// struct MyStruct {
> +///     work_field: Work<Pin<Box<MyStruct>>>,
> +/// }
> +///
> +/// kernel::impl_has_work! {
> +///     impl HasWork<Pin<Box<MyStruct>>> for MyStruct { self.work_field }
> +/// }
> +///
> +/// impl BoxWorkItem for MyStruct {
> +///     fn run(self: Pin<Box<MyStruct>>) {
> +///         pr_info!("Executing MyStruct on a workqueue.");
> +///     }
> +/// }
> +/// ```
> +///
> +/// [`Box`]: alloc::boxed::Box
> +/// [`Pin`]: core::pin::Pin
> +pub trait BoxWorkItem {
> +    /// Called when this work item is executed.
> +    fn run(self: Pin<Box<Self>>);
> +}
> +
> +unsafe impl<T> WorkItem for Pin<Box<T>>
> +where
> +    T: BoxWorkItem + HasWork<Self> + ?Sized,
> +{
> +    // When a box is in a workqueue, the workqueue has exclusive ownership of the box. Therefore,
> +    // it's not possible to enqueue a box while it is in a workqueue.
> +    type EnqueueOutput = ();
> +
> +    unsafe fn __enqueue<F>(self, queue_work_on: F)
> +    where
> +        F: FnOnce(*mut bindings::work_struct) -> bool,
> +    {
> +        // SAFETY: We will not used the contents in an unpinned manner.
> +        let ptr = unsafe { Box::into_raw(Pin::into_inner_unchecked(self)) };
> +
> +        // Using `get_work_offset` here for object-safety.
> +        //
> +        // SAFETY: The pointer is valid since we just got it from `into_raw`.
> +        let off = unsafe { (&*ptr).get_work_offset() };
> +
> +        // SAFETY: The `HasWork` impl promises that this offset gives us a field of type
> +        // `Work<Self>` in the same allocation.
> +        let work_ptr = unsafe { (ptr as *mut u8).add(off) as *mut Work<Self> };
> +        // SAFETY: The pointer is not dangling.
> +        let work_ptr = unsafe { Work::raw_get(work_ptr) };
> +
> +        match (queue_work_on)(work_ptr) {
> +            true => {}
> +            // SAFETY: This method requires exclusive ownership of the box, so it cannot be in a
> +            // workqueue.
> +            false => unsafe { core::hint::unreachable_unchecked() },
> +        }
> +    }
> +}
> +
> +// Let `Work<Pin<Box<T>>>` be usable with types that are `BoxWorkItem`.
> +//
> +// We do not allow unsized types here. The `Work<Pin<Box<T>>>` field should always specify the actual
> +// concrete type stored in the `Box`.
> +//
> +// SAFETY: The `Work<Pin<Box<T>>>` field must be initialized with this run method because the `Work`
> +// struct prevents you from initializing it in any other way. The `__enqueue` trait uses the
> +// same `Work<Pin<Box<T>>>` field because `HasWork` promises to always return the same field.
> +unsafe impl<T> WorkItemAdapter for Pin<Box<T>>
> +where
> +    T: BoxWorkItem + HasWork<Self> + Sized,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct) {
> +        // SAFETY: The `__enqueue` method always uses a `work_struct` stored in a `Work<Self>`.
> +        let ptr = ptr as *mut Work<Self>;
> +        // 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 `Box::into_raw` and we've been given back ownership.
> +        // The box was originally pinned, so pinning it again is ok.
> +        let boxed = unsafe { Pin::new_unchecked(Box::from_raw(ptr)) };
> +
> +        boxed.run();
> +    }
> +}
> +
> +/// Helper macro for structs with several `Work` fields that can be in several queues at once.
> +///
> +/// For each `Work` field in your type `T`, a newtype struct that wraps an `Arc<T>` or
> +/// `Pin<Box<T>>` should be defined.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// struct MyStruct {
> +///     work1: Work<MyStructWork1>,
> +///     work2: Work<MyStructWork2>,
> +/// }
> +///
> +/// impl_has_work! {
> +///     impl HasWork<MyStructWork1> for MyStruct { self.work1 }
> +///     impl HasWork<MyStructWork2> for MyStruct { self.work2 }
> +/// }
> +///
> +/// define_work_adapter_newtype! {
> +///     struct MyStructWork1(Arc<MyStruct>);
> +///     struct MyStructWork2(Arc<MyStruct>);
> +/// }
> +///
> +/// impl MyStructWork1 {
> +///     fn run(self) {
> +///         // ...
> +///     }
> +/// }
> +///
> +/// impl MyStructWork2 {
> +///     fn run(self) {
> +///         // ...
> +///     }
> +/// }
> +/// ```
> +///
> +/// This will let you push a `MyStructWork1(arc)` or `MyStructWork2(arc)` to a work queue. The [`Arc`]
> +/// can be in two work queues at the same time, and the `run` method on the wrapper type is called
> +/// when the work item is called.
> +///
> +/// [`Arc`]: crate::sync::Arc
> +#[macro_export]
> +macro_rules! define_work_adapter_newtype {
> +    (
> +        $(#[$outer:meta])*
> +        $pub:vis struct $name:ident(
> +            $(#[$innermeta:meta])*
> +            $fpub:vis Arc<$inner:ty> $(,)?
> +        );
> +        $($rest:tt)*
> +    ) => {
> +        $(#[$outer])*
> +        $pub struct $name($(#[$innermeta])* $fpub $crate::sync::Arc<$inner>);
> +
> +        unsafe impl $crate::workqueue::WorkItem for $name {
> +            type EnqueueOutput = ::core::result::Result<(), $name>;
> +
> +            unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
> +            where
> +                F: ::core::ops::FnOnce(*mut $crate::bindings::work_struct) -> bool,
> +            {
> +                let ptr = $crate::sync::Arc::into_raw(self.0);
> +
> +                // SAFETY: The pointer is not dangling since we just got it from Arc::into_raw.
> +                let work_ptr = unsafe { <$inner as $crate::workqueue::HasWork::<$name>>::raw_get_work(ptr.cast_mut()) };
> +
> +                // SAFETY: The pointer is not dangling.
> +                let work_ptr = unsafe { $crate::workqueue::Work::raw_get(work_ptr) };
> +
> +                match (queue_work_on)(work_ptr) {
> +                    true => Ok(()),
> +                    // SAFETY: The work queue has not taken ownership of the pointer.
> +                    false => Err($name(unsafe { $crate::sync::Arc::from_raw(ptr) })),
> +                }
> +            }
> +        }
> +
> +        unsafe impl $crate::workqueue::WorkItemAdapter for $name {
> +            unsafe extern "C" fn run(ptr: *mut $crate::bindings::work_struct) {
> +                // SAFETY: The `__enqueue` method always uses a `work_struct` stored in a `Work<Self>`.
> +                let ptr = ptr as *mut $crate::workqueue::Work<Self>;
> +                // SAFETY: This computes the pointer that `__enqueue` got from `Arc::into_raw`.
> +                let ptr = unsafe { <$inner as $crate::workqueue::HasWork::<$name>>::work_container_of(ptr) };
> +                // SAFETY: This pointer comes from `Arc::into_raw` and we've been given back ownership.
> +                let arc = unsafe { $crate::sync::Arc::from_raw(ptr) };
> +
> +                $name::run($name(arc));
> +            }
> +        }
> +
> +        define_work_adapter_newtype! { $($rest)* }
> +    };
> +
> +    (
> +        $(#[$outer:meta])*
> +        $pub:vis struct $name:ident(
> +            $(#[$innermeta:meta])*
> +            $fpub:vis Pin<Box<$inner:ty>> $(,)?
> +        );
> +        $($rest:tt)*
> +    ) => {
> +        $(#[$outer])*
> +        $pub struct $name($(#[$innermeta])* $fpub ::core::pin::Pin<::alloc::boxed::Box<$inner>>);
> +
> +        unsafe impl $crate::workqueue::WorkItem for $name {
> +            type EnqueueOutput = ();
> +
> +            unsafe fn __enqueue<F>(self, queue_work_on: F)
> +            where
> +                F: ::core::ops::FnOnce(*mut $crate::bindings::work_struct) -> bool,
> +            {
> +                // SAFETY: We will not used the contents in an unpinned manner.
> +                let boxed = unsafe { ::core::pin::Pin::into_inner_unchecked(self.0) };
> +                let ptr = ::alloc::boxed::Box::into_raw(boxed);
> +
> +                // SAFETY: The pointer is not dangling since we just got it from Box::into_raw.
> +                let work_ptr = unsafe { <$inner as $crate::workqueue::HasWork::<$name>>::raw_get_work(ptr) };
> +
> +                // SAFETY: The pointer is not dangling.
> +                let work_ptr = unsafe { $crate::workqueue::Work::raw_get(work_ptr) };
> +
> +                match (queue_work_on)(work_ptr) {
> +                    true => {},
> +                    // SAFETY: This method requires exclusive ownership of the box, so it cannot be in a
> +                    // workqueue.
> +                    false => unsafe { ::core::hint::unreachable_unchecked() },
> +                }
> +            }
> +        }
> +
> +        unsafe impl $crate::workqueue::WorkItemAdapter for $name {
> +            unsafe extern "C" fn run(ptr: *mut $crate::bindings::work_struct) {
> +                // SAFETY: The `__enqueue` method always uses a `work_struct` stored in a `Work<Self>`.
> +                let ptr = ptr as *mut $crate::workqueue::Work<Self>;
> +                // SAFETY: This computes the pointer that `__enqueue` got from `Arc::into_raw`.
> +                let ptr = unsafe { <$inner as $crate::workqueue::HasWork::<$name>>::work_container_of(ptr) };
> +                // SAFETY: This pointer comes from `Box::into_raw` and we've been given back ownership.
> +                let boxed = unsafe { ::alloc::boxed::Box::from_raw(ptr) };
> +                // SAFETY: The box was originally pinned, so pinning it again is ok.
> +                let boxed = unsafe { ::core::pin::Pin::new_unchecked(boxed) };
> +
> +                $name::run($name(boxed));
> +            }
> +        }
> +
> +        define_work_adapter_newtype! { $($rest)* }
> +    };
> +
> +    // After processing the last definition, we call ourselves with no input.
> +    () => {};
> +}
> +
>  /// Returns the system work queue (`system_wq`).
>  ///
>  /// It is the one used by `schedule[_delayed]_work[_on]()`. Multi-CPU multi-threaded. There are

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