[v3,6/9] rust: workqueue: add helper for defining work_struct fields

  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>
---
v2 -> v3:
 * Use __INIT_WORK_WITH_KEY instead of __INIT_WORK.
 * Expand documentation for WorkItemPointer.
 * Expand documentation for HasWork.
 * Removed PhantomPinned from Work. This depends on [1] for correctness.
 * Dropped Reviewed-by annotations due to changes.

[1]: https://lore.kernel.org/all/20230630150216.109789-1-benno.lossin@proton.me/

 rust/helpers.c           |   8 ++
 rust/kernel/workqueue.rs | 240 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 247 insertions(+), 1 deletion(-)
  

On 7/11/23 06:33, Alice Ryhl wrote:
> 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: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
  

> 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>
> ---
> v2 -> v3:
>  * Use __INIT_WORK_WITH_KEY instead of __INIT_WORK.
>  * Expand documentation for WorkItemPointer.
>  * Expand documentation for HasWork.
>  * Removed PhantomPinned from Work. This depends on [1] for correctness.
>  * Dropped Reviewed-by annotations due to changes.
> 
> [1]: https://lore.kernel.org/all/20230630150216.109789-1-benno.lossin@proton.me/
> 
>  rust/helpers.c           |   8 ++
>  rust/kernel/workqueue.rs | 240 ++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 247 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index bb594da56137..f7a04a266f46 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -27,6 +27,7 @@
>  #include <linux/spinlock.h>
>  #include <linux/sched/signal.h>
>  #include <linux/wait.h>
> +#include <linux/workqueue.h>
> 
>  __noreturn void rust_helper_BUG(void)
>  {
> @@ -135,6 +136,13 @@ void rust_helper_put_task_struct(struct task_struct *t)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
> 
> +void rust_helper___INIT_WORK_WITH_KEY(struct work_struct *work, work_func_t func,
> +				      bool on_stack, struct lock_class_key *key)
> +{
> +	__INIT_WORK_WITH_KEY(work, func, on_stack, key);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper___INIT_WORK_WITH_KEY);
> +
>  /*
>   * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
>   * as the Rust `usize` type, so we can use it in contexts where Rust
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index cad039dfc5a9..a7c972bda0ed 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -2,9 +2,34 @@
> 
>  //! Work queues.
>  //!
> +//! This file has two components: The raw work item API, and the safe work item API.
> +//!
> +//! One pattern that is used in both APIs is the `ID` const generic, which exists to allow a single
> +//! type to define multiple `work_struct` fields. This is done by choosing an id for each field,
> +//! and using that id to specify which field you wish to use. (The actual value doesn't matter, as
> +//! long as you use different values for different fields of the same struct.) Since these IDs are
> +//! generic, they are used only at compile-time, so they shouldn't exist in the final binary.
> +//!
> +//! # The raw API
> +//!
> +//! The raw API consists of the `RawWorkItem` trait, where the work item needs to provide an
> +//! arbitrary function that knows how to enqueue the work item. It should usually not be used
> +//! directly, but if you want to, you can use it without using the pieces from the safe API.

"but if you want to" -> "but if you really have to"

And I would swap the raw API section with the safe API section, this will
help guide people to the safe API.

> +//!
> +//! # The safe API
> +//!
> +//! The safe API is used via the `Work` struct and `WorkItem` traits. Furthermore, it also includes
> +//! a trait called `WorkItemPointer`, which is usually not used directly by the user.
> +//!
> +//!  * The `Work` struct is the Rust wrapper for the C `work_struct` type.
> +//!  * The `WorkItem` trait is implemented for structs that can be enqueued to a workqueue.
> +//!  * The `WorkItemPointer` trait is implemented for the pointer type that points at a something
> +//!    that implements `WorkItem`.
> +//!
>  //! C header: [`include/linux/workqueue.h`](../../../../include/linux/workqueue.h)
> 
> -use crate::{bindings, types::Opaque};
> +use crate::{bindings, prelude::*, sync::LockClassKey, types::Opaque};
> +use core::marker::PhantomData;
> 
>  /// A kernel work queue.
>  ///
> @@ -107,6 +132,219 @@ unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
>          F: FnOnce(*mut bindings::work_struct) -> bool;
>  }
> 
> +/// Defines the method that should be called directly when a work item is executed.
> +///
> +/// This trait is implemented by `Pin<Box<T>>` and `Arc<T>`, and is mainly intended to be
> +/// implemented for smart pointer types. For your own structs, you would implement [`WorkItem`]
> +/// instead. The `run` method on this trait will usually just perform the appropriate
> +/// `container_of` translation and then call into the `run` method from the [`WorkItem`] trait.
> +///
> +/// This trait is used when the `work_struct` field is defined using the [`Work`] helper.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that [`__enqueue`] uses a `work_struct` initialized with the [`run`]
> +/// method of this trait as the function pointer.
> +///
> +/// [`__enqueue`]: RawWorkItem::__enqueue
> +/// [`run`]: WorkItemPointer::run
> +pub unsafe trait WorkItemPointer<const ID: u64>: RawWorkItem<ID> {
> +    /// Run this work item.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The provided `work_struct` pointer must originate from a previous call to `__enqueue` where
> +    /// the `queue_work_on` closure returned true, and the pointer must still be valid.
> +    unsafe extern "C" fn run(ptr: *mut bindings::work_struct);
> +}
> +
> +/// Defines the method that should be called when this work item is executed.
> +///
> +/// This trait is used when the `work_struct` field is defined using the [`Work`] helper.
> +pub trait WorkItem<const ID: u64 = 0> {
> +    /// The pointer type that this struct is wrapped in. This will typically be `Arc<Self>` or
> +    /// `Pin<Box<Self>>`.
> +    type Pointer: WorkItemPointer<ID>;
> +
> +    /// The method that should be called when this work item is executed.
> +    fn run(this: Self::Pointer);
> +}
> +
> +/// Links for a work item.
> +///
> +/// This struct contains a function pointer to the `run` function from the [`WorkItemPointer`]
> +/// trait, and defines the linked list pointers necessary to enqueue a work item in a workqueue.
> +///
> +/// Wraps the kernel's C `struct work_struct`.
> +///
> +/// This is a helper type used to associate a `work_struct` with the [`WorkItem`] that uses it.
> +#[repr(transparent)]
> +pub struct Work<T: ?Sized, const ID: u64 = 0> {
> +    work: Opaque<bindings::work_struct>,
> +    _inner: PhantomData<T>,

Should this really be `PhantomData<T>`? Are you dropping `T`s in the 
destructor of `Work<T>`? I do not think so `PhantomData<fn() -> Box<T>>`
should do the trick.

> +}
> +
> +// SAFETY: Kernel work items are usable from any thread.
> +//
> +// We do not need to constrain `T` since the work item does not actually contain a `T`.
> +unsafe impl<T: ?Sized, const ID: u64> Send for Work<T, ID> {}
> +// SAFETY: Kernel work items are usable from any thread.
> +//
> +// We do not need to constrain `T` since the work item does not actually contain a `T`.
> +unsafe impl<T: ?Sized, const ID: u64> Sync for Work<T, ID> {}
> +
> +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(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, key.as_ptr());
> +                Ok(())
> +            })
> +        }

Please do 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, key.as_ptr())
            }),
            _inner: PhantomData,
        })
```

> +    }
> +
> +    /// Get a pointer to the inner `work_struct`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The provided pointer must not be dangling and must be properly aligned. (But the memory
> +    /// need not be initialized.)
> +    #[inline]
> +    pub unsafe fn raw_get(ptr: *const Self) -> *mut bindings::work_struct {
> +        // SAFETY: The caller promises that the pointer is aligned and not dangling.
> +        //
> +        // A pointer cast would also be ok due to `#[repr(transparent)]`. We use `addr_of!` so that
> +        // the compiler does not complain that the `work` field is unused.
> +        unsafe { Opaque::raw_get(core::ptr::addr_of!((*ptr).work)) }
> +    }
> +}
> +
> +/// Declares that a type has a [`Work<T, ID>`] field.
> +///
> +/// The intended way of using this trait is via the [`impl_has_work!`] macro. You can use the macro
> +/// like this:
> +///
> +/// ```no_run
> +/// use kernel::prelude::*;
> +/// use kernel::workqueue::Work;
> +///
> +/// 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.
> +///
> +/// # Safety
> +///
> +/// The [`OFFSET`] constant must be the offset of a field in Self of type [`Work<T, ID>`]. The methods on
> +/// this trait must have exactly the behavior that the definitions given below have.
> +///
> +/// [`Work<T, ID>`]: Work
> +/// [`impl_has_work!`]: crate::impl_has_work
> +/// [`OFFSET`]: HasWork::OFFSET
> +pub unsafe trait HasWork<T, const ID: u64 = 0> {

Is there a reason that `T` is `Sized` here?

--
Cheers,
Benno

> +    /// The offset of the [`Work<T, ID>`] field.
> +    ///
> +    /// [`Work<T, ID>`]: Work
> +    const OFFSET: usize;
> +
> +    /// Returns the offset of the [`Work<T, ID>`] field.
> +    ///
> +    /// This method exists because the [`OFFSET`] constant cannot be accessed if the type is not Sized.
> +    ///
> +    /// [`Work<T, ID>`]: Work
> +    /// [`OFFSET`]: HasWork::OFFSET
> +    #[inline]
> +    fn get_work_offset(&self) -> usize {
> +        Self::OFFSET
> +    }
> +
> +    /// Returns a pointer to the [`Work<T, ID>`] field.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The provided pointer must point at a valid struct of type `Self`.
> +    ///
> +    /// [`Work<T, ID>`]: Work
> +    #[inline]
> +    unsafe fn raw_get_work(ptr: *mut Self) -> *mut Work<T, ID> {
> +        // SAFETY: The caller promises that the pointer is valid.
> +        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut Work<T, ID> }
> +    }
> +
> +    /// Returns a pointer to the struct containing the [`Work<T, ID>`] field.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The pointer must point at a [`Work<T, ID>`] field in a struct of type `Self`.
> +    ///
> +    /// [`Work<T, ID>`]: Work
> +    #[inline]
> +    unsafe fn work_container_of(ptr: *mut Work<T, ID>) -> *mut Self
> +    where
> +        Self: Sized,
> +    {
> +        // SAFETY: The caller promises that the pointer points at a field of the right type in the
> +        // right kind of struct.
> +        unsafe { (ptr as *mut u8).sub(Self::OFFSET) as *mut Self }
> +    }
> +}
> +
> +/// Used to safely implement the [`HasWork<T, ID>`] trait.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::sync::Arc;
> +///
> +/// struct MyStruct {
> +///     work_field: Work<MyStruct, 17>,
> +/// }
> +///
> +/// impl_has_work! {
> +///     impl HasWork<MyStruct, 17> for MyStruct { self.work_field }
> +/// }
> +/// ```
> +///
> +/// [`HasWork<T, ID>`]: HasWork
> +#[macro_export]
> +macro_rules! impl_has_work {
> +    ($(impl$(<$($implarg:ident),*>)?
> +       HasWork<$work_type:ty $(, $id:tt)?>
> +       for $self:ident $(<$($selfarg:ident),*>)?
> +       { self.$field:ident }
> +    )*) => {$(
> +        // SAFETY: The implementation of `raw_get_work` only compiles if the field has the right
> +        // type.
> +        unsafe impl$(<$($implarg),*>)? $crate::workqueue::HasWork<$work_type $(, $id)?> for $self $(<$($selfarg),*>)? {
> +            const OFFSET: usize = $crate::offset_of!(Self, $field) as usize;
> +
> +            #[inline]
> +            unsafe fn raw_get_work(ptr: *mut Self) -> *mut $crate::workqueue::Work<$work_type $(, $id)?> {
> +                // SAFETY: The caller promises that the pointer is not dangling.
> +                unsafe {
> +                    ::core::ptr::addr_of_mut!((*ptr).$field)
> +                }
> +            }
> +        }
> +    )*};
> +}
> +
>  /// Returns the system work queue (`system_wq`).
>  ///
>  /// It is the one used by `schedule[_delayed]_work[_on]()`. Multi-CPU multi-threaded. There are
> --
> 2.41.0.255.g8b1d071c50-goog
>
  

Benno Lossin <benno.lossin@proton.me> writes:
>> +/// Links for a work item.
>> +///
>> +/// This struct contains a function pointer to the `run` function from the [`WorkItemPointer`]
>> +/// trait, and defines the linked list pointers necessary to enqueue a work item in a workqueue.
>> +///
>> +/// Wraps the kernel's C `struct work_struct`.
>> +///
>> +/// This is a helper type used to associate a `work_struct` with the [`WorkItem`] that uses it.
>> +#[repr(transparent)]
>> +pub struct Work<T: ?Sized, const ID: u64 = 0> {
>> +    work: Opaque<bindings::work_struct>,
>> +    _inner: PhantomData<T>,
>
> Should this really be `PhantomData<T>`? Are you dropping `T`s in the 
> destructor of `Work<T>`? I do not think so `PhantomData<fn() -> Box<T>>`
> should do the trick.

Hmm. The `work_struct` effectively has a field of type `fn(T::Pointer)`,
so arguably, that's the type that should be used for the `PhantomData`.
This makes `Work` contravariant instead.

That said, the type `T` is going to be the struct itself, so, for
example, a `MyStruct<'a>` might have a `&'a i32` and a
`Work<MyStruct<'a>>` field, so it ends up being invariant.

By that argument, perhaps we should just make `Work` invariant instead?
We can do that with `PhantomData<*mut T>`.

>> +pub unsafe trait HasWork<T, const ID: u64 = 0> {
> 
> Is there a reason that `T` is `Sized` here?

Hmm, that's an oversight. In v1 of this patchset, the Sized requirement
was correct since T was the pointer type, but now it isn't anymore.

Alice