rust: workqueue: add low-level workqueue bindings

Define basic low-level bindings to a kernel workqueue. The API defined
here can only be used unsafely. Later commits will provide safe
wrappers.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>

rust/kernel/lib.rs

@@ -47,6 +47,7 @@ pub mod str;
 pub mod sync;
 pub mod task;
 pub mod types;
+pub mod workqueue;
 
 #[doc(hidden)]
 pub use bindings;

rust/kernel/workqueue.rs

@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Work queues.
+//!
+//! C header: [`include/linux/workqueue.h`](../../../../include/linux/workqueue.h)
+
+use crate::{bindings, types::Opaque};
+
+/// A kernel work queue.
+///
+/// Wraps the kernel's C `struct workqueue_struct`.
+///
+/// It allows work items to be queued to run on thread pools managed by the kernel. Several are
+/// always available, for example, `system`, `system_highpri`, `system_long`, etc.
+#[repr(transparent)]
+pub struct Queue(Opaque<bindings::workqueue_struct>);
+
+// SAFETY: Kernel workqueues are usable from any thread.
+unsafe impl Send for Queue {}
+unsafe impl Sync for Queue {}
+
+impl Queue {
+    /// Use the provided `struct workqueue_struct` with Rust.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the provided raw pointer is not dangling, that it points at a
+    /// valid workqueue, and that it remains valid until the end of 'a.
+    pub unsafe fn from_raw<'a>(ptr: *const bindings::workqueue_struct) -> &'a Queue {
+        // SAFETY: The `Queue` type is `#[repr(transparent)]`, so the pointer cast is valid. The
+        // caller promises that the pointer is not dangling.
+        unsafe { &*(ptr as *const Queue) }
+    }
+
+    /// Enqueues a work item.
+    ///
+    /// This may fail if the work item is already enqueued in a workqueue.
+    pub fn enqueue<T: WorkItem + Send + 'static>(&self, w: T) -> T::EnqueueOutput {
+        let queue_ptr = self.0.get();
+
+        // SAFETY: There are two cases.
+        //
+        //  1. If `queue_work_on` returns false, then we failed to push the work item to the queue.
+        //     In this case, we don't touch the work item again.
+        //
+        //  2. If `queue_work_on` returns true, then we pushed the work item to the queue. The work
+        //     queue will call the function pointer in the `work_struct` at some point in the
+        //     future. We require `T` to be static, so the type has no lifetimes annotated on it.
+        //     We require `T` to be send, so there are no thread-safety issues to take care of.
+        //
+        // In either case we follow the safety requirements of `__enqueue`.
+        unsafe {
+            w.__enqueue(move |work_ptr| {
+                bindings::queue_work_on(bindings::WORK_CPU_UNBOUND as _, queue_ptr, work_ptr)
+            })
+        }
+    }
+}
+
+/// A work item.
+///
+/// This is the low-level trait that is designed for being as general as possible.
+///
+/// # Safety
+///
+/// Implementers must ensure that `__enqueue` behaves as documented.
+pub unsafe trait WorkItem {
+    /// The return type of [`Queue::enqueue`].
+    type EnqueueOutput;
+
+    /// Enqueues this work item on a queue using the provided `queue_work_on` method.
+    ///
+    /// # Safety
+    ///
+    /// Calling this method guarantees that the provided closure will be called with a raw pointer
+    /// to a `struct work_struct`. The closure should behave in the following way:
+    ///
+    ///  1. If the `struct work_struct` cannot be pushed to a workqueue because its already in one,
+    ///     then the closure should return `false`. It may not access the pointer after returning
+    ///     `false`.
+    ///  2. If the `struct work_struct` is successfully added to a workqueue, then the closure
+    ///     should return `true`. When the workqueue executes the work item, it will do so by
+    ///     calling the function pointer stored in the `struct work_struct`. The work item ensures
+    ///     that the raw pointer remains valid until that happens.
+    ///
+    /// This method may not have any other failure cases than the closure returning `false`. The
+    /// output type should reflect this, but it may also be an infallible type if the work item
+    /// statically ensures that pushing the `struct work_struct` will succeed.
+    ///
+    /// If the work item type is annotated with any lifetimes, then the workqueue must call the
+    /// function pointer before any such lifetime expires. (Or it may forget the work item and
+    /// never call the function pointer at all.)
+    ///
+    /// If the work item type is not [`Send`], then the work item must be executed on the same
+    /// thread as the call to `__enqueue`.
+    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
+    where
+        F: FnOnce(*mut bindings::work_struct) -> bool;
+}