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.

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>
Reviewed-by: "Andreas Hindborg (Samsung)" <nmi@metaspace.dk>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

rust/bindings/bindings_helper.h

@@ -12,6 +12,7 @@
 #include <linux/refcount.h>
 #include <linux/wait.h>
 #include <linux/sched.h>
+#include <linux/workqueue.h>
 
 /* `bindgen` gets confused at certain things. */
 const size_t BINDINGS_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;

rust/kernel/lib.rs

@@ -46,6 +46,7 @@ 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 +1,109 @@
+// 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: Accesses to workqueues used by [`Queue`] are thread-safe.
+unsafe impl Send for Queue {}
+// SAFETY: Accesses to workqueues used by [`Queue`] are thread-safe.
+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.
+    ///
+    /// The work item will be submitted using `WORK_CPU_UNBOUND`.
+    pub fn enqueue<W, const ID: u64>(&self, w: W) -> W::EnqueueOutput
+    where
+        W: RawWorkItem<ID> + Send + 'static,
+    {
+        let queue_ptr = self.0.get();
+
+        // SAFETY: We only return `false` if the `work_struct` is already in a workqueue. The other
+        // `__enqueue` requirements are not relevant since `W` is `Send` and static.
+        //
+        // The call to `bindings::queue_work_on` will dereference the provided raw pointer, which
+        // is ok because `__enqueue` guarantees that the pointer is valid for the duration of this
+        // closure.
+        //
+        // Furthermore, if the C workqueue code accesses the pointer after this call to
+        // `__enqueue`, then the work item was successfully enqueued, and `bindings::queue_work_on`
+        // will have returned true. In this case, `__enqueue` promises that the raw pointer will
+        // stay valid until we call the function pointer in the `work_struct`, so the access is ok.
+        unsafe {
+            w.__enqueue(move |work_ptr| {
+                bindings::queue_work_on(bindings::WORK_CPU_UNBOUND as _, queue_ptr, work_ptr)
+            })
+        }
+    }
+}
+
+/// A raw work item.
+///
+/// This is the low-level trait that is designed for being as general as possible.
+///
+/// The `ID` parameter to this trait exists so that a single type can provide multiple
+/// implementations of this trait. For example, if a struct has multiple `work_struct` fields, then
+/// you will implement this trait once for each field, using a different id for each field. The
+/// actual value of the id is not important as long as you use different ids for different fields
+/// of the same struct. (Fields of different structs need not use different ids.)
+///
+/// Note that the id is used only to select the right method to call during compilation. It wont be
+/// part of the final executable.
+///
+/// # Safety
+///
+/// Implementers must ensure that any pointers passed to a `queue_work_on` closure by `__enqueue`
+/// remain valid for the duration specified in the guarantees section of the documentation for
+/// `__enqueue`.
+pub unsafe trait RawWorkItem<const ID: u64> {
+    /// The return type of [`Queue::enqueue`].
+    type EnqueueOutput;
+
+    /// Enqueues this work item on a queue using the provided `queue_work_on` method.
+    ///
+    /// # Guarantees
+    ///
+    /// If this method calls the provided closure, then the raw pointer is guaranteed to point at a
+    /// valid `work_struct` for the duration of the call to the closure. If the closure returns
+    /// true, then it is further guaranteed that the pointer remains valid until someone calls the
+    /// function pointer stored in the `work_struct`.
+    ///
+    /// # Safety
+    ///
+    /// The provided closure may only return `false` if the `work_struct` is already in a workqueue.
+    ///
+    /// If the work item type is annotated with any lifetimes, then you must not call the function
+    /// pointer after any such lifetime expires. (Never calling the function pointer is okay.)
+    ///
+    /// If the work item type is not [`Send`], then the function pointer must be called 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;
+}