[PATCH v10 03/13] rust: hrtimer: implement `HrTimerPointer` for `Arc`

[Andreas Hindborg <a.hindborg@kernel.org>]

Allow the use of intrusive `hrtimer` fields in structs that are managed by
an `Arc` by implementing `HrTimerPointer` and `RawTimerCallbck` for `Arc`.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/time/hrtimer.rs     |  4 +-
 rust/kernel/time/hrtimer/arc.rs | 94 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index 7d7d490f8b6f..bb46602a7749 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -150,7 +150,6 @@ unsafe fn raw_get(this: *const Self) -> *mut bindings::hrtimer {
     /// # Safety
     ///
     /// `this` must point to a valid `Self`.
-    #[allow(dead_code)]
     pub(crate) unsafe fn raw_cancel(this: *const Self) -> bool {
         // SAFETY: `this` points to an allocation of at least `HrTimer` size.
         let c_timer_ptr = unsafe { HrTimer::raw_get(this) };
@@ -357,3 +356,6 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
         }
     }
 }
+
+mod arc;
+pub use arc::ArcHrTimerHandle;
diff --git a/rust/kernel/time/hrtimer/arc.rs b/rust/kernel/time/hrtimer/arc.rs
new file mode 100644
index 000000000000..5c916489fc13
--- /dev/null
+++ b/rust/kernel/time/hrtimer/arc.rs
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::HasHrTimer;
+use super::HrTimer;
+use super::HrTimerCallback;
+use super::HrTimerHandle;
+use super::HrTimerPointer;
+use super::RawHrTimerCallback;
+use crate::sync::Arc;
+use crate::sync::ArcBorrow;
+use crate::time::Ktime;
+
+/// A handle for an `Arc<HasHrTimer<T>>` returned by a call to
+/// [`HrTimerPointer::start`].
+pub struct ArcHrTimerHandle<T>
+where
+    T: HasHrTimer<T>,
+{
+    pub(crate) inner: Arc<T>,
+}
+
+// SAFETY: We implement drop below, and we cancel the timer in the drop
+// implementation.
+unsafe impl<T> HrTimerHandle for ArcHrTimerHandle<T>
+where
+    T: HasHrTimer<T>,
+{
+    fn cancel(&mut self) -> bool {
+        let self_ptr = Arc::as_ptr(&self.inner);
+
+        // SAFETY: As we obtained `self_ptr` from a valid reference above, it
+        // must point to a valid `T`.
+        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self_ptr) };
+
+        // SAFETY: As `timer_ptr` points into `T` and `T` is valid, `timer_ptr`
+        // must point to a valid `HrTimer` instance.
+        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<T> Drop for ArcHrTimerHandle<T>
+where
+    T: HasHrTimer<T>,
+{
+    fn drop(&mut self) {
+        self.cancel();
+    }
+}
+
+impl<T> HrTimerPointer for Arc<T>
+where
+    T: 'static,
+    T: Send + Sync,
+    T: HasHrTimer<T>,
+    T: for<'a> HrTimerCallback<Pointer<'a> = Self>,
+    Arc<T>: for<'a> RawHrTimerCallback<CallbackTarget<'a> = ArcBorrow<'a, T>>,
+{
+    type TimerHandle = ArcHrTimerHandle<T>;
+
+    fn start(self, expires: Ktime) -> ArcHrTimerHandle<T> {
+        // SAFETY:
+        //  - We keep `self` alive by wrapping it in a handle below.
+        //  - Since we generate the pointer passed to `start` from a valid
+        //    reference, it is a valid pointer.
+        unsafe { T::start(Arc::as_ptr(&self), expires) };
+        ArcHrTimerHandle { inner: self }
+    }
+}
+
+impl<T> RawHrTimerCallback for Arc<T>
+where
+    T: 'static,
+    T: HasHrTimer<T>,
+    T: for<'a> HrTimerCallback<Pointer<'a> = Self>,
+{
+    type CallbackTarget<'a> = ArcBorrow<'a, T>;
+
+    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
+        // `HrTimer` is `repr(C)`
+        let timer_ptr = ptr.cast::<super::HrTimer<T>>();
+
+        // SAFETY: By C API contract `ptr` is the pointer we passed when
+        // queuing the timer, so it is a `HrTimer<T>` embedded in a `T`.
+        let data_ptr = unsafe { T::timer_container_of(timer_ptr) };
+
+        // SAFETY: `data_ptr` points to the `T` that was used to queue the
+        // timer. This `T` is contained in an `Arc`.
+        let receiver = unsafe { ArcBorrow::from_raw(data_ptr) };
+
+        T::run(receiver);
+
+        bindings::hrtimer_restart_HRTIMER_NORESTART
+    }
+}

-- 
2.47.0