[PATCH v1 2/4] rust: hrtimer: Introduce HrTimerPin to make arming exclusive

[FUJITA Tomonori <tomo@flapping.org>]

From: FUJITA Tomonori <fujita.tomonori@gmail.com>

Pin<&T> has the same hole that Arc<T> had: it is Copy and
ScopedHrTimerPointer::start_scoped() is safe, so a copy captured by the
closure can arm a timer while it is already armed and its callback may
be running.

Split the right to arm out of Pin<&T> into HrTimerPin<'a, T>, which is
created from a Pin<&'a mut T> and consumed by start_scoped(). The borrow
checker supplies the exclusivity here, and the closure keeps reading the
object through the shared pinned reference returned by
HrTimerPin::as_ref().

All four pointer types now separate sharing an object from arming its
timer, so the restart operation no longer exists in the safe API. Drop
it from the documentation.

Fixes: 3f2a5ba784b8 ("rust: hrtimer: Add HrTimerCallbackContext and ::forward()")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/time/hrtimer.rs     |  50 +++++++--------
 rust/kernel/time/hrtimer/pin.rs | 105 +++++++++++++++++++++++---------
 2 files changed, 98 insertions(+), 57 deletions(-)

diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index a7587db1d552..d94275f2e93f 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -9,15 +9,15 @@
 //!
 //! States:
 //!
-//! - Stopped: initialized but not started, or cancelled, or not restarted.
-//! - Started: initialized and started or restarted.
+//! - Stopped: initialized but not started, cancelled, or the callback returned
+//!   `NoRestart`.
+//! - Started: initialized and started, or the callback returned `Restart`.
 //! - Running: executing the callback.
 //!
 //! Operations:
 //!
 //! * Start
 //! * Cancel
-//! * Restart
 //!
 //! Events:
 //!
@@ -42,11 +42,7 @@
 //! --------->|    Stopped      |                 |      Started     +---------->|     Running     |
 //!           |                 |     Cancel      |                  |           |                 |
 //!           |                 |<----------------+                  |           |                 |
-//!           +-----------------+                 +---------------+--+           +-----------------+
-//!                                                     ^         |
-//!                                                     |         |
-//!                                                     +---------+
-//!                                                      Restart
+//!           +-----------------+                 +------------------+           +-----------------+
 //! ```
 //!
 //!
@@ -60,16 +56,13 @@
 //! by the `cancel` operation. A timer that is cancelled enters the **stopped**
 //! state.
 //!
-//! A `cancel` or `restart` operation on a timer in the **running** state takes
-//! effect after the handler has returned and the timer has transitioned
-//! out of the **running** state.
+//! A `cancel` operation on a timer in the **running** state takes effect after
+//! the handler has returned and the timer has transitioned out of the
+//! **running** state.
 //!
-//! A `restart` operation on a timer in the **stopped** state is equivalent to a
-//! `start` operation.
-//!
-//! When a type implements both `HrTimerPointer` and `Clone`, it is possible to
-//! issue the `start` operation while the timer is in the **started** state. In
-//! this case the `start` operation is equivalent to the `restart` operation.
+//! The `start` operation consumes the pointer it is called on, so a timer in the
+//! **started** or **running** state cannot be started again. It has to be
+//! **cancelled** first.
 //!
 //! # Examples
 //!
@@ -253,8 +246,8 @@
 //! #     },
 //! #     time::{
 //! #         hrtimer::{
-//! #             ScopedHrTimerPointer, HrTimer, HrTimerCallback, HrTimerPointer, HrTimerRestart,
-//! #             HasHrTimer, RelativeMode, HrTimerCallbackContext
+//! #             ScopedHrTimerPointer, HrTimer, HrTimerCallback, HrTimerPin, HrTimerPointer,
+//! #             HrTimerRestart, HasHrTimer, RelativeMode, HrTimerCallbackContext
 //! #         },
 //! #         Delta, Monotonic,
 //! #     },
@@ -282,7 +275,7 @@
 //! }
 //!
 //! impl HrTimerCallback for IntrusiveHrTimer {
-//!     type Pointer<'a> = Pin<&'a Self>;
+//!     type Pointer<'a> = HrTimerPin<'a, Self>;
 //!
 //!     fn run(this: Pin<&Self>, _ctx: HrTimerCallbackContext<'_, Self>) -> HrTimerRestart {
 //!         pr_info!("Timer called\n");
@@ -301,9 +294,12 @@
 //! }
 //!
 //! stack_pin_init!( let has_timer = IntrusiveHrTimer::new() );
-//! has_timer.as_ref().start_scoped(Delta::from_micros(200), || {
-//!     while has_timer.flag.load(ordering::Relaxed) != 1 {
-//!         has_timer.cond.wait_for_completion();
+//! let timer_pin = HrTimerPin::new(has_timer);
+//! let shared = timer_pin.as_ref();
+//!
+//! timer_pin.start_scoped(Delta::from_micros(200), || {
+//!     while shared.flag.load(ordering::Relaxed) != 1 {
+//!         shared.cond.wait_for_completion();
 //!     }
 //! });
 //!
@@ -618,7 +614,8 @@ pub trait HrTimerPointer: Sync + Sized {
 /// [`HrTimerHandle`] returned by `start` would be unsound. This is the case for
 /// stack allocated timers.
 ///
-/// Typical implementers are pinned references such as [`Pin<&T>`].
+/// Typical implementers are [`HrTimerPin`] and pinned references such as
+/// [`Pin<&mut T>`].
 ///
 /// # Safety
 ///
@@ -640,8 +637,7 @@ pub unsafe trait UnsafeHrTimerPointer: Sync + Sized {
     /// until the timer is stopped and the callback has completed.
     type TimerHandle: HrTimerHandle;
 
-    /// Start the timer after `expires` time units. If the timer was already
-    /// running, it is restarted at the new expiry time.
+    /// Start the timer after `expires` time units.
     ///
     /// # Safety
     ///
@@ -1111,7 +1107,7 @@ unsafe fn timer_container_of(
 mod arc;
 pub use arc::{ArcHrTimerHandle, HrTimerArc};
 mod pin;
-pub use pin::PinHrTimerHandle;
+pub use pin::{HrTimerPin, PinHrTimerHandle};
 mod pin_mut;
 pub use pin_mut::PinMutHrTimerHandle;
 // `box` is a reserved keyword, so prefix with `t` for timer
diff --git a/rust/kernel/time/hrtimer/pin.rs b/rust/kernel/time/hrtimer/pin.rs
index 4d39ef781697..f44ac07cb722 100644
--- a/rust/kernel/time/hrtimer/pin.rs
+++ b/rust/kernel/time/hrtimer/pin.rs
@@ -10,50 +10,58 @@
 use super::UnsafeHrTimerPointer;
 use core::pin::Pin;
 
-/// A handle for a `Pin<&HasHrTimer>`. When the handle exists, the timer might be
-/// running.
-pub struct PinHrTimerHandle<'a, T>
+/// A wrapper around a pinned shared reference that's guaranteed unique.
+///
+/// The `HrTimerPin` type can be thought of as a special pinned reference to an object that
+/// owns the permission to arm the [`HrTimer`] stored in the object. By ensuring that each
+/// object has only one `HrTimerPin`, the owner of it is assured exclusive access to the arming
+/// operation. Starting a timer consumes the `HrTimerPin`, and the returned
+/// [`PinHrTimerHandle`] keeps the object borrowed, so the timer cannot be armed again until the
+/// handle is dropped.
+///
+/// While this `HrTimerPin` is unique, shared pinned references to the object can still be
+/// obtained with [`HrTimerPin::as_ref`].
+///
+/// # Invariants
+///
+/// * Each object has at most one `HrTimerPin`.
+pub struct HrTimerPin<'a, T>
 where
     T: HasHrTimer<T>,
 {
-    pub(crate) inner: Pin<&'a T>,
+    pin: Pin<&'a T>,
 }
 
-// SAFETY: We cancel the timer when the handle is dropped. The implementation of
-// the `cancel` method will block if the timer handler is running.
-unsafe impl<'a, T> HrTimerHandle for PinHrTimerHandle<'a, T>
+impl<'a, T> HrTimerPin<'a, T>
 where
     T: HasHrTimer<T>,
 {
-    fn cancel(&mut self) -> bool {
-        let self_ptr: *const T = self.inner.get_ref();
-
-        // SAFETY: As we got `self_ptr` from a 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` is derived from a reference, it must point to
-        // a valid and initialized `HrTimer`.
-        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
+    /// Create a `HrTimerPin` from an exclusive pinned reference to a `T`.
+    #[inline]
+    pub fn new(inner: Pin<&'a mut T>) -> Self {
+        // INVARIANT: We have an exclusive reference, so there is no `HrTimerPin` for this
+        // object.
+        Self {
+            pin: inner.into_ref(),
+        }
     }
-}
 
-impl<'a, T> Drop for PinHrTimerHandle<'a, T>
-where
-    T: HasHrTimer<T>,
-{
-    fn drop(&mut self) {
-        self.cancel();
+    /// Get a shared pinned reference to the object.
+    ///
+    /// The returned reference can be used to access the object, but not to arm its timer.
+    #[inline]
+    pub fn as_ref(&self) -> Pin<&'a T> {
+        self.pin
     }
 }
 
 // SAFETY: We capture the lifetime of `Self` when we create a `PinHrTimerHandle`,
 // so `Self` will outlive the handle.
-unsafe impl<'a, T> UnsafeHrTimerPointer for Pin<&'a T>
+unsafe impl<'a, T> UnsafeHrTimerPointer for HrTimerPin<'a, T>
 where
     T: Send + Sync,
     T: HasHrTimer<T>,
-    T: HrTimerCallback<Pointer<'a> = Self>,
+    T: HrTimerCallback<Pointer<'a> = HrTimerPin<'a, T>>,
 {
     type TimerMode = <T as HasHrTimer<T>>::TimerMode;
     type TimerHandle = PinHrTimerHandle<'a, T>;
@@ -63,7 +71,7 @@ unsafe fn start(
         expires: <<T as HasHrTimer<T>>::TimerMode as HrTimerMode>::Expires,
     ) -> Self::TimerHandle {
         // Cast to pointer
-        let self_ptr: *const T = self.get_ref();
+        let self_ptr: *const T = self.pin.get_ref();
 
         // SAFETY:
         //  - As we derive `self_ptr` from a reference above, it must point to a
@@ -71,16 +79,53 @@ unsafe fn start(
         //  - We keep `self` alive by wrapping it in a handle below.
         unsafe { T::start(self_ptr, expires) };
 
-        PinHrTimerHandle { inner: self }
+        PinHrTimerHandle { inner: self.pin }
+    }
+}
+
+/// A handle for a `Pin<&HasHrTimer>`. When the handle exists, the timer might be
+/// running.
+pub struct PinHrTimerHandle<'a, T>
+where
+    T: HasHrTimer<T>,
+{
+    pub(crate) inner: Pin<&'a T>,
+}
+
+// SAFETY: We cancel the timer when the handle is dropped. The implementation of
+// the `cancel` method will block if the timer handler is running.
+unsafe impl<'a, T> HrTimerHandle for PinHrTimerHandle<'a, T>
+where
+    T: HasHrTimer<T>,
+{
+    fn cancel(&mut self) -> bool {
+        let self_ptr: *const T = self.inner.get_ref();
+
+        // SAFETY: As we got `self_ptr` from a 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` is derived from a reference, it must point to
+        // a valid and initialized `HrTimer`.
+        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<'a, T> Drop for PinHrTimerHandle<'a, T>
+where
+    T: HasHrTimer<T>,
+{
+    fn drop(&mut self) {
+        self.cancel();
     }
 }
 
-impl<'a, T> RawHrTimerCallback for Pin<&'a T>
+impl<'a, T> RawHrTimerCallback for HrTimerPin<'a, T>
 where
     T: HasHrTimer<T>,
     T: HrTimerCallback<Pointer<'a> = Self>,
 {
-    type CallbackTarget<'b> = Self;
+    type CallbackTarget<'b> = Pin<&'a T>;
 
     unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
         // `HrTimer` is `repr(C)`
-- 
2.43.0