linux-kernel - Re: [PATCH v10 08/13] rust: hrtimer: implement `UnsafeHrTimerPointer` for `Pin<&mut T>`

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Message-ID: <87h6453qmc.fsf@kernel.org>
Date: Fri, 07 Mar 2025 14:41:15 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Benno Lossin" <benno.lossin@...ton.me>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Anna-Maria Behnsen"
 <anna-maria@...utronix.de>,  "Frederic Weisbecker" <frederic@...nel.org>,
  "Thomas Gleixner" <tglx@...utronix.de>,  "Danilo Krummrich"
 <dakr@...nel.org>,  "Alex Gaynor" <alex.gaynor@...il.com>,  "Boqun Feng"
 <boqun.feng@...il.com>,  "Gary Guo" <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor
 Gross" <tmgross@...ch.edu>,  "Lyude Paul" <lyude@...hat.com>,  "Guangbo
 Cui" <2407018371@...com>,  "Dirk Behme" <dirk.behme@...il.com>,  "Daniel
 Almeida" <daniel.almeida@...labora.com>,  "Tamir Duberstein"
 <tamird@...il.com>,  "Markus Elfring" <Markus.Elfring@....de>,
  <rust-for-linux@...r.kernel.org>,  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v10 08/13] rust: hrtimer: implement
 `UnsafeHrTimerPointer` for `Pin<&mut T>`

"Benno Lossin" <benno.lossin@...ton.me> writes:

> On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
>> Allow pinned mutable references to structs that contain a `HrTimer` node to
>> be scheduled with the `hrtimer` subsystem.
>>
>> Acked-by: Frederic Weisbecker <frederic@...nel.org>
>> Reviewed-by: Lyude Paul <lyude@...hat.com>
>> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
>> ---
>>  rust/kernel/time/hrtimer.rs         |   2 +
>>  rust/kernel/time/hrtimer/pin_mut.rs | 101 ++++++++++++++++++++++++++++++++++++
>>  2 files changed, 103 insertions(+)
>>
>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>> index 2ca56397eade..d2791fd624b7 100644
>> --- a/rust/kernel/time/hrtimer.rs
>> +++ b/rust/kernel/time/hrtimer.rs
>> @@ -441,3 +441,5 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
>>  pub use arc::ArcHrTimerHandle;
>>  mod pin;
>>  pub use pin::PinHrTimerHandle;
>> +mod pin_mut;
>> +pub use pin_mut::PinMutHrTimerHandle;
>> diff --git a/rust/kernel/time/hrtimer/pin_mut.rs b/rust/kernel/time/hrtimer/pin_mut.rs
>> new file mode 100644
>> index 000000000000..4f4a9e9602d8
>> --- /dev/null
>> +++ b/rust/kernel/time/hrtimer/pin_mut.rs
>> @@ -0,0 +1,101 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +use super::HasHrTimer;
>> +use super::HrTimer;
>> +use super::HrTimerCallback;
>> +use super::HrTimerHandle;
>> +use super::RawHrTimerCallback;
>> +use super::UnsafeHrTimerPointer;
>> +use crate::time::Ktime;
>> +use core::pin::Pin;
>> +
>> +/// A handle for a `Pin<&mut HasHrTimer>`. When the handle exists, the timer might
>> +/// be running.
>> +pub struct PinMutHrTimerHandle<'a, T>
>> +where
>> +    T: HasHrTimer<T>,
>> +{
>> +    pub(crate) inner: Pin<&'a mut 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 PinMutHrTimerHandle<'a, T>
>> +where
>> +    T: HasHrTimer<T>,
>> +{
>> +    fn cancel(&mut self) -> bool {
>> +        // SAFETY: We are not moving out of `self` or handing out mutable
>> +        // references to `self`.
>> +        let self_ptr = unsafe { self.inner.as_mut().get_unchecked_mut() as *mut T };
>> +
>> +        // 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 PinMutHrTimerHandle<'a, T>
>> +where
>> +    T: HasHrTimer<T>,
>> +{
>> +    fn drop(&mut self) {
>> +        self.cancel();
>> +    }
>> +}
>> +
>> +// SAFETY: We capture the lifetime of `Self` when we create a
>> +// `PinMutHrTimerHandle`, so `Self` will outlive the handle.
>> +unsafe impl<'a, T> UnsafeHrTimerPointer for Pin<&'a mut T>
>> +where
>> +    T: Send + Sync,
>> +    T: HasHrTimer<T>,
>> +    T: HrTimerCallback<Pointer<'a> = Self>,
>> +    Pin<&'a mut T>: RawHrTimerCallback<CallbackTarget<'a> = Self>,
>> +{
>> +    type TimerHandle = PinMutHrTimerHandle<'a, T>;
>> +
>> +    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle {
>> +        // Cast to pointer
>> +        let self_ptr: *const T = <Self as core::ops::Deref>::deref(&self);
>
> You cannot go through a shared reference here, since you convert the
> pointer obtained here in the `run` function later back into a mutable
> reference. You will have to use `get_unchecked_mut` or
> `into_inner_unchecked`.

Thanks, will fix.


Best regards,
Andreas Hindborg