Re: [PATCH v10 10/13] rust: hrtimer: implement `HrTimerPointer` for `Pin<Box<T>>`

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

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

> On Fri Mar 7, 2025 at 3:01 PM CET, Andreas Hindborg wrote:
>> "Benno Lossin" <benno.lossin@proton.me> writes:
>>
>>> On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
>>>> Allow `Pin<Box<T>>` to be the target of a timer callback.
>>>>
>>>> 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      |   3 ++
>>>>  rust/kernel/time/hrtimer/tbox.rs | 109 +++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 112 insertions(+)
>>>>
>>>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>>>> index d2791fd624b7..991d37b0524a 100644
>>>> --- a/rust/kernel/time/hrtimer.rs
>>>> +++ b/rust/kernel/time/hrtimer.rs
>>>> @@ -443,3 +443,6 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
>>>>  pub use pin::PinHrTimerHandle;
>>>>  mod pin_mut;
>>>>  pub use pin_mut::PinMutHrTimerHandle;
>>>> +// `box` is a reserved keyword, so prefix with `t` for timer
>>>> +mod tbox;
>>>> +pub use tbox::BoxHrTimerHandle;
>>>> diff --git a/rust/kernel/time/hrtimer/tbox.rs b/rust/kernel/time/hrtimer/tbox.rs
>>>> new file mode 100644
>>>> index 000000000000..a3b2ed849050
>>>> --- /dev/null
>>>> +++ b/rust/kernel/time/hrtimer/tbox.rs
>>>> @@ -0,0 +1,109 @@
>>>> +// 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::prelude::*;
>>>> +use crate::time::Ktime;
>>>> +use core::mem::ManuallyDrop;
>>>> +use core::ptr::NonNull;
>>>> +
>>>> +/// A handle for a [`Box<HasHrTimer<T>>`] returned by a call to
>>>> +/// [`HrTimerPointer::start`].
>>>> +pub struct BoxHrTimerHandle<T, A>
>>>
>>> Should this type implement `Send` and `Sync` depending on `T`?
>>
>> Yes. In practice `T` will always be `Send` and `Sync` because of bounds
>> on other traits.
>>
>> I don't think we have to require `T: Sync`, because the handle does not ever
>> create shared references to the underlying `T`?
>
> Oh I meant to do:
>
>     unsafe impl<T: Send + Sync, A> Send for BoxHrTimerHandle<T, A> {}
>
> But since you don't have it, it might be unnecessary.
>
>>>> +where
>>>> +    T: HasHrTimer<T>,
>>>> +    A: crate::alloc::Allocator,
>>>> +{
>>>> +    pub(crate) inner: NonNull<T>,
>>>> +    _p: core::marker::PhantomData<A>,
>>>> +}
>>>> +
>>>> +// SAFETY: We implement drop below, and we cancel the timer in the drop
>>>> +// implementation.
>>>> +unsafe impl<T, A> HrTimerHandle for BoxHrTimerHandle<T, A>
>>>> +where
>>>> +    T: HasHrTimer<T>,
>>>> +    A: crate::alloc::Allocator,
>>>> +{
>>>> +    fn cancel(&mut self) -> bool {
>>>> +        // SAFETY: As we obtained `self.inner` from a valid reference when we
>>>> +        // created `self`, it must point to a valid `T`.
>>>> +        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self.inner.as_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, A> Drop for BoxHrTimerHandle<T, A>
>>>> +where
>>>> +    T: HasHrTimer<T>,
>>>> +    A: crate::alloc::Allocator,
>>>> +{
>>>> +    fn drop(&mut self) {
>>>> +        self.cancel();
>>>> +        // SAFETY: `self.inner` came from a `Box::into_raw` call
>>>
>>> Please add this as an invariant to `Self`.
>>
>> OK.
>>
>>>
>>>> +        drop(unsafe { Box::<T, A>::from_raw(self.inner.as_ptr()) })
>>>> +    }
>>>> +}
>>>> +
>>>> +impl<T, A> HrTimerPointer for Pin<Box<T, A>>
>>>> +where
>>>> +    T: 'static,
>>>> +    T: Send + Sync,
>>>> +    T: HasHrTimer<T>,
>>>> +    T: for<'a> HrTimerCallback<Pointer<'a> = Pin<Box<T, A>>>,
>>>> +    Pin<Box<T, A>>: for<'a> RawHrTimerCallback<CallbackTarget<'a> = Pin<&'a T>>,
>>>
>>> I don't think this is necessary.
>>
>> Should I remove it? I feel like it communicates intent.
>
> What intent?
>
>>>> +    A: crate::alloc::Allocator,
>>>> +{
>>>> +    type TimerHandle = BoxHrTimerHandle<T, A>;
>>>> +
>>>> +    fn start(self, expires: Ktime) -> Self::TimerHandle {
>>>> +        // SAFETY:
>>>> +        //  - We will not move out of this box during timer callback (we pass an
>>>> +        //    immutable reference to the callback).
>>>> +        //  - `Box::into_raw` is guaranteed to return a valid pointer.
>>>> +        let inner =
>>>> +            unsafe { NonNull::new_unchecked(Box::into_raw(Pin::into_inner_unchecked(self))) };
>>>> +
>>>> +        // 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(inner.as_ptr(), expires) };
>>>> +
>>>> +        BoxHrTimerHandle {
>>>> +            inner,
>>>> +            _p: core::marker::PhantomData,
>>>> +        }
>>>> +    }
>>>> +}
>>>> +
>>>> +impl<T, A> RawHrTimerCallback for Pin<Box<T, A>>
>>>> +where
>>>> +    T: 'static,
>>>> +    T: HasHrTimer<T>,
>>>> +    T: for<'a> HrTimerCallback<Pointer<'a> = Pin<Box<T, A>>>,
>>>> +    A: crate::alloc::Allocator,
>>>> +{
>>>> +    type CallbackTarget<'a> = Pin<&'a T>;
>>>
>>> Why isn't this `Pin<&'a mut T>`?
>>
>> I don't think it matters much? There can be no other mutable references
>> while the callback is running, so why not a shared ref?
>
> IIUC there can be no references to the value, since the user used a
> `Pin<Box<T>>` to schedule the timer.
>
> I thought it might make sense to give a pinned mutable reference, since
> you explicitly implement the `RawHrTimerCallback` for `Pin<&mut T>`.
> Which made me believe one sometimes needs to modify the `T` from the
> callback.
>
> Since we're able to do that when the user used a `Box`, I think we
> should just do it.

I see your point, I will change it.

>
>>>> +
>>>> +    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: We called `Box::into_raw` when we queued the timer.
>>>> +        let tbox = ManuallyDrop::new(Box::into_pin(unsafe { Box::<T, A>::from_raw(data_ptr) }));
>>>
>>> Since you turn this into a reference below and never run the drop, why
>>> not turn the pointer directly into a reference?
>>
>> You mean replace with `unsafe {&*data_ptr};`? I guess that could work,
>> but it hinges on `Box` being transparent which is more subtle than going
>> through the API.
>
> I think that's cleaner. Also why does that rely on `Box` being
> transparent?

I did not think of `Box::into_raw` as returning a pointer to the
underlying `T`, but looking at the signature, it does. I thought it was
returning a pointer to a `Box`.

I will change it as you suggest.


Best regards,
Andreas Hindborg