Re: [PATCH] rust: hrtimer: introduce hrtimer support

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Apr 26 2024 at 07:52, Benno Lossin wrote:
> On 25.04.24 11:46, Andreas Hindborg wrote:
>> +#[pinned_drop]
>> +impl<T> PinnedDrop for Timer<T> {
>> +    fn drop(self: Pin<&mut Self>) {
>> +        // SAFETY: By struct invariant `self.timer` was initialized by
>> +        // `hrtimer_init` so by C API contract it is safe to call
>> +        // `hrtimer_cancel`.
>> +        unsafe {
>> +            bindings::hrtimer_cancel(self.timer.get());
>> +        }
>> +    }
>> +}
>
> Why is this needed? The only way to schedule a timer using this API is
> by having an `Arc` with a timer-containing struct inside. But to
> schedule the `Arc`, you consume one refcount which is then sent to the
> timer subsystem. So it is impossible for the refcount to drop below zero
> while the timer is scheduled, but not yet running.
> Do you need to call `hrtimer_cancel` after/while a timer is running?
>
> Also is it ok to call `hrtimer_cancel` inside the timer callback? Since
> that can happen when the timer callback owns the last refcount.

You cannot invoke hrtimer_cancel() from within the callback. That
deadlocks because hrtimer_cancel() waits for the callback to complete.

Thanks,

        tglx