linux-kernel - Re: [PATCH] rust: hrtimer: introduce hrtimer support

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Date: Tue, 30 Apr 2024 19:05:37 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg
 <nmi@...aspace.dk>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor
 <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker
 <frederic@...nel.org>
Cc: Andreas Hindborg <a.hindborg@...sung.com>, Boqun Feng
 <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn
 Roy Baron
 <bjorn3_gh@...tonmail.com>, Alice Ryhl <aliceryhl@...gle.com>,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: hrtimer: introduce hrtimer support

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