Re: [PATCH 2/6] rust: hrtimer: Add HrTimerCallbackContext and ::forward()

Lyude Paul
2025-04-09T16:59:04Z

On Wed, 2025-04-09 at 09:49 +0200, Andreas Hindborg wrote:
> "Lyude Paul" <lyude@redhat.com> writes:
>
> > On Tue, 2025-04-08 at 13:47 +0200, Andreas Hindborg wrote:
> > > "Lyude Paul" <lyude@redhat.com> writes:
> > >
> > > > With Linux's hrtimer API, certain functions require we either acquire
> > > > proper locking to call specific methods - or that we call said methods from
> > > > the context of the timer callback. hrtimer_forward() is one of these
> > > > functions, so we start by adding a new HrTimerCallbackContext type which
> > > > provides a way of calling these methods that is inaccessible outside of
> > > > hrtimer callbacks.
> > >
> > > Based on tglx comment, we should be able to call this function if the
> > > timer is stopped and we have a unique ownership of the timer. Do you
> > > want to add that? If not, could you add a note about this somewhere?
> >
> >
> > Happy to! So, I think if we were to add a function for this I assume we would
> > want something like this?
> >
> > fn forward(&mut self, now: Instant, interval: Duration) -> u64 {
> > self.cancel();
> > /* Do actual forward stuff here */
> > }
> >
> > Of course with some documentation pointing out that this function will stop
> > the timer if required.
>
> Yes, something like that. My first thought was to check if the timer was
> running and return `Err` if that is the case. But it might be more
> simple to just call `cancel`. What do you think?

Yeah - I considered doing that too, but I think there's a bit more then meets
the eye to what's needed here. I think we might actually want to introduce
some kind of UniqueHrTimerPointer trait. Recall We mentioned earlier that we
have two requirements for this:

* Timer must not be running
* We must have unique ownership of the timer

Basically, I think this limits where we could have a context-less forward() to
types where:

* We can fallibly convert the type into a unique version (e.g. Arc<> ->
UniqueArc<>). In this case, the unique variant would implement both
HrTimerPointer and UniqueHrTimerPoiner.
* The type is unique by nature - e.g. Pin<&'a mut T> and Pin<Box<T, A>>. Here
we'd just implement UniqueHrTimerPointer.

Pin<&'a T> is noticeably absent, because I'm not sure it could fulfill these
requirements. That being said - assuming we fulfill the unique ownership
requirement, I believe that for all the unique aforementioned types it
wouldn't be possible to take out a timer handle when they're in scope anyhow.
So we probably could skip the cancel() call?

>
>
> Best regards,
> Andreas Hindborg
>
>

--
Cheers, Lyude Paul (she/her) Software Engineer at Red Hat Ignore all previous
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