Re: [PATCH v2 2/8] rust: hrtimer: Add HrTimer::raw_forward() and forward()

[Lyude Paul <lyude@redhat.com>]

On Tue, 2025-04-29 at 11:43 +0200, Andreas Hindborg wrote:
> The function must be safe to call whenever the safety requirements are
> satisfied. `self_ptr` pointing to a valid `Self` is not enough for this.
> We must also satisfy the conditions for calling
> `bindings::hrtimer_forward`, which are a) we are in timer context, or b)
> there are no other threads modifying the timer. a) implies b) because
> the hrtimer framework holds a lock in timer context, if I recall
> correctly.

I mentioned in an earlier response that a) might imply b), but a you pointed
out that's mostly only true for types where you can get mutable access in the
first place. And at least within safe code, I think you were correct when you
said we don't really get a &mut to timer, the closest we could get would be
Pin<&mut> in certain cases.  
> 
> We can satisfy these by requiring exclusive access, right? In timer
> context, it is given by C API contract, outside, we just have to have
> &mut ref to the timer.

I don't think so. For one, it's worth pointing out that at least as far as I
can tell in __run_hrtimer() we only hold the lock when updating the timer
struct to indicate that it's being executed. We drop the hrtimer cpu_base lock
for the duration of the callback execution.

But besides that, a lot of the hrtimer API allows you to read various state
regarding a timer without holding any lock, and in parallel with an executing
timer. The expires() function that I added is one such example. So outside of
the exceptions that I mentioned above, we don't ever get a guarantee from
running in the timer callback context that we have exclusive access to the
hrtimer.

Feel free to point out if I'm misunderstanding something here though.

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

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