Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Feb 21, 2025 at 09:46:08AM -0500, Tamir Duberstein wrote:
> On Fri, Feb 21, 2025 at 9:40 AM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > Hmm... if you mean:
> >
> > trait HasHrTimer {
> >     unsafe fn start(&self, expires: Ktime) {
> >         ...
> >     }
> > }
> >
> > Then it'll be problematic because the pointer derived from `&self`
> > doesn't have write provenance, therefore in a timer callback, the
> > pointer cannot be used for write, which means for example you cannot
> > convert the pointer back into a `Pin<Box<HasTimer>>`.
> >
> > To answer Tamir's question, pointers are heavily used here because we
> > need to preserve the provenance.
> 
> Wouldn't the natural implication be that &mut self is needed? Maybe

For an `Arc<HasTimer>`, you cannot get `&mut self`.

> you can help me understand why pointers can express a contract that
> references can't?

I assume you already know what a pointer provenance is?

	http://doc.rust-lang.org/std/ptr/index.html#provenance

Passing a pointer (including offset operation on it) preserves the
provenance (determined as derive time), however, deriving a pointer from
a reference gives the pointer a provenance based on the reference type.
For example, let's say we have an `Arc<i32>` and a clone:

	let arc = Arc::new(42);
	let clone = arc.clone();

you can obviously do a into_raw() + from_raw() pair:

	let ptr = Arc::into_raw(arc);
	let arc = unsafe { Arc::from_raw(arc) };

however, if you create a reference based on `Arc::into_raw()`, and then
derive a pointer from that, you change the provenance, therefore the
below code would generate UB:

	// cannot mutably borrow because of clone.
	let ptr = unsafe { &*Arc::into_raw(arc) } as *const i32;
    
	let arc = unsafe { Arc::from_raw(ptr) };


(playground code snippet for this example)

	https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=15e051db46c3886b29ed02e579562278

As you already know, the whole thing about pointers/references here is
passing the value to the callback and the callback can "reconstruct" the
data, in such a case, reborrowing in the middle of the chain into a
reference is not necessary, and as the above shows, it can be
problematic.

Hope this could be helpful.

Regards,
Boqun