Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

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

On Sat, Jul 05, 2025 at 12:05:48AM +0200, Benno Lossin wrote:
[..]
> >> 
> >> I don't think there is a big difference between `Opaque<T>` and
> >> `Opaque<T::Repr>` if we have the transmute equivalence between the two.
> >> From a safety perspective, you don't gain or lose anything by using the
> >> first over the second one. They both require the invariant that they are
> >> valid (as `Opaque` removes that... we should really be using
> >> `UnsafeCell` here instead... why aren't we doing that?).
> >> 
> >
> > I need the `UnsafePinned`-like behavior of `Atomic<*mut T>` to support
> > Rcu<T>, and I will replace it with `UnsafePinned`, once that's is
> > available.
> 
> Can you expand on this? What do you mean by "`UnsafePinned`-like
> behavior"? And what does `Rcu<T>` have to do with atomics?
> 

`Rcu<T>` is an RCU protected (atomic) pointer, the its definition is

    pub struct Rcu<T>(Atomic<*mut T>);

I need Pin<&mut Rcu<T>> and &Rcu<T> able to co-exist: an updater will
have the access to Pin<&mut Rcu<T>>, and all the readers will have the
access to &Rcu<T>, for that I need `Atomic<*mut T>` to be
`UnsafePinned`, because `Pin<&mut Rcu<T>>` cannot imply noalias.

> > Maybe that also means `UnsafePinned<T>` make more sense? Because if `T`
> > is a pointer, it's easy to prove the provenance is there. (Note a
> > `&Atomic<*mut T>` may come from a `*mut *mut T`, may be a field in C
> > struct)
> 
> Also don't understand this.
> 

One of the usage of the atomic is being able to communicate with C side,
for example, if we have a struct foo:

    struct foo {
        struct bar *b;
    }

and writer can do this at C side:

   struct foo *f = ...;
   struct bar *b = kcalloc(*b, ...);

   // init b;

   smp_store_release(&f->b, b);

and a reader at Rust side can do:

    #[repr(transparent)]
    struct Bar(binding::bar);
    struct Foo(Opaque<bindings::foo>);

    fn get_bar(foo: &Foo) {
        let foo_ptr = foo.0.get();

	let b: *mut *mut Bar = unsafe { &raw mut (*foo_ptr).b }.cast();
	// SAFETY: C side accessing this pointer with atomics.
        let b = unsafe { Atomic::<*mut Bar>::from_ptr(b) };

        // Acquire pairs with the Release from C side;
	let bar_ptr = b.load(Acquire);

	// accessing bar.
    }

This is the case we must support if we want to write any non-trivial
synchronization code communicate with C side.

And in this case, it's generally easier to reason why we can convert a
*mut *mut Bar to &UnsafePinned<*mut Bar>.

Regards,
Boqun

> ---
> Cheers,
> Benno