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

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

On Sat, Jul 05, 2025 at 12:49:09AM +0200, Benno Lossin wrote:
> On Sat Jul 5, 2025 at 12:30 AM CEST, Boqun Feng wrote:
> > 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.
> 
> Then `Rcu` should be
>     
>     pub struct Rcu<T>(UnsafePinned<Atomic<*mut T>>);
> 
> And `Atomic` shouldn't wrap `UnsafePinned<T>`. Because that prevents
> `&mut Atomic<i32>` to be tagged with `noalias` and that should be fine.
> You should only pay for what you need :)
> 

Fair enough. Changing it to UnsafeCell then.

> >> > 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 a nice example, might be a good idea to put this on
> `Atomic::from_ptr`.
> 

I have something similar in the doc comment of `Atomic::from_ptr()`,
just not an `Atomic<*mut T>`.

> > 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>.
> 
> What does that have to do with `UnsafePinned`? `UnsafeCell` should
> suffice.
> 

I was talking about things like UnsafeCell<*mut T> vs UnsafeCell<isize>
not comparing between UnsafePinned and UnsafeCell.

Regards,
Boqun

> Also where does the provenance interact with `UnsafePinned`?
> 
> ---
> Cheers,
> Benno