Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

[Alice Ryhl]

On 9/26/23 20:20, Boqun Feng wrote:
> On Tue, Sep 26, 2023 at 05:15:52PM +0000, Benno Lossin wrote:
> [...]
> > > But here the difference it that we only derive a `*mut` from a `&`,
> > > rather than transmute to a `&mut`, right? We only use `&` to get a
> > > pointer value (a usize), so I don't think that rule applies here? Or in
> > > other words, does the following implemenation look good to you?
> > >
> > > 	impl<T: ?Sized> Arc<T> {
> > > 	    pub fn as_with_ref(&self) -> &WithRef<T> {
> > > 		// expose
> > > 		let _ = self.ptr.as_ptr() as usize;
> > > 		unsafe { self.ptr.as_ref() }
> > > 	    }
> > > 	}
> > >
> > > 	impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
> > > 	    fn from(b: &WithRef<T>) -> Self {
> > > 		// from exposed
> > > 		let ptr = unsafe { NonNull::new_unchecked(b as *const _ as usize as *mut _) };
> > > 		// SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
> > > 		// guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
> > > 		// increment.
> > > 		ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
> > > 		    .deref()
> > > 		    .clone()
> > > 	    }
> > > 	}
> > >
> > >
> > > An equivalent code snippet is as below (in case anyone wants to try it
> > > in miri):
> > > ```rust
> > >       let raw = Box::into_raw(arc);
> > >
> > >       // as_with_ref()
> > >       let _ = raw as usize;
> > >       let reference = unsafe { &*raw };
> > >
> > >       // from()
> > >       let raw: *mut T = reference as *const _ as usize as  *mut _ ;
> > >
> > >       // drop()
> > >       let arc = unsafe { Box::from_raw(raw) };
> > > ```
> >
> > I don't understand why we are trying to use ptr2int to fix this.
> > Simply wrapping the `T` field inside `WithRef` with `UnsafeCell`
> > should be enough.
>
> BTW, how do you fix this with only wrapping `T` field in `WithRef`?
>
> Let say `WithRef` is defined as:
>
> struct WithRef<T> {
>      refcount: Opaque<bindings::refcount_t>,
>      data: UnsafeCell<T>,
> }
>
>
> impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
>      fn from(b: &WithRef<T>) -> Self {
>          let data_ptr: *mut T = b.data.get();
>
> 	let ptr = ?; // how to get a pointer to `WithRef<T>` with the
>                       // provenance to the whole data?
>
>          ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
>              .deref()
>              .clone()
>      }
> }
>
> The `data_ptr` above only has provenance to part of the struct for the
> similar reason that my proposal of (ab)using `b.refcount.get()`. Am I
> missing something here?

Easiest is to wrap the entire thing:

struct WithRef<T>(UnsafeCell<ArcInner<T>>);

struct ArcInner<T> {
    refcount: Opaque<bindings::refcount_t>,
    data: T,
}

Alice