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Message-ID: <ZRIB0hXNvmJtmyak@boqun-archlinux>
Date: Mon, 25 Sep 2023 14:55:30 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
Wedson Almeida Filho <wedsonaf@...il.com>,
rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>,
Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Andreas Hindborg <a.hindborg@...sung.com>,
linux-kernel@...r.kernel.org,
Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of
`WithRef`
On Mon, Sep 25, 2023 at 09:03:52PM +0000, Benno Lossin wrote:
> On 25.09.23 20:51, Boqun Feng wrote:
> > On Mon, Sep 25, 2023 at 05:00:45PM +0000, Benno Lossin wrote:
> >> On 25.09.23 18:16, Boqun Feng wrote:
> >>> On Mon, Sep 25, 2023 at 03:07:44PM +0000, Benno Lossin wrote:
> >>>> ```rust
> >>>> struct MutatingDrop {
> >>>> value: i32,
> >>>> }
> >>>>
> >>>> impl Drop for MutatingDrop {
> >>>> fn drop(&mut self) {
> >>>> self.value = 0;
> >>>> }
> >>>> }
> >>>>
> >>>> let arc = Arc::new(MutatingDrop { value: 42 });
> >>>> let wr = arc.as_with_ref(); // this creates a shared `&` reference to the MutatingDrop
> >>>> let arc2: Arc<MutatingDrop> = wr.into(); // increments the reference count to 2
> >>>
> >>> More precisely, here we did a
> >>>
> >>> &WithRef<_> -> NonNull<WithRef<_>>
> >>>
> >>> conversion, and later on, we may use the `NonNull<WithRef<_>>` in
> >>> `drop` to get a `Box<WithRef<_>>`.
> >>
> >> Indeed.
> >>
> >
> > Can we workaround this issue by (ab)using the `UnsafeCell` inside
> > `WithRef<T>`?
> >
> > impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
> > fn from(b: &WithRef<T>) -> Self {
> > // SAFETY: The existence of the references proves that
> > // `b.refcount.get()` is a valid pointer to `WithRef<T>`.
> > let ptr = unsafe { NonNull::new_unchecked(b.refcount.get().cast::<WithRef<T>>()) };
> >
> > // SAFETY: see the SAFETY above `let ptr = ..` line.
> > ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
> > .deref()
> > .clone()
> > }
> > }
> >
> > This way, the raw pointer in the new Arc no longer derives from the
> > reference of `WithRef<T>`.
>
> No, the code above only obtains a pointer that has provenance valid
> for a `bindings::refcount_t` (or type with the same layout, such as
> `Opaque<bindings::refcount_t>`). But not the whole `WithRef<T>`, so accessing
> it by reading/writing will still be UB.
>
Hmm... but we do the similar thing in `Arc::from_raw()`, right?
pub unsafe fn from_raw(ptr: *const T) -> Self {
..
}
, what we have is a pointer to T, and we construct a pointer to
`ArcInner<T>/WithRef<T>`, in that function. Because the `sub` on pointer
gets away from provenance? If so, we can also do a sub(0) in the above
code.
Regards,
Boqun
> --
> Cheers,
> Benno
>
>
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