Messages in this thread | | | Date | Mon, 25 Sep 2023 11:51:21 -0700 | From | Boqun Feng <> | Subject | Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef` |
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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>`.
Regards, Boqun
> > > >> drop(arc); // this decrements the reference count to 1 > >> drop(arc2); // this decrements the reference count to 0, so it will drop it > >> ``` > >> When dropping `arc2` it will run the destructor for `MutatingDrop`, > >> which mutates `value`. This is a problem, because the mutable reference > >> supplied was derived from a `&`, that is not allowed in Rust. > >> > > > > Is this an UB? I kinda wonder what's the real damage we can get, because > > in this case, we just use a reference to carry a value of a pointer, > > i.e. > > > > ptr -> reference -> ptr > > > > I cannot think of any real damage compiler can make, but I'm happy to be > > surprised ;-) > > This is UB, so anything can happen :) > > -- > Cheers, > Benno > >
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