linux-kernel - Re: [PATCH v2 3/6] rust: arc: split unsafe block, add missing comment

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Message-ID: <CAH5fLgi=n-v5C4hH-+uozbiwwWCU_QGzhxfdTXfTyyy2ejJR+w@mail.gmail.com>
Date: Fri, 8 Nov 2024 13:37:33 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] rust: arc: split unsafe block, add missing comment

On Fri, Nov 8, 2024 at 1:30 PM Tamir Duberstein <tamird@...il.com> wrote:
>
> On Fri, Nov 8, 2024 at 6:38 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
> > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > > index af383bcd003e1122ebe1b62a49fe40279458e379..9adea755a5ad1a7b03f7fc30a7abc76c1f966c6c 100644
> > > --- a/rust/kernel/sync/arc.rs
> > > +++ b/rust/kernel/sync/arc.rs
> > > @@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
> > >
> > >  impl<T: ?Sized> Clone for Arc<T> {
> > >      fn clone(&self) -> Self {
> > > +        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> > > +        // safe to dereference it.
> > > +        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
> >
> > I would normally prefer to avoid creating a reference to the entire
> > ArcInner, but in this particular case it is okay due to the specifics
> > of how Arc works.
>
> Note that this particular line appears also in the Drop impl just
> below. That said, can you help me understand the concern with creating
> a reference to the entire ArcInner?

Creating a shared reference to the entire ArcInner is an assertion
that no &mut exists to any part of the ArcInner, even though you only
access the refcount field. In this particular case, asserting shared
access to the `data` field is not a problem due to how Arc works, so
it's okay, but the pattern is problematic in other cases.

You could write `unsafe { (*self.ptr.as_ptr()).refcount.get() }` to
avoid the as_ref call.

Alice