Messages in this thread | | | Date | Mon, 25 Sep 2023 09:14:50 +0000 | From | Benno Lossin <> | Subject | Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef` |
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On 25.09.23 08:29, Alice Ryhl wrote: > On Sat, Sep 23, 2023 at 4:50 PM Wedson Almeida Filho <wedsonaf@gmail.com> wrote: >> >> From: Wedson Almeida Filho <walmeida@microsoft.com> >> >> With GATs, we don't need a separate type to represent a borrowed object >> with a refcount, we can just use Rust's regular shared borrowing. In >> this case, we use `&WithRef<T>` instead of `ArcBorrow<'_, T>`. >> >> Co-developed-by: Boqun Feng <boqun.feng@gmail.com> >> Signed-off-by: Boqun Feng <boqun.feng@gmail.com> >> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com> >> --- >> rust/kernel/sync.rs | 2 +- >> rust/kernel/sync/arc.rs | 134 ++++++++++++---------------------------- >> 2 files changed, 39 insertions(+), 97 deletions(-) > > I'm concerned about this change, because an `&WithRef<T>` only has > immutable permissions for the allocation. No pointer derived from it > may be used to modify the value in the Arc, however, the drop > implementation of Arc will do exactly that.
That is indeed a problem. We could put the value in an `UnsafeCell`, but that would lose us niche optimizations and probably also other optimizations.
> It also means that we > can't convert an Arc with refcount 1 into a UniqueArc.
I think you still can, since to do that you would consume the `Arc<T>` by value, thus guaranteeing that no references (and thus no `&WithRef<T>`) exist. So I think this would still be fine.
-- Cheers, Benno
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