Messages in this thread | | | Date | Mon, 25 Sep 2023 07:49:06 -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 09:14:50AM +0000, Benno Lossin wrote: > 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. >
Not sure I understand the problem here, why do we allow modifying the value in the Arc if you only have a shared ownership? Also I fail to see why `ArcBorrow` doesn't have the problem. Maybe I'm missing something subtle here? Could you provide an example?
Regards, Boqun
> > 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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