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From:   Wedson Almeida Filho <wedsonaf@gmail.com>
Date:   Fri, 3 Feb 2023 16:22:44 -0300
Message-ID: <CANeycqo088O+yCA91hujZtxZzbR4UyYrj_yvGYHp0MgWatxmpA@mail.gmail.com>
Subject: Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()
To:     Josh Stone <jistone@redhat.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Gary Guo <gary@garyguo.net>,
        Peter Zijlstra <peterz@infradead.org>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
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On Fri, 3 Feb 2023 at 16:17, Josh Stone <jistone@redhat.com> wrote:
>
> On 2/2/23 11:38 PM, Greg KH wrote:
> > How?  Because you have an implicit reference on it already?  If so, then
> > why does reading from it matter at all, as if you have a reference, you
> > know it isn't 0, and that's all that you can really care about.  You
> > don't care about any number other than 0 for a reference count, as by
> > definition, that's what a reference count does :)
>
> There is an additional ability for 1, mentioned up thread -- if you have
> &mut Arc<T>, and the inner count is 1, then you *know* there aren't any
> other Arc<T> handles anywhere else. Then it is safe to return an
> exclusive &mut T, like the upstream Arc::get_mut and Arc::make_mut. This
> can also be used for owned Arc<T> like the upstream Arc::try_unwrap.

The `Arc<T>` in the kernel is pinned so we can't return an `&mut T`,
though a `Pin<&mut T>` would be ok.