linux-kernel - Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()

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Date:   Fri, 3 Feb 2023 11:17:47 -0800
From:   Josh Stone <jistone@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Peter Zijlstra <peterz@...radead.org>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>
Subject: Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()

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.