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Date:   Fri, 3 Feb 2023 11:17:47 -0800
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Subject: Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()
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To:     Greg KH <gregkh@linuxfoundation.org>,
        Boqun Feng <boqun.feng@gmail.com>
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        Gary Guo <gary@garyguo.net>,
        Peter Zijlstra <peterz@infradead.org>,
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        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
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        Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
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From:   Josh Stone <jistone@redhat.com>
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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.