Feedback-ID: iad51458e:Fastmail
Date:   Fri, 3 Feb 2023 00:01:37 -0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     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>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
        Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Subject: Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()
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On Fri, Feb 03, 2023 at 08:38:25AM +0100, Greg KH wrote:
[...]
> > Needless to say that ArcInner::count() can guarantee not reading 0
> 
> How?  Because you have an implicit reference on it already?  If so, then

Yes, roughly a reference ("&") in Rust can be treated as a
compile-time reference count, so the existence of "&" in fact prevents
the underlying data from going away, or in Rust term, being "drop"ped.

To get a "&ArcInner<T>", we need a "&Arc<T>", and as long as there is
a reference to an "Arc<T>", the object won't be dropped, that's the
proof of the underly object still being referenced.

Other folks may explain this better and accurate, but that's the basic
idea ;-)

Regards,
Boqun

> 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 :)
> 

[...]

> thanks,
> 
> greg k-h