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

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Date:   Fri, 3 Feb 2023 00:01:37 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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 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