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Date:   Mon, 28 Sep 2020 13:34:31 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Shuah Khan <skhan@linuxfoundation.org>, corbet@lwn.net,
        gregkh@linuxfoundation.org, shuah@kernel.org, rafael@kernel.org,
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Subject: Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters
Message-ID: <202009281331.444F36A7B@keescook>
References: <cover.1601073127.git.skhan@linuxfoundation.org>
 <20200927233526.GA500818@google.com>
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On Sun, Sep 27, 2020 at 07:35:26PM -0400, Joel Fernandes wrote:
> On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
> > This patch series is a result of discussion at the refcount_t BOF
> > the Linux Plumbers Conference. In this discussion, we identified
> > a need for looking closely and investigating atomic_t usages in
> > the kernel when it is used strictly as a counter without it
> > controlling object lifetimes and state changes.
> > 
> > There are a number of atomic_t usages in the kernel where atomic_t api
> > is used strictly for counting and not for managing object lifetime. In
> > some cases, atomic_t might not even be needed.
> >     
> > The purpose of these counters is twofold: 1. clearly differentiate
> > atomic_t counters from atomic_t usages that guard object lifetimes,
> > hence prone to overflow and underflow errors. It allows tools that scan
> > for underflow and overflow on atomic_t usages to detect overflow and
> > underflows to scan just the cases that are prone to errors. 2. provides
> > non-atomic counters for cases where atomic isn't necessary.
> 
> Nice series :)
> 
> It appears there is no user of counter_simple in this series other than the
> selftest. Would you be planning to add any conversions in the series itself,
> for illustration of use? Sorry if I missed a usage.
> 
> Also how do we guard against atomicity of counter_simple RMW operations? Is
> the implication that it should be guarded using other synchronization to
> prevent lost-update problem?
> 
> Some more comments:
> 
> 1.  atomic RMW operations that have a return value are fully ordered. Would
>     you be adding support to counter_simple for such ordering as well, for
>     consistency?

No -- there is no atomicity guarantee for counter_simple. I would prefer
counter_simple not exist at all, specifically for this reason.

> 2. I felt counter_atomic and counter_atomic64 would be nice equivalents to
>    the atomic and atomic64 naming currently used (i.e. dropping the '32').
>    However that is just my opinion and I am ok with either naming.

I had asked that they be size-named to avoid any confusion (i.e. we're
making a new API).

-- 
Kees Cook