Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters

Kees Cook
2020-09-28T20:34:38Z

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