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

[Shuah Khan]

On 9/28/20 3:17 PM, Joel Fernandes wrote:
> On Mon, Sep 28, 2020 at 01:34:31PM -0700, Kees Cook wrote:
> > 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 :)

Thanks.

> > > 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.
>
> Yeah I am ok with it not existing, especially also as there are no examples
> of its conversion/usage in the series.

No. counter_simple is just for counting when there is no need for
atomicity with the premise that there might be some use-cases. You
are right that this patch series doesn't use these. My hunch is though
that atomic_t is overused and it isn't needed in all cases.

I will do some research to look for any places that can use
counter_simple before I spin v2. If I don't find any, I can drop them.

thanks,
-- Shuah