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

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:

quoted

On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:

quoted

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.

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

quoted

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).

Works for me.

Cheers,

 - Joel