Re: [PATCH v3 00/11] Introduce Simple atomic counters

Peter Zijlstra
2020-10-10T22:58:43Z

On Fri, Oct 09, 2020 at 01:45:43PM -0700, Kees Cook wrote:
> On Fri, Oct 09, 2020 at 09:37:46PM +0200, Peter Zijlstra wrote:
> > On Fri, Oct 09, 2020 at 09:55:55AM -0600, Shuah Khan wrote:
> > > Simple atomic counters api provides interfaces for simple atomic counters
> > > that just count, and don't guard resource lifetimes. The interfaces are
> > > built on top of atomic_t api, providing a smaller subset of atomic_t
> > > interfaces necessary to support simple counters.
> >
> > To what actual purpose?!? AFACIT its pointless wrappery, it gets us
> > nothing.
>
> It's not pointless. There is value is separating types for behavioral
> constraint to avoid flaws. atomic_t provides a native operation. We gained
> refcount_t for the "must not wrap" type, and this gets us the other side
> of that behavioral type, which is "wrapping is expected". Separating the
> atomic_t uses allows for a clearer path to being able to reason about
> code flow, whether it be a human or a static analyzer.

refcount_t got us actual rutime exceptions that atomic_t doesn't. This
propsal gets us nothing.

atomic_t is very much expected to wrap.

> The counter wrappers add nothing to the image size, and only serve to
> confine the API to one that cannot be used for lifetime management.

It doesn't add anything period. It doesn't get us new behaviour, it
splits a 'can wrap' use-case from a 'can wrap' type. That's sodding
pointless.

Worse, it mixes 2 unrelated cases into one type, which just makes a
mockery of things (all the inc_return users are not statistics, some
might even mis-behave if they wrap).

> Once conversions are done, we have a clean line between refcounting
> and statistical atomics, which means we have a much lower chance of
> introducing new flaws (and maybe we'll fix flaws during the conversion,
> which we've certainly seen before when doing this stricter type/language
> changes).
>
> I don't see why this is an objectionable goal.

People can and will always find a way to mess things up.

Only add types when you get behavioural changes, otherwise it's
pointless noise.

My NAK stands.