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

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Date:   Fri, 9 Oct 2020 13:45:43 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Shuah Khan <skhan@...uxfoundation.org>, corbet@....net,
        gregkh@...uxfoundation.org, shuah@...nel.org, rafael@...nel.org,
        johannes@...solutions.net, lenb@...nel.org, james.morse@....com,
        tony.luck@...el.com, bp@...en8.de, arve@...roid.com,
        tkjos@...roid.com, maco@...roid.com, joel@...lfernandes.org,
        christian@...uner.io, hridya@...gle.com, surenb@...gle.com,
        minyard@....org, arnd@...db.de, mchehab@...nel.org,
        rric@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-acpi@...r.kernel.org, devel@...verdev.osuosl.org,
        openipmi-developer@...ts.sourceforge.net,
        linux-edac@...r.kernel.org, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 00/11] Introduce Simple atomic counters

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.

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.

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.

-- 
Kees Cook