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

Peter Zijlstra
2020-10-09T19:38:31Z

On Fri, Oct 09, 2020 at 09:55:55AM -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.

Then the right patch is to not user atomic_t in those cases.

> The purpose of these counters is to 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.

Guarding lifetimes is what we got refcount_t for.

> 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.

> Counter wraps around to INT_MIN when it overflows and should not be used
> to guard resource lifetimes, device usage and open counts that control
> state changes, and pm states. Overflowing to INT_MIN is consistent with
> the atomic_t api, which it is built on top of.
>
> Using counter_atomic* to guard lifetimes could lead to use-after free
> when it overflows and undefined behavior when used to manage state
> changes and device usage/open states.
>
> This patch series introduces Simple atomic counters. Counter atomic ops
> leverage atomic_t and provide a sub-set of atomic_t ops.

Thanks for Cc'ing the atomic maintainers :/

NAK.