Re: [PATCH v2] creds: Convert cred.usage to refcount_t

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Fri, Aug 18, 2023 at 04:10:49PM -0400, Jeff Layton wrote:
>> [...]
>> extra checks (supposedly) compile down to nothing. It should be possible
>> to build alternate refcount_t handling functions that are just wrappers
>> around atomic_t with no extra checks, for folks who want to really run
>> "fast and loose".
>
> No -- there's no benefit for this. We already did all this work years
> ago with the fast vs full break-down. All that got tossed out since it
> didn't matter. We did all the performance benchmarking and there was no
> meaningful difference -- refcount _is_ atomic with an added check that
> is branch-predicted away. Peter Zijlstra and Will Deacon spent a lot of
> time making it run smoothly. :)

Since you did all of the work should the text size of be growing by a
kilobyte for this change?

Is that expected?

That is a valid concern with this change and it really should be
justified in the change long as someone brought it up.

Eric