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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Jeff Layton <jlayton@kernel.org>,
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Date:   Fri, 18 Aug 2023 16:07:49 -0500
In-Reply-To: <202308181317.66E6C9A5@keescook> (Kees Cook's message of "Fri, 18
        Aug 2023 13:24:58 -0700")
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Subject: Re: [PATCH v2] creds: Convert cred.usage to refcount_t
Precedence: bulk

Kees Cook <keescook@chromium.org> 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