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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Kees Cook' <keescook@chromium.org>,
        Jeff Layton <jlayton@kernel.org>
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        Hans Liljestrand <ishkamiel@gmail.com>,
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        Anna Schumaker <anna@kernel.org>,
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Subject: RE: [PATCH v2] creds: Convert cred.usage to refcount_t
Thread-Topic: [PATCH v2] creds: Convert cred.usage to refcount_t
Thread-Index: AQHZ0hIjCu0dtWFxxUKez0i3A14O7q/0jIjg
Date:   Mon, 21 Aug 2023 10:18:13 +0000
Message-ID: <d2bb7f4a8b214e1f8d50176e6d94b118@AcuMS.aculab.com>
References: <20230818041740.gonna.513-kees@kernel.org>
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From: Kees Cook
> Sent: Friday, August 18, 2023 9:25 PM
> 
> 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.

Hmmm IIRC recent Intel x86 cpu never do static branch prediction.
So you can't avoid mis-predicted branches in cold code.

	David

> Peter Zijlstra and Will Deacon spent a lot of
> time making it run smoothly. :)
> 
> -Kees
> 
> --
> Kees Cook

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