netdev - RE: [PATCH v2] creds: Convert cred.usage to refcount

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Date: Mon, 21 Aug 2023 10:18:13 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Kees Cook' <keescook@...omium.org>, Jeff Layton <jlayton@...nel.org>
CC: Andrew Morton <akpm@...ux-foundation.org>, Jann Horn <jannh@...gle.com>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>, "Elena
 Reshetova" <elena.reshetova@...el.com>, David Windsor <dwindsor@...il.com>,
	Hans Liljestrand <ishkamiel@...il.com>, Trond Myklebust
	<trond.myklebust@...merspace.com>, Anna Schumaker <anna@...nel.org>, "Chuck
 Lever" <chuck.lever@...cle.com>, Neil Brown <neilb@...e.de>, Olga
 Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey
	<tom@...pey.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Sergey Senozhatsky <senozhatsky@...omium.org>, "Alexey
 Gladkov" <legion@...nel.org>, "Eric W. Biederman" <ebiederm@...ssion.com>,
	"Yu Zhao" <yuzhao@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-nfs@...r.kernel.org"
	<linux-nfs@...r.kernel.org>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>
Subject: RE: [PATCH v2] creds: Convert cred.usage to refcount_t

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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