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Message-ID: <d2bb7f4a8b214e1f8d50176e6d94b118@AcuMS.aculab.com> 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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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