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Date: Mon, 24 Jul 2017 10:44:55 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jann Horn <jannh@...gle.com>, Eric Biggers <ebiggers3@...il.com>,
	Elena Reshetova <elena.reshetova@...el.com>,
	Hans Liljestrand <ishkamiel@...il.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
	Davidlohr Bueso <dave@...olabs.net>,
	Manfred Spraul <manfred@...orfullife.com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	"David S. Miller" <davem@...emloft.net>,
	Rik van Riel <riel@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection

On Mon, Jul 24, 2017 at 04:38:06PM +1000, Michael Ellerman wrote:

> What I'm not entirely clear on is what the best trade off is in terms of
> overhead vs checks. The summary of behaviour between the fast and full
> versions you promised Ingo will help there I think.

That's something that's probably completely different for PPC than it is
for x86. Both because your primitive is LL/SC and thus the saturation
semantics we need a cmpxchg loop for are more natural in your case
anyway, and the fact that your LL/SC is horrendously slow in any case.

Also, I still haven't seen an actual benchmark where our cmpxchg loop
actually regresses anything, just a lot of yelling about potential
regressions :/

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