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Date: Mon, 24 Jul 2017 10:44:55 +0200
From: Peter Zijlstra <>
To: Michael Ellerman <>
Cc: Kees Cook <>,
	Andrew Morton <>,
	Ingo Molnar <>,
	Josh Poimboeuf <>,
	Christoph Hellwig <>,
	"Eric W. Biederman" <>,
	Jann Horn <>, Eric Biggers <>,
	Elena Reshetova <>,
	Hans Liljestrand <>,
	Greg KH <>,
	Alexey Dobriyan <>,
	"Serge E. Hallyn" <>,,
	Davidlohr Bueso <>,
	Manfred Spraul <>,
	"" <>,
	James Bottomley <>,
	"" <>, Arnd Bergmann <>,
	"David S. Miller" <>,
	Rik van Riel <>, LKML <>,
	linux-arch <>,
	"" <>
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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