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Date: Mon, 24 Jul 2017 14:23:27 +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 10:09:32PM +1000, Michael Ellerman wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > anyway, and the fact that your LL/SC is horrendously slow in any case.
> 
> Boo :/

:-)

> Just kidding. I suspect you're right that we can probably pack a
> reasonable amount of tests in the body of the LL/SC and not notice.
> 
> > Also, I still haven't seen an actual benchmark where our cmpxchg loop
> > actually regresses anything, just a lot of yelling about potential
> > regressions :/
> 
> Heh yeah. Though I have looked at the code it generates on PPC and it's
> not sleek, though I guess that's not a benchmark is it :)

Oh for sure, GCC still can't sanely convert a cmpxchg loop (esp. if the
cmpxchg is implemented using asm) into a native LL/SC sequence, so the
generic code will end up looking pretty horrendous.

A native implementation of the same semantics should look loads better.


One thing that might help you is that refcount_dec_and_test() is weaker
than atomic_dec_and_test() wrt ordering, so that might help some
(RELEASE vs fully ordered).

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