From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752926AbdGXIpu (ORCPT <rfc822;w@1wt.eu>);
        Mon, 24 Jul 2017 04:45:50 -0400
Received: from merlin.infradead.org ([205.233.59.134]:45150 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751637AbdGXIpn (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 24 Jul 2017 04:45:43 -0400
Date: Mon, 24 Jul 2017 10:44:55 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>, Eric Biggers <ebiggers3@gmail.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Hans Liljestrand <ishkamiel@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "Serge E. Hallyn" <serge@hallyn.com>, arozansk@redhat.com,
        Davidlohr Bueso <dave@stgolabs.net>,
        Manfred Spraul <manfred@colorfullife.com>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        "x86@kernel.org" <x86@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        "David S. Miller" <davem@davemloft.net>,
        Rik van Riel <riel@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection
Message-ID: <20170724084455.o5asa55fckgyjri2@hirez.programming.kicks-ass.net>
References: <1500422614-94821-1-git-send-email-keescook@chromium.org>
 <20170720091106.kigtr6zy7pjgk2s6@gmail.com>
 <20170721142255.586224f0db9cf0714e654859@linux-foundation.org>
 <CAGXu5j+QU7OXWA_oMaLUsDnS4yyfNx8-1f_ywHX=o88kqE7=JQ@mail.gmail.com>
 <87zibujq5t.fsf@concordia.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87zibujq5t.fsf@concordia.ellerman.id.au>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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 :/