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 S1754194AbbIGJag (ORCPT <rfc822;w@1wt.eu>);
	Mon, 7 Sep 2015 05:30:36 -0400
Received: from foss.arm.com ([217.140.101.70]:50067 "EHLO foss.arm.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753533AbbIGJac (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 7 Sep 2015 05:30:32 -0400
Date: Mon, 7 Sep 2015 10:30:29 +0100
From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>, Oleg Nesterov <oleg@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        "mtk.manpages@gmail.com" <mtk.manpages@gmail.com>,
        "dvhart@infradead.org" <dvhart@infradead.org>,
        "dave@stgolabs.net" <dave@stgolabs.net>,
        "Vineet.Gupta1@synopsys.com" <Vineet.Gupta1@synopsys.com>,
        "ralf@linux-mips.org" <ralf@linux-mips.org>,
        "ddaney@caviumnetworks.com" <ddaney@caviumnetworks.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Richard Henderson <rth@twiddle.net>
Subject: Re: futex atomic vs ordering constraints
Message-ID: <20150907093029.GC14244@arm.com>
References: <20150826181659.GW16853@twins.programming.kicks-ass.net>
 <20150901163140.GK1612@arm.com>
 <20150901164247.GO16853@twins.programming.kicks-ass.net>
 <alpine.DEB.2.11.1509012104350.15006@nanos>
 <20150902125555.GT16853@twins.programming.kicks-ass.net>
 <55E71F92.4000001@ezchip.com>
 <20150902170008.GU19282@twins.programming.kicks-ass.net>
 <CA+55aFzbycJ5UXYvwROcr6RG5_+Xxy2YaMn+Q3DksdNp+T3+bg@mail.gmail.com>
 <20150905175302.GV3644@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150905175302.GV3644@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sat, Sep 05, 2015 at 06:53:02PM +0100, Peter Zijlstra wrote:
> On Wed, Sep 02, 2015 at 02:18:53PM -0700, Linus Torvalds wrote:
> > So I think we could possibly relax the requirements (and document this
> > very clearly) to say that the futex operation must be totally ordered
> > wrt any other _user_space_ accesses by that thread. I suspect a lot of
> > architectures can then say "we may be very weakly ordered, but kernel
> > entry/exit implies enough synchronization that we do not need any
> > futher memory barriers".
> 
> Right, so before sending this email I actually spoke to Ralf about this
> option, and he said that this is not actually well defined for MIPS.
> 
> But we could certainly document it such and let archs for which this is
> well documented (I would expect this to be most) choose that
> implementation.

Whilst a control-dependency + exception return forms a barrier of sorts
on arm/arm64, it's not required to be transitive [1], so I wouldn't be
comfortable making that relaxation on the futex path.

Will

[1] See, for example, "ISA2+dmb+ctrlisb+dmb" at
    https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html#ARM