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 S1754346AbbJPPSf (ORCPT <rfc822;w@1wt.eu>);
	Fri, 16 Oct 2015 11:18:35 -0400
Received: from casper.infradead.org ([85.118.1.10]:34513 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754111AbbJPPSe (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 16 Oct 2015 11:18:34 -0400
Date: Fri, 16 Oct 2015 17:18:30 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: Q: schedule() and implied barriers on arm64
Message-ID: <20151016151830.GZ3816@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Hi,

IIRC Paul relies on schedule() implying a full memory barrier with
strong transitivity for RCU.

If not, ignore this email.

If so, however, I suspect AARGH64 is borken and would need (just like
PPC):

#define smp_mb__before_spinlock()	smp_mb()

The problem is that schedule() (when a NO-OP) does:

	smp_mb__before_spinlock();
	LOCK rq->lock

	clear_bit()

	UNLOCK rq->lock

And nothing there implies a full barrier on AARGH64, since
smp_mb__before_spinlock() defaults to WMB, LOCK is an "ldaxr" or
load-acquire, UNLOCK is "stlrh" or store-release and clear_bit() isn't
anything.

Pretty much every other arch has LOCK implying a full barrier, either
because its strongly ordered or because it needs one for the ACQUIRE
semantics.