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 S1755158AbcA1Sfi (ORCPT <rfc822;w@1wt.eu>);
	Thu, 28 Jan 2016 13:35:38 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:51166 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752271AbcA1Sff (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 28 Jan 2016 13:35:35 -0500
Date: Thu, 28 Jan 2016 19:35:26 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [RFC PATCH 1/3] lib/list_batch: A simple list insertion/deletion
 batching facility
Message-ID: <20160128183526.GX6356@twins.programming.kicks-ass.net>
References: <1453824219-51437-1-git-send-email-Waiman.Long@hpe.com>
 <1453824219-51437-2-git-send-email-Waiman.Long@hpe.com>
 <20160127163450.GU6357@twins.programming.kicks-ass.net>
 <56A926FB.80609@hpe.com>
 <20160127205400.GZ6357@twins.programming.kicks-ass.net>
 <56AA45B4.20401@hpe.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <56AA45B4.20401@hpe.com>
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

On Thu, Jan 28, 2016 at 11:45:40AM -0500, Waiman Long wrote:
> Using xchg_release() looks OK to me. As this feature is enabled on x86 only
> for this patch, we can make the change and whoever enabling it for other
> architectures that have a real release function will have to test it.

Ah, I was more thinking about:

	/*
	 * We rely on the memory barrier implied by xchg() below to
	 * ensure the node initialization is complete before its
	 * published.
	 */

And then use xchg() like you already do.


> >READ/WRITE_ONCE() provide _no_ order what so ever. And the issue here is
> >that we must not do any other stores to nptr after the state_done.
> >
> 
> I thought if those macros are accessing the same cacheline, the compiler
> won't change the ordering and the hardware will take care of the proper
> ordering. Anyway, I do intended to change to use smp_store_release() for
> safety.

The macros use a volatile cast, and that ensures the compiler must emit
the store and must emit it as a single store. I'm not 100% sure on the
rules of the compiler reordering volatile accesses, they are not a
compiler barrier.