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 S1757189AbcH2Ks3 (ORCPT <rfc822;w@1wt.eu>);
        Mon, 29 Aug 2016 06:48:29 -0400
Received: from merlin.infradead.org ([205.233.59.134]:39562 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750860AbcH2Ks2 (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 29 Aug 2016 06:48:28 -0400
Date: Mon, 29 Aug 2016 12:48:15 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Manfred Spraul <manfred@colorfullife.com>
Cc: benh@kernel.crashing.org, paulmck@linux.vnet.ibm.com,
        Ingo Molnar <mingo@elte.hu>, Boqun Feng <boqun.feng@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, 1vier1@web.de,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 1/4] spinlock: Document memory barrier rules
Message-ID: <20160829104815.GI10153@twins.programming.kicks-ass.net>
References: <1472385376-8801-1-git-send-email-manfred@colorfullife.com>
 <1472385376-8801-2-git-send-email-manfred@colorfullife.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1472385376-8801-2-git-send-email-manfred@colorfullife.com>
User-Agent: Mutt/1.5.23.1 (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 Sun, Aug 28, 2016 at 01:56:13PM +0200, Manfred Spraul wrote:
> Right now, the spinlock machinery tries to guarantee barriers even for
> unorthodox locking cases, which ends up as a constant stream of updates
> as the architectures try to support new unorthodox ideas.
> 
> The patch proposes to reverse that:
> spin_lock is ACQUIRE, spin_unlock is RELEASE.
> spin_unlock_wait is also ACQUIRE.
> Code that needs further guarantees must use appropriate explicit barriers.
> 
> Architectures that can implement some barriers for free can define the
> barriers as NOPs.
> 
> As the initial step, the patch converts ipc/sem.c to the new defines:
> - no more smp_rmb() after spin_unlock_wait(), that is part of
>   spin_unlock_wait()
> - smp_mb__after_spin_lock() instead of a direct smp_mb().
> 

Why? This does not explain why..