From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933631AbbJAMY5 (ORCPT ); Thu, 1 Oct 2015 08:24:57 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:52497 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933262AbbJAMYy (ORCPT ); Thu, 1 Oct 2015 08:24:54 -0400 Date: Thu, 1 Oct 2015 14:24:40 +0200 From: Peter Zijlstra To: Boqun Feng Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Ingo Molnar , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Thomas Gleixner , Will Deacon , "Paul E. McKenney" , Waiman Long Subject: Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants Message-ID: <20151001122440.GP2881@worktop.programming.kicks-ass.net> References: <1442418575-12297-1-git-send-email-boqun.feng@gmail.com> <1442418575-12297-5-git-send-email-boqun.feng@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1442418575-12297-5-git-send-email-boqun.feng@gmail.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 16, 2015 at 11:49:32PM +0800, Boqun Feng wrote: > Implement xchg_relaxed and define atomic{,64}_xchg_* as xchg_relaxed, > based on these _relaxed variants, release/acquire variants can be built. > > Note that xchg_relaxed and atomic_{,64}_xchg_relaxed are not compiler > barriers. Hmm, and I note your previous patch creating the regular _relaxed thingies also removes the memory clobber. And looking at the ARM _relaxed patch from Will, I see their _relaxed ops are also not a compiler barrier. I must say I'm somewhat surprised by this level of relaxation, I had expected to only loose SMP barriers, not the program order ones. Is there a good argument for this?