Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752079AbdLDWPL (ORCPT ); Mon, 4 Dec 2017 17:15:11 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:37606 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751380AbdLDWPI (ORCPT ); Mon, 4 Dec 2017 17:15:08 -0500 Date: Mon, 4 Dec 2017 14:15:01 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: David Howells , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies smp_barrier_depends() Reply-To: paulmck@linux.vnet.ibm.com References: <1512157876-24665-1-git-send-email-paulmck@linux.vnet.ibm.com> <20171201195053.GA23494@linux.vnet.ibm.com> <15076.1512401936@warthog.procyon.org.uk> <20171204185215.GB7829@linux.vnet.ibm.com> <20171204215448.GY3326@worktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171204215448.GY3326@worktop> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17120422-0056-0000-0000-000003F3B2B6 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008152; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000243; SDB=6.00955478; UDB=6.00482910; IPR=6.00735515; BA=6.00005727; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018347; XFM=3.00000015; UTC=2017-12-04 22:15:02 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17120422-0057-0000-0000-0000082AE660 Message-Id: <20171204221501.GH7829@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-12-04_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1712040311 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1313 Lines: 32 On Mon, Dec 04, 2017 at 10:54:48PM +0100, Peter Zijlstra wrote: > On Mon, Dec 04, 2017 at 10:52:15AM -0800, Paul E. McKenney wrote: > > On Mon, Dec 04, 2017 at 03:38:56PM +0000, David Howells wrote: > > > Paul E. McKenney wrote: > > > > > > > - Q = READ_ONCE(P); smp_read_barrier_depends(); D = READ_ONCE(*Q); > > > > + Q = READ_ONCE(P); D = READ_ONCE(*Q); > > > > > > > > the CPU will issue the following memory operations: > > > > > > > > Q = LOAD P, D = LOAD *Q > > > > > > The CPU may now issue two barriers in addition to the loads, so should we show > > > this? E.g.: > > > > > > Q = LOAD P, BARRIER, D = LOAD *Q, BARRIER > > > > Good point! How about as shown in the updated patch below? > > Humm, I thought the idea was to completely remove read_barrier_depends > from the lkmm and memory-barriers.txt, making it an Alpha implementation > detail. That was indeed my hope, but a too-abrupt departure of DEC Alpha seemed to be causing some confusion, so I jumped on David's suggested change. My hope now is to slowly remove mention of DEC Alpha from the documentation. Hmmm... Maybe we need an LWN article on how we are weaning the memory model from its historical DEC Alpha influences? That might get the word out more effectively. Thanx, Paul