From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753389AbdJKQTm (ORCPT ); Wed, 11 Oct 2017 12:19:42 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:53012 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751988AbdJKQTk (ORCPT ); Wed, 11 Oct 2017 12:19:40 -0400 Date: Wed, 11 Oct 2017 09:19:33 -0700 From: "Paul E. McKenney" To: David Howells Cc: Will Deacon , linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, mark.rutland@arm.com, linux-arch@vger.kernel.org, peterz@infradead.org, Jonathan Corbet , Alexander Kuleshov , dvyukov@google.com Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Reply-To: paulmck@linux.vnet.ibm.com References: <20171011155948.GE3521@linux.vnet.ibm.com> <20171011122217.GD11106@arm.com> <20171010155042.GD3521@linux.vnet.ibm.com> <1507594969-8347-12-git-send-email-paulmck@linux.vnet.ibm.com> <20171010001951.GA6476@linux.vnet.ibm.com> <8079.1507628146@warthog.procyon.org.uk> <26455.1507724399@warthog.procyon.org.uk> <6309.1507735045@warthog.procyon.org.uk> <7484.1507738025@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7484.1507738025@warthog.procyon.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17101116-0048-0000-0000-000001F4174D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007878; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000236; SDB=6.00929656; UDB=6.00467921; IPR=6.00709927; BA=6.00005634; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017492; XFM=3.00000015; UTC=2017-10-11 16:19:37 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17101116-0049-0000-0000-000042D64AB3 Message-Id: <20171011161933.GH3521@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-10-11_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1710110225 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 11, 2017 at 05:07:05PM +0100, David Howells wrote: > Paul E. McKenney wrote: > > > It does not. In most cases, the barriered version would be > > smp_store_release(). > > Ummm... Is that good enough? Is: > > WRITE_ONCE(x, 1); > WRITE_ONCE(x, 2); > > equivalent to: > > smp_store_release(x, 1); > smp_store_release(x, 2); > > if CONFIG_SMP=n? smp_store_release(&x, 1); smp_store_release(&x, 2); But yes, give or take that smp_store_release() potentially disables more compiler optimizations than does WRITE_ONCE(). > (Consider what happens if an interrupt messes with x). OK, I will bite... What is your scenario in which an interrupt gives different results for CONFIG_SMP=n? The barriers > If it is good enough, should we be using smp_load_acquire() rather than > READ_ONCE()? On x86, that might be OK, give or take that smp_load_acquire() potentially disables more optimizations than does READ_ONCE(). But on ARM, PowerPC, MIPS, and so on, smp_load_acquire() emits a memory-barrier instruction and READ_ONCE() does not. Thanx, Paul