From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752497AbdJKP74 (ORCPT ); Wed, 11 Oct 2017 11:59:56 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:43110 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751968AbdJKP7y (ORCPT ); Wed, 11 Oct 2017 11:59:54 -0400 Date: Wed, 11 Oct 2017 08:59:48 -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: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6309.1507735045@warthog.procyon.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17101115-0052-0000-0000-000002701846 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007878; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000236; SDB=6.00929649; UDB=6.00467917; IPR=6.00709921; 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=3.00017491; XFM=3.00000015; UTC=2017-10-11 15:59:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17101115-0053-0000-0000-000052493C19 Message-Id: <20171011155948.GE3521@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-1710110221 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 11, 2017 at 04:17:25PM +0100, David Howells wrote: > Will Deacon wrote: > > > FWIW, that's exactly what my patches do, this fixup looks a bit weird > > because it removes a prior barrier which suggests that either (a) it's in > > the wrong place to start with, or (b) we're annotating the wrong load. > > There is a loop involved. The barrier is against the read in the previous > iteration of the loop. IIRC, the reason I did it this way is to avoid the > need for the barrier if there's nothing on the 'after-side' - ie. we examine > the pointer and see that it's NULL or a leaf. However, I'm not sure that's a > particularly necessary optimisation. Given that smp_read_barrier_depends() is nothingness on anything other than DEC Alpha, I would argue that this optimization is not necessary. > So if READ_ONCE() issues a smp_read_barrier_depends() after the read, then > I've no problem with the removal of these explicit barriers. Very good! > I will, however, quibble with the appropriateness of the name READ_ONCE()... > I still think it's not sufficiently obvious that this is a barrier and the > barrier is after. Maybe READ_AND_BARRIER()? Linus was unhappy with READ_ONCE_CTRL() to tag control dependencies, but indicated that he might consider it if it helped code-analysis tools. Adding Dmitry Vyukov for his thoughts on whether tagging READ_ONCE() for dependencies would help. Me, I would suggest READ_ONCE_DEP(), but let's figure out if the bikeshed needs to be painted before arguing over the color. ;-) > Also, does WRITE_ONCE() imply a preceding barrier? It does not. In most cases, the barriered version would be smp_store_release(). Thanx, Paul