From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753738AbeENNYR (ORCPT ); Mon, 14 May 2018 09:24:17 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:42254 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752296AbeENNYQ (ORCPT ); Mon, 14 May 2018 09:24:16 -0400 Date: Mon, 14 May 2018 06:25:41 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Xiao Guangrong , rostedt@goodmis.org, Lai Jiangshan , stefani@seibold.net, "linux-kernel@vger.kernel.org" Subject: Re: Is read barrier missed in kfifo? Reply-To: paulmck@linux.vnet.ibm.com References: <5382d3a8-08e0-5429-0cd3-36f9c69197e1@gmail.com> <20180511083242.GJ12217@hirez.programming.kicks-ass.net> <20180511162053.GY26088@linux.vnet.ibm.com> <20180514065755.GM12217@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180514065755.GM12217@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18051413-0052-0000-0000-000002EE3FB7 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009024; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000260; SDB=6.01032180; UDB=6.00527662; IPR=6.00811312; MB=3.00021105; MTD=3.00000008; XFM=3.00000015; UTC=2018-05-14 13:24:12 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18051413-0053-0000-0000-00005CADF361 Message-Id: <20180514132541.GS26088@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-14_03:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=9 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-1805140137 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 14, 2018 at 08:57:55AM +0200, Peter Zijlstra wrote: > On Fri, May 11, 2018 at 09:20:53AM -0700, Paul E. McKenney wrote: > > On Fri, May 11, 2018 at 10:32:42AM +0200, Peter Zijlstra wrote: > > > On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote: > > > > > > > > Hi, > > > > > > > > Currently, there is no read barrier between reading the index > > > > (kfifo.in) and fetching the real data from the fifo. > > > > > > > > I am afraid that will cause the vfifo is observed as not empty > > > > however the data is not actually ready for read. Right? > > > > > > That code is decidedly dodgy indeed. I can only see smp_wmb() but no > > > matching barriers at all -- therefore the code is almost certainly as > > > good as not having any barriers at all. > > > > > > I would suggest you try and convert the code to smp_store_release() and > > > smp_load_acquire() while you're at it. > > > > Isn't this one of the places where we rely on control dependencies? > > Then it bloody well should have a comment. But at least one side of the > fifo needs a read barrier I think. We can rely on a ctrl-dep on the > write side, where we read the head/tail values, compute space and then > conditionally allow writes to happen. > > But on the read side it's all reads and ctrl-dep doesn't help anything. Agreed, for a control depdendency to help, there does need to be a control dependency. ;-) Thanx, Paul