From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753821AbeENG6J (ORCPT ); Mon, 14 May 2018 02:58:09 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:55388 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753782AbeENG6C (ORCPT ); Mon, 14 May 2018 02:58:02 -0400 Date: Mon, 14 May 2018 08:57:55 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Xiao Guangrong , rostedt@goodmis.org, Lai Jiangshan , stefani@seibold.net, "linux-kernel@vger.kernel.org" Subject: Re: Is read barrier missed in kfifo? Message-ID: <20180514065755.GM12217@hirez.programming.kicks-ass.net> References: <5382d3a8-08e0-5429-0cd3-36f9c69197e1@gmail.com> <20180511083242.GJ12217@hirez.programming.kicks-ass.net> <20180511162053.GY26088@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180511162053.GY26088@linux.vnet.ibm.com> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.