From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753334AbcBXVMN (ORCPT ); Wed, 24 Feb 2016 16:12:13 -0500 Received: from mail.efficios.com ([78.47.125.74]:44935 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750779AbcBXVML (ORCPT ); Wed, 24 Feb 2016 16:12:11 -0500 Date: Wed, 24 Feb 2016 21:12:04 +0000 (UTC) From: Mathieu Desnoyers To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Lai Jiangshan , dipankar@in.ibm.com, Andrew Morton , josh@joshtriplett.org, Thomas Gleixner , Peter Zijlstra , rostedt , David Howells , edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, Oleg Nesterov , bobby prani Message-ID: <467632883.7240.1456348324456.JavaMail.zimbra@efficios.com> In-Reply-To: <1456290047-16654-2-git-send-email-paulmck@linux.vnet.ibm.com> References: <20160224050021.GA14616@linux.vnet.ibm.com> <1456290047-16654-2-git-send-email-paulmck@linux.vnet.ibm.com> Subject: Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [78.47.125.74] X-Mailer: Zimbra 8.6.0_GA_1178 (ZimbraWebClient - FF44 (Linux)/8.6.0_GA_1178) Thread-Topic: documentation: Fix control dependency and identical stores Thread-Index: AJf+2VVxTmD6N/szFKn+zNfh6myw3w== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Feb 24, 2016, at 12:00 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote: > The summary of the "CONTROL DEPENDENCIES" section incorrectly states that > barrier() may be used to prevent compiler reordering when more than one > leg of the control-dependent "if" statement start with identical stores. > This is incorrect at high optimization levels. This commit therefore > updates the summary to match the detailed description. > > Reported by: Jianyu Zhan > Signed-off-by: Paul E. McKenney > --- > Documentation/memory-barriers.txt | 10 +++++++--- > 1 file changed, 7 insertions(+), 3 deletions(-) > > diff --git a/Documentation/memory-barriers.txt > b/Documentation/memory-barriers.txt > index 904ee42d078e..e26058d3e253 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -800,9 +800,13 @@ In summary: > use smp_rmb(), smp_wmb(), or, in the case of prior stores and > later loads, smp_mb(). > > - (*) If both legs of the "if" statement begin with identical stores > - to the same variable, a barrier() statement is required at the > - beginning of each leg of the "if" statement. > + (*) If both legs of the "if" statement begin with identical stores to > + the same variable, then those stores must be ordered, either by > + preceding both of them with smp_mb() or by using smp_store_release() > + to carry out the stores. Please note that it is -not- sufficient > + to use barrier() at beginning of each leg of the "if" statement, > + as optimizing compilers do not necessarily respect barrier() > + in this case. Hrm, I really don't understand this one. One caveat, as stated here, would be that optimizing compilers can reorder instruction with respect to barrier() placed at the beginning of if/else legs that start with identical stores. It goes on stating that "smp_mb() or smp_store_release()" should be used rather than barrier() in those cases. I don't get how, from a compiler optimization perspective, barrier() is any different from smp_mb(). #define barrier() __asm__ __volatile__("": : :"memory") vs #define mb() asm volatile("mfence":::"memory") What the compiler would observe is a "memory" clobber in both cases. Now if the stated cause of this issue would have been internal reordering of those identical stores within the processor, I would understand that smp_mb() has an effect which differs from the compiler barrier, but since the paragraph begins by stating that this is purely for compiler optimizations, I'm confused. What am I missing there ? Thanks, Mathieu > > (*) Control dependencies require at least one run-time conditional > between the prior load and the subsequent store, and this > -- > 2.5.2 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com