Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760120AbcCDVFc (ORCPT ); Fri, 4 Mar 2016 16:05:32 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:43882 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759326AbcCDVFa (ORCPT ); Fri, 4 Mar 2016 16:05:30 -0500 Date: Fri, 4 Mar 2016 13:05:24 -0800 From: Darren Hart To: Jianyu Zhan , Paul McKenney Cc: LKML , Peter Zijlstra , Thomas Gleixner , dave@stgolabs.net, Andrew Morton , Ingo Molnar , Rasmus Villemoes , dvhart@linux.intel.com, Christian Borntraeger , Fengguang Wu , bigeasy@linutronix.de Subject: Re: [PATCH] futex: replace bare barrier() with more lightweight READ_ONCE() Message-ID: <20160304210524.GF1092@dvhart-mobl5.amr.corp.intel.com> References: <1457019485-26441-1-git-send-email-nasa4836@gmail.com> <20160303170532.GC1092@dvhart-mobl5.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2476 Lines: 67 On Fri, Mar 04, 2016 at 09:12:31AM +0800, Jianyu Zhan wrote: > On Fri, Mar 4, 2016 at 1:05 AM, Darren Hart wrote: > > I thought I provided a corrected comment block.... maybe I didn't. We have been > > working on improving the futex documentation, so we're paying close attention to > > terminology as well as grammar. This one needs a couple minor tweaks. I suggest: > > > > /* > > * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and > > * optimizing lock_ptr out of the logic below. > > */ > > > > The bit about q->lock_ptr possibly changing is already covered by the large > > comment block below the spin_lock(lock_ptr) call. > > The large comment block is explaining the why the retry logic is required. > To achieve this semantic requirement, the READ_ONCE is needed to prevent > compiler optimizing it by doing double loads. > > So I think the comment above should explain this tricky part. Fair point. Consider: /* * q->lock_ptr can change between this read and the following spin_lock. * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and * optimizing lock_ptr out of the logic below. */ > > > /* Use READ_ONCE to forbid the compiler from reloading q->lock_ptr in spin_lock() */ > > And as for preventing from optimizing the lock_ptr out of the retry > code block, I have consult > Paul Mckenney, he suggests one more READ_ONCE should be added here: Let's keep this discussion together so we have a record of the justification. +Paul McKenney Paul, my understanding was that spin_lock was a CPU memory barrier, which in turn is an implicit compiler barrier (aka barrier()), of which READ_ONCE is described as a weaker form. Reviewing this, I realize the scope of barrier() wasn't clear to me. It seems while barrier() ensures ordering, it does not offer the same guarantee regarding reloading that READ_ONCE offers. So READ_ONCE is not strictly a weaker form of barrier() as I had gathered from a spotty reading of memory-barriers.txt, but it also offers guarantees regarding memory references that barrier() does not. Correct? > > if (unlikely(lock_ptr != READ_ONCE(q->lock_ptr))) { > <------------------------------ > spin_unlock(lock_ptr); > goto retry; > } > > And I think this are two problem, and should be separated into two patches? Yes (pending results of the conversation above). -- Darren Hart Intel Open Source Technology Center