From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com (foss.arm.com [217.140.101.70]) by lists.ozlabs.org (Postfix) with ESMTP id CF55E1A0050 for ; Fri, 15 Jan 2016 21:24:35 +1100 (AEDT) Date: Fri, 15 Jan 2016 10:24:32 +0000 From: Will Deacon To: "Paul E. McKenney" Cc: Leonid Yegoshin , Peter Zijlstra , "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andrew Cooper , Russell King - ARM Linux , virtualization@lists.linux-foundation.org, Stefano Stabellini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Joe Perches , David Miller , linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-metag@vger.kernel.org, linux-mips@linux-mips.org, x86@kernel.org, user-mode-linux-devel@lists.sourceforge.net, adi-buildroot-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, xen-devel@lists.xenproject.org, Ralf Baechle , Ingo Molnar , ddaney.cavm@gmail.com, james.hogan@imgtec.com, Michael Ellerman Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Message-ID: <20160115102431.GB2131@arm.com> References: <569565DA.2010903@imgtec.com> <20160113104516.GE25458@arm.com> <5696CF08.8080700@imgtec.com> <20160114121449.GC15828@arm.com> <5697F6D2.60409@imgtec.com> <20160114203430.GC3818@linux.vnet.ibm.com> <56980C91.1010403@imgtec.com> <20160114212913.GF3818@linux.vnet.ibm.com> <569814F2.50801@imgtec.com> <20160114225510.GJ3818@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160114225510.GJ3818@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Jan 14, 2016 at 02:55:10PM -0800, Paul E. McKenney wrote: > On Thu, Jan 14, 2016 at 01:36:50PM -0800, Leonid Yegoshin wrote: > > On 01/14/2016 01:29 PM, Paul E. McKenney wrote: > > > > > >>On 01/14/2016 12:34 PM, Paul E. McKenney wrote: > > >>> > > >>>The WRC+addr+addr is OK because data dependencies are not required to be > > >>>transitive, in other words, they are not required to flow from one CPU to > > >>>another without the help of an explicit memory barrier. > > >>I don't see any reliable way to fit WRC+addr+addr into "DATA > > >>DEPENDENCY BARRIERS" section recommendation to have data dependency > > >>barrier between read of a shared pointer/index and read the shared > > >>data based on that pointer. If you have this two reads, it doesn't > > >>matter the rest of scenario, you should put the dependency barrier > > >>in code anyway. If you don't do it in WRC+addr+addr scenario then > > >>after years it can be easily changed to different scenario which > > >>fits some of scenario in "DATA DEPENDENCY BARRIERS" section and > > >>fails. > > >The trick is that lockless_dereference() contains an > > >smp_read_barrier_depends(): > > > > > >#define lockless_dereference(p) \ > > >({ \ > > > typeof(p) _________p1 = READ_ONCE(p); \ > > > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \ > > > (_________p1); \ > > >}) > > > > > >Or am I missing your point? > > > > WRC+addr+addr has no any barrier. lockless_dereference() has a > > barrier. I don't see a common points between this and that in your > > answer, sorry. > > Me, I am wondering what WRC+addr+addr has to do with anything at all. See my earlier reply [1] (but also, your WRC Linux example looks more like a variant on WWC and I couldn't really follow it). > > > OK, so it looks like Will was asking not about WRC+addr+addr, but instead > about WRC+sync+addr. This would drop an smp_mb() into cpu2() in my > earlier example, which needs to provide ordering. > > I am guessing that the manual's "Older instructions which must be globally > performed when the SYNC instruction completes" provides the equivalent > of ARM/Power A-cumulativity, which can be thought of as transitivity > backwards in time. I couldn't make that leap. In particular, the manual's "Detailed Description" sections explicitly refer to program-order: Every synchronizable specified memory instruction (loads or stores or both) that occurs in the instruction stream before the SYNC instruction must reach a stage in the load/store datapath after which no instruction re-ordering is possible before any synchronizable specified memory instruction which occurs after the SYNC instruction in the instruction stream reaches the same stage in the load/store datapath. Will [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/399765.html