From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e35.co.us.ibm.com (e35.co.us.ibm.com [32.97.110.153]) (using TLSv1 with cipher CAMELLIA256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 1E95B1A1910 for ; Wed, 27 Jan 2016 08:59:56 +1100 (AEDT) Received: from localhost by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 26 Jan 2016 14:59:55 -0700 Received: from b03cxnp08026.gho.boulder.ibm.com (b03cxnp08026.gho.boulder.ibm.com [9.17.130.18]) by d03dlp01.boulder.ibm.com (Postfix) with ESMTP id 481C21FF0021 for ; Tue, 26 Jan 2016 14:48:02 -0700 (MST) Received: from d03av05.boulder.ibm.com (d03av05.boulder.ibm.com [9.17.195.85]) by b03cxnp08026.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u0QLxqjY22020344 for ; Tue, 26 Jan 2016 14:59:52 -0700 Received: from d03av05.boulder.ibm.com (localhost [127.0.0.1]) by d03av05.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u0QLxWO8008450 for ; Tue, 26 Jan 2016 14:59:51 -0700 Date: Tue, 26 Jan 2016 12:13:39 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Will Deacon , Leonid Yegoshin , "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: <20160126201339.GW4503@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <56980C91.1010403@imgtec.com> <20160114212913.GF3818@linux.vnet.ibm.com> <20160115085554.GF3421@worktop> <20160115091348.GA27936@worktop> <20160115174612.GV3818@linux.vnet.ibm.com> <20160115212714.GM3421@worktop> <20160115215853.GC3818@linux.vnet.ibm.com> <20160125164242.GF22927@arm.com> <20160126060322.GJ4503@linux.vnet.ibm.com> <20160126101927.GD6357@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160126101927.GD6357@twins.programming.kicks-ass.net> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Jan 26, 2016 at 11:19:27AM +0100, Peter Zijlstra wrote: > On Mon, Jan 25, 2016 at 10:03:22PM -0800, Paul E. McKenney wrote: > > On Mon, Jan 25, 2016 at 04:42:43PM +0000, Will Deacon wrote: > > > On Fri, Jan 15, 2016 at 01:58:53PM -0800, Paul E. McKenney wrote: > > > > On Fri, Jan 15, 2016 at 10:27:14PM +0100, Peter Zijlstra wrote: > > > > > > Yes, that seems a good start. But yesterday you raised the 'fun' point > > > > > of two globally ordered sequences connected by a single local link. > > > > > > > > The conclusion that I am slowly coming to is that litmus tests should > > > > not be thought of as linear chains, but rather as cycles. If you think > > > > of it as a cycle, then it doesn't matter where the local link is, just > > > > how many of them and how they are connected. > > > > > > Do you have some examples of this? I'm struggling to make it work in my > > > mind, or are you talking specifically in the context of the kernel > > > memory model? > > > > Now that you mention it, maybe it would be best to keep the transitive > > and non-transitive separate for the time being anyway. Just because it > > might be possible to deal with does not necessarily mean that we should > > be encouraging it. ;-) > > So isn't smp_mb__after_unlock_lock() exactly such a scenario? And would > not someone trying to implement RCsc locks using locally transitive > RELEASE/ACQUIRE operations need exactly this stuff? > > That is, I am afraid we need to cover the mix of local and global > transitive operations at least in overview. True, but we haven't gotten to locking yet. That said, I would argue that smp_mb__after_unlock_lock() upgrades locks to transitive, and thus would not be an exception to the "no combining transitive and non-transitive steps in cycles" rule. Thanx, Paul