From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Tue, 26 Jan 2016 22:59:53 +0100 (CET) Received: from e31.co.us.ibm.com ([32.97.110.149]:51778 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S27011640AbcAZV7vhr1Ci (ORCPT ); Tue, 26 Jan 2016 22:59:51 +0100 Received: from localhost by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 26 Jan 2016 14:59:45 -0700 Received: from d03dlp03.boulder.ibm.com (9.17.202.179) by e31.co.us.ibm.com (192.168.1.131) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Tue, 26 Jan 2016 14:59:42 -0700 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-mips@linux-mips.org;ralf@linux-mips.org Received: from b03cxnp08025.gho.boulder.ibm.com (b03cxnp08025.gho.boulder.ibm.com [9.17.130.17]) by d03dlp03.boulder.ibm.com (Postfix) with ESMTP id 6C1E719D8042; Tue, 26 Jan 2016 14:47:42 -0700 (MST) Received: from d03av05.boulder.ibm.com (d03av05.boulder.ibm.com [9.17.195.85]) by b03cxnp08025.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u0QLxgja29622424; Tue, 26 Jan 2016 14:59:42 -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 u0QLxVDh008353; Tue, 26 Jan 2016 14:59:41 -0700 Received: from paulmck-ThinkPad-W541 (paulmck-thinkpad-w541.au.ibm.com [9.192.250.130]) by d03av05.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVin) with ESMTP id u0QLxTm3008208; Tue, 26 Jan 2016 14:59:29 -0700 Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id BA71F16C2B4C; Tue, 26 Jan 2016 11:58:20 -0800 (PST) Date: Tue, 26 Jan 2016 11:58:20 -0800 From: "Paul E. McKenney" To: Will Deacon Cc: Peter Zijlstra , 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: <20160126195820.GS4503@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> <20160126121608.GE21553@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160126121608.GE21553@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16012621-8236-0000-0000-0000158A4856 Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 51432 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: paulmck@linux.vnet.ibm.com Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On Tue, Jan 26, 2016 at 12:16:09PM +0000, Will Deacon 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: > > > > PPC Overlapping Group-B sets version 4 > > > > "" > > > > (* When the Group-B sets from two different barriers involve instructions in > > > > the same thread, within that thread one set must contain the other. > > > > > > > > P0 P1 P2 > > > > Rx=1 Wy=1 Wz=2 > > > > dep. lwsync lwsync > > > > Ry=0 Wz=1 Wx=1 > > > > Rz=1 > > > > > > > > assert(!(z=2)) > > > > > > > > Forbidden by ppcmem, allowed by herd. > > > > *) > > > > { > > > > 0:r1=x; 0:r2=y; 0:r3=z; > > > > 1:r1=x; 1:r2=y; 1:r3=z; 1:r4=1; > > > > 2:r1=x; 2:r2=y; 2:r3=z; 2:r4=1; 2:r5=2; > > > > } > > > > P0 | P1 | P2 ; > > > > lwz r6,0(r1) | stw r4,0(r2) | stw r5,0(r3) ; > > > > xor r7,r6,r6 | lwsync | lwsync ; > > > > lwzx r7,r7,r2 | stw r4,0(r3) | stw r4,0(r1) ; > > > > lwz r8,0(r3) | | ; > > > > > > > > exists > > > > (z=2 /\ 0:r6=1 /\ 0:r7=0 /\ 0:r8=1) > > > > > > That really hurts. Assuming that the "assert(!(z=2))" is actually there > > > to constrain the coherence order of z to be {0->1->2}, then I think that > > > this test is forbidden on arm using dmb instead of lwsync. That said, I > > > also don't think the Rz=1 in P0 changes anything. > > > > What about the smp_wmb() variant of dmb that orders only stores? > > Tricky, but I think it still works out if the coherence order of z is as > I described above. The line of reasoning is weird though -- I ended up > considering the two cases where P0 reads z before and after it reads x > and what that means for the read of y. By "works out" you mean that ARM prohibits the outcome? BTW, I never have seen a real-world use for this case. At the moment it is mostly a cautionary tale about memory-model corner cases and tools. Thanx, Paul