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 3727B1A0174 for ; Wed, 27 Jan 2016 21:25:53 +1100 (AEDT) Date: Wed, 27 Jan 2016 10:25:46 +0000 From: Will Deacon To: "Paul E. McKenney" 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: <20160127102546.GD2390@arm.com> References: <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> <20160126195820.GS4503@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160126195820.GS4503@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Jan 26, 2016 at 11:58:20AM -0800, Paul E. McKenney wrote: > 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? Yes, that's my understanding. Will