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 A95241A0FDC for ; Tue, 26 Jan 2016 23:10:20 +1100 (AEDT) Date: Tue, 26 Jan 2016 12:10:10 +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: <20160126121010.GD21553@arm.com> References: <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> <20160115102431.GB2131@arm.com> <20160115175401.GW3818@linux.vnet.ibm.com> <20160115192845.GA12510@linux.vnet.ibm.com> <20160125144133.GB22927@arm.com> <20160126010646.GH4503@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160126010646.GH4503@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Mon, Jan 25, 2016 at 05:06:46PM -0800, Paul E. McKenney wrote: > On Mon, Jan 25, 2016 at 02:41:34PM +0000, Will Deacon wrote: > > On Fri, Jan 15, 2016 at 11:28:45AM -0800, Paul E. McKenney wrote: > > > On Fri, Jan 15, 2016 at 09:54:01AM -0800, Paul E. McKenney wrote: > > > > On Fri, Jan 15, 2016 at 10:24:32AM +0000, Will Deacon wrote: > > > > > 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). > > > > > > > > I will revisit my WRC Linux example. And yes, creating litmus tests > > > > that use non-fake dependencies is still a bit of an undertaking. :-/ > > > > I am sure that it will seem more natural with time and experience... > > > > > > Hmmm... You are quite right, I did do WWC. I need to change cpu2()'s > > > last access from a store to a load to get WRC. Plus the levels of > > > indirection definitely didn't match up, did they? > > > > Nope, it was pretty baffling! > > "It is a service that I provide." ;-) > > > > struct foo { > > > struct foo *next; > > > }; > > > struct foo a; > > > struct foo b; > > > struct foo c = { &a }; > > > struct foo d = { &b }; > > > struct foo x = { &c }; > > > struct foo y = { &d }; > > > struct foo *r1, *r2, *r3; > > > > > > void cpu0(void) > > > { > > > WRITE_ONCE(x.next, &y); > > > } > > > > > > void cpu1(void) > > > { > > > r1 = lockless_dereference(x.next); > > > WRITE_ONCE(r1->next, &x); > > > } > > > > > > void cpu2(void) > > > { > > > r2 = lockless_dereference(y.next); > > > r3 = READ_ONCE(r2->next); > > > } > > > > > > In this case, it is legal to end the run with: > > > > > > r1 == &y && r2 == &x && r3 == &c > > > > > > Please see below for a ppcmem litmus test. > > > > > > So, did I get it right this time? ;-) > > > > The code above looks correct to me (in that it matches WRC+addrs), > > but your litmus test: > > > > > PPC WRCnf+addrs > > > "" > > > { > > > 0:r2=x; 0:r3=y; > > > 1:r2=x; 1:r3=y; > > > 2:r2=x; 2:r3=y; > > > c=a; d=b; x=c; y=d; > > > } > > > P0 | P1 | P2 ; > > > stw r3,0(r2) | lwz r8,0(r2) | lwz r8,0(r3) ; > > > | stw r2,0(r3) | lwz r9,0(r8) ; > > > exists > > > (1:r8=y /\ 2:r8=x /\ 2:r9=c) > > > > Seems to be missing the address dependency on P1. > > You are quite correct! How about the following? I think that's it! > As before, both herd and ppcmem say that the cycle is allowed, as > expected, given non-transitive ordering. To prohibit the cycle, P1 > needs a suitable memory-barrier instruction. > > ------------------------------------------------------------------------ > > PPC WRCnf+addrs > "" > { > 0:r2=x; 0:r3=y; > 1:r2=x; 1:r3=y; > 2:r2=x; 2:r3=y; > c=a; d=b; x=c; y=d; > } > P0 | P1 | P2 ; > stw r3,0(r2) | lwz r8,0(r2) | lwz r8,0(r3) ; > | stw r2,0(r8) | lwz r9,0(r8) ; > exists > (1:r8=y /\ 2:r8=x /\ 2:r9=c) Agreed. Will