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 973B81A0018 for ; Tue, 26 Jan 2016 05:03:06 +1100 (AEDT) Date: Mon, 25 Jan 2016 18:02:34 +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: <20160125180234.GA26732@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> <20160115085554.GF3421@worktop> <20160115173912.GU3818@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160115173912.GU3818@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Hi Paul, On Fri, Jan 15, 2016 at 09:39:12AM -0800, Paul E. McKenney wrote: > On Fri, Jan 15, 2016 at 09:55:54AM +0100, Peter Zijlstra wrote: > > On Thu, Jan 14, 2016 at 01:29:13PM -0800, Paul E. McKenney wrote: > > > So smp_mb() provides transitivity, as do pairs of smp_store_release() > > > and smp_read_acquire(), > > > > But they provide different grades of transitivity, which is where all > > the confusion lays. > > > > smp_mb() is strongly/globally transitive, all CPUs will agree on the order. > > > > Whereas the RCpc release+acquire is weakly so, only the two cpus > > involved in the handover will agree on the order. > > Good point! > > Using grace periods in place of smp_mb() also provides strong/global > transitivity, but also insanely high latencies. ;-) > > The patch below updates Documentation/memory-barriers.txt to define > local vs. global transitivity. The corresponding ppcmem litmus test > is included below as well. > > Should we start putting litmus tests for the various examples > somewhere, perhaps in a litmus-tests directory within each participating > architecture? I have a pile of powerpc-related litmus tests on my laptop, > but they probably aren't doing all that much good there. I too would like to have the litmus tests in the kernel so that we can refer to them from memory-barriers.txt. Ideally they wouldn't be targetted to a particular arch, however. > PPC local-transitive > "" > { > 0:r1=1; 0:r2=u; 0:r3=v; 0:r4=x; 0:r5=y; 0:r6=z; > 1:r1=1; 1:r2=u; 1:r3=v; 1:r4=x; 1:r5=y; 1:r6=z; > 2:r1=1; 2:r2=u; 2:r3=v; 2:r4=x; 2:r5=y; 2:r6=z; > 3:r1=1; 3:r2=u; 3:r3=v; 3:r4=x; 3:r5=y; 3:r6=z; > } > P0 | P1 | P2 | P3 ; > lwz r9,0(r4) | lwz r9,0(r5) | lwz r9,0(r6) | stw r1,0(r3) ; > lwsync | lwsync | lwsync | sync ; > stw r1,0(r2) | lwz r8,0(r3) | stw r1,0(r7) | lwz r9,0(r2) ; > lwsync | lwz r7,0(r2) | | ; > stw r1,0(r5) | lwsync | | ; > | stw r1,0(r6) | | ; > exists > (* (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r8=0 /\ 3:r9=0) *) > (* (0:r9=1 /\ 1:r9=1 /\ 2:r9=1) *) > (* (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r7=0) *) > (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r7=0) i.e. we should rewrite this using READ_ONCE/WRITE_ONCE and smp_mb() etc. > ------------------------------------------------------------------------ > > commit 2cb4e83a1b5c89c8e39b8a64bd89269d05913e41 > Author: Paul E. McKenney > Date: Fri Jan 15 09:30:42 2016 -0800 > > documentation: Distinguish between local and global transitivity > > The introduction of smp_load_acquire() and smp_store_release() had > the side effect of introducing a weaker notion of transitivity: > The transitivity of full smp_mb() barriers is global, but that > of smp_store_release()/smp_load_acquire() chains is local. This > commit therefore introduces the notion of local transitivity and > gives an example. > > Reported-by: Peter Zijlstra > Reported-by: Will Deacon > Signed-off-by: Paul E. McKenney > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index c66ba46d8079..d8109ed99342 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -1318,8 +1318,82 @@ or a level of cache, CPU 2 might have early access to CPU 1's writes. > General barriers are therefore required to ensure that all CPUs agree > on the combined order of CPU 1's and CPU 2's accesses. > > -To reiterate, if your code requires transitivity, use general barriers > -throughout. > +General barriers provide "global transitivity", so that all CPUs will > +agree on the order of operations. In contrast, a chain of release-acquire > +pairs provides only "local transitivity", so that only those CPUs on > +the chain are guaranteed to agree on the combined order of the accesses. Thanks for having a go at this. I tried defining something axiomatically, but got stuck pretty quickly. In my scheme, I used "data-directed transitivity" instead of "local transitivity", since the latter seems to be a bit of a misnomer. > +For example, switching to C code in deference to Herman Hollerith: > + > + int u, v, x, y, z; > + > + void cpu0(void) > + { > + r0 = smp_load_acquire(&x); > + WRITE_ONCE(u, 1); > + smp_store_release(&y, 1); > + } > + > + void cpu1(void) > + { > + r1 = smp_load_acquire(&y); > + r4 = READ_ONCE(v); > + r5 = READ_ONCE(u); > + smp_store_release(&z, 1); > + } > + > + void cpu2(void) > + { > + r2 = smp_load_acquire(&z); > + smp_store_release(&x, 1); > + } > + > + void cpu3(void) > + { > + WRITE_ONCE(v, 1); > + smp_mb(); > + r3 = READ_ONCE(u); > + } > + > +Because cpu0(), cpu1(), and cpu2() participate in a local transitive > +chain of smp_store_release()/smp_load_acquire() pairs, the following > +outcome is prohibited: > + > + r0 == 1 && r1 == 1 && r2 == 1 > + > +Furthermore, because of the release-acquire relationship between cpu0() > +and cpu1(), cpu1() must see cpu0()'s writes, so that the following > +outcome is prohibited: > + > + r1 == 1 && r5 == 0 > + > +However, the transitivity of release-acquire is local to the participating > +CPUs and does not apply to cpu3(). Therefore, the following outcome > +is possible: > + > + r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 I think you should be completely explicit and include r5 == 1 here, too. Also -- where would you add the smp_mb__after_release_acquire to fix (i.e. forbid) this? Immediately after cpu1()'s read of y? > +Although cpu0(), cpu1(), and cpu2() will see their respective reads and > +writes in order, CPUs not involved in the release-acquire chain might > +well disagree on the order. This disagreement stems from the fact that > +the weak memory-barrier instructions used to implement smp_load_acquire() > +and smp_store_release() are not required to order prior stores against > +subsequent loads in all cases. This means that cpu3() can see cpu0()'s > +store to u as happening -after- cpu1()'s load from v, even though > +both cpu0() and cpu1() agree that these two operations occurred in the > +intended order. > + > +However, please keep in mind that smp_load_acquire() is not magic. > +In particular, it simply reads from its argument with ordering. It does > +-not- ensure that any particular value will be read. Therefore, the > +following outcome is possible: > + > + r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0 > + > +Note that this outcome can happen even on a mythical sequentially > +consistent system where nothing is ever reordered. I'm not sure this last bit is strictly necessary. If somebody thinks that acquire/release involve some sort of implicit synchronisation, I think they may have bigger problems with memory-barriers.txt. Will