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 A83E71A009A for ; Fri, 15 Jan 2016 09:21:04 +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 ; Thu, 14 Jan 2016 15:21:02 -0700 Received: from b03cxnp08025.gho.boulder.ibm.com (b03cxnp08025.gho.boulder.ibm.com [9.17.130.17]) by d03dlp01.boulder.ibm.com (Postfix) with ESMTP id EFD2DC40002 for ; Thu, 14 Jan 2016 15:09:10 -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 u0EML0PU29032692 for ; Thu, 14 Jan 2016 15:21:00 -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 u0EMKmAJ000862 for ; Thu, 14 Jan 2016 15:20:59 -0700 Date: Thu, 14 Jan 2016 14:20:46 -0800 From: "Paul E. McKenney" To: Leonid Yegoshin Cc: Will Deacon , 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: <20160114222046.GH3818@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20160112114111.GB15737@arm.com> <569565DA.2010903@imgtec.com> <20160113104516.GE25458@arm.com> <56969F4B.7070001@imgtec.com> <20160113204844.GV6357@twins.programming.kicks-ass.net> <5696BA6E.4070508@imgtec.com> <20160114120445.GB15828@arm.com> <56980145.5030901@imgtec.com> <20160114204827.GE3818@linux.vnet.ibm.com> <56981212.7050301@imgtec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <56981212.7050301@imgtec.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote: > On 01/14/2016 12:48 PM, Paul E. McKenney wrote: > > > >So SYNC_RMB is intended to implement smp_rmb(), correct? > Yes. > > > >You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > >smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > If smp_read_barrier_depends() is used to separate not only two reads > but read pointer and WRITE basing on that pointer (example below) - > yes. I just doesn't see any example of this in famous > Documentation/memory-barriers.txt and had no chance to know what you > use it in this way too. Well, Documentation/memory-barriers.txt was intended as a guide for Linux kernel hackers, and not for hardware architects. The need for something more precise has become clear over the past year or two, and I am working on it with some heavy-duty memory-model folks. But all previous memory models have been for a specific CPU architecture, so doing one for the intersection of several is offering up some complications. I therefore cannot yet provide a completion date. That said, I still suggest use of SYNC_ACQUIRE for read_barrier_depends(). > >The reason for this is that smp_read_barrier_depends() must order the > >pointer load against any subsequent read or write through a dereference > >of that pointer. > > I can't see that requirement anywhere in Documents directory. I mean > - the words "write through a dereference of that pointer" or similar > for smp_read_barrier_depends. No worries, I will add one. Please see the end of this message for an initial patch. Please understand that Documentation/memory-barriers.txt is a living document: v4.4: Two changes v4.3: Three changes v4.2: Six changes v4.1: Three changes v4.0: Two changes It tends to change as we locate corner cases either in hardware or in software use cases/APIs. > > For example: > > > > p = READ_ONCE(gp); > > smp_rmb(); > > r1 = p->a; /* ordered by smp_rmb(). */ > > p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */ > > r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */ > > > >In contrast: > > > > p = READ_ONCE(gp); > > smp_read_barrier_depends(); > > r1 = p->a; /* ordered by smp_read_barrier_depends(). */ > > p->b = 42; /* ordered by smp_read_barrier_depends(). */ > > r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */ > > > >Again, if your hardware maintains local ordering for address > >and data dependencies, you can have read_barrier_depends() and > >smp_read_barrier_depends() be no-ops like they are for most > >architectures. > > It is not so simple, I mean "local ordering for address and data > dependencies". Local ordering is NOT enough. It happens that current > MIPS R6 doesn't require in your example smp_read_barrier_depends() > but in discussion it comes out that it may not. Because without > smp_read_barrier_depends() your example can be a part of Will's > WRC+addr+addr and we found some design which easily can bump into > this test. And that design actually performs "local ordering for > address and data dependencies" too. As noted in another email in this thread, I do not believe that WRC+addr+addr needs to be prohibited. Sounds like Will and I need to get our story straight, though. Will? Thanx, Paul ------------------------------------------------------------------------ commit 955720966e216b00613fcf60188d507c103f0e80 Author: Paul E. McKenney Date: Thu Jan 14 14:17:04 2016 -0800 documentation: Subsequent writes ordered by rcu_dereference() The current memory-barriers.txt does not address the possibility of a write to a dereferenced pointer. This should be rare, but when it happens, we need that write -not- to be clobbered by the initialization. This commit therefore adds an example showing a data dependency ordering a later data-dependent write. Reported-by: Leonid Yegoshin Signed-off-by: Paul E. McKenney diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index f49c15f7864f..c66ba46d8079 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -555,6 +555,30 @@ between the address load and the data load: This enforces the occurrence of one of the two implications, and prevents the third possibility from arising. +A data-dependency barrier must also order against dependent writes: + + CPU 1 CPU 2 + =============== =============== + { A == 1, B == 2, C = 3, P == &A, Q == &C } + B = 4; + + WRITE_ONCE(P, &B); + Q = READ_ONCE(P); + + *Q = 5; + +The data-dependency barrier must order the read into Q with the store +into *Q. This prohibits this outcome: + + (Q == B) && (B == 4) + +Please note that this pattern should be rare. After all, the whole point +of dependency ordering is to -prevent- writes to the data structure, along +with the expensive cache misses associated with those writes. This pattern +can be used to record rare error conditions and the like, and the ordering +prevents such records from being lost. + + [!] Note that this extremely counterintuitive situation arises most easily on machines with split caches, so that, for example, one cache bank processes even-numbered cache lines and the other bank processes odd-numbered cache