From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Date: Fri, 15 Jan 2016 09:57:57 +0000 Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Message-Id: <20160115095756.GA2131@arm.com> List-Id: References: <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> <20160114222046.GH3818@linux.vnet.ibm.com> In-Reply-To: <20160114222046.GH3818@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Paul E. McKenney" Cc: linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, "Michael S. Tsirkin" , Peter Zijlstra , virtualization@lists.linux-foundation.org, "H. Peter Anvin" , sparclinux@vger.kernel.org, Ingo Molnar , linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Russell King - ARM Linux , user-mode-linux-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, Michael Ellerman , x86@kernel.org, xen-devel@lists.xenproject.org, Ingo Molnar , linux-xtensa@linux-xtensa.org, james.hogan@imgtec.com, Arnd Bergmann , Stefano Stabellini , adi-buildroot-devel@lists.sourceforge.net, Leonid Yegoshin , ddaney.cavm@gmail.com, Thomas Gleixner , linux-metag@vger.kernel. Paul, On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote: > On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote: > > 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. I think you figured this out while I was sleeping, but just to confirm: 1. The MIPS64 ISA doc [1] talks about SYNC in a way that applies only to memory accesses appearing in *program-order* before the SYNC 2. We need WRC+sync+addr to work, which means that the SYNC in P1 must also capture the store in P0 as being "before" the barrier. Leonid reckons it works, but his explanation [2] focussed on the address dependency in P2 as to why this works. If that is the case (i.e. address dependency provides global transitivity), then WRC+addr+addr should also work (even though its not required). 3. It seems that WRC+addr+addr doesn't work, so I'm still suspicious about WRC+sync+addr, because neither the architecture document or Leonid's explanation tell me that it should be forbidden. Will [1] https://imgtec.com/?do-downloadC02 [2] http://lkml.kernel.org/r/569565DA.2010903@imgtec.com (scroll to the end)