From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep Date: Fri, 3 Jun 2016 14:45:53 +0100 Message-ID: <20160603134552.GL9915@arm.com> References: <20160524143649.523586684@infradead.org> <57451581.6000700@hpe.com> <20160525045329.GQ4148@linux.vnet.ibm.com> <5745C2CA.4040003@hpe.com> <20160525155747.GE3789@linux.vnet.ibm.com> <57514B6E.6010001@synopsys.com> <20160603093834.GI3190@twins.programming.kicks-ass.net> <20160603120827.GT5231@linux.vnet.ibm.com> <20160603122310.GM3190@twins.programming.kicks-ass.net> <20160603133238.GV5231@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Peter Zijlstra , Vineet Gupta , Waiman Long , linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, manfred@colorfullife.com, dave@stgolabs.net, boqun.feng@gmail.com, tj@kernel.org, pablo@netfilter.org, kaber@trash.net, davem@davemloft.net, oleg@redhat.com, netfilter-devel@vger.kernel.org, sasha.levin@oracle.com, hofrat@osadl.org To: "Paul E. McKenney" Return-path: Received: from foss.arm.com ([217.140.101.70]:52832 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932538AbcFCNpj (ORCPT ); Fri, 3 Jun 2016 09:45:39 -0400 Content-Disposition: inline In-Reply-To: <20160603133238.GV5231@linux.vnet.ibm.com> Sender: netfilter-devel-owner@vger.kernel.org List-ID: On Fri, Jun 03, 2016 at 06:32:38AM -0700, Paul E. McKenney wrote: > On Fri, Jun 03, 2016 at 02:23:10PM +0200, Peter Zijlstra wrote: > > On Fri, Jun 03, 2016 at 05:08:27AM -0700, Paul E. McKenney wrote: > > > On Fri, Jun 03, 2016 at 11:38:34AM +0200, Peter Zijlstra wrote: > > > > On Fri, Jun 03, 2016 at 02:48:38PM +0530, Vineet Gupta wrote: > > > > > On Wednesday 25 May 2016 09:27 PM, Paul E. McKenney wrote: > > > > > > For your example, but keeping the compiler in check: > > > > > > > > > > > > if (READ_ONCE(a)) > > > > > > WRITE_ONCE(b, 1); > > > > > > smp_rmb(); > > > > > > WRITE_ONCE(c, 2); > > > > > > > > So I think it example is broken. The store to @c is not in fact > > > > dependent on the condition of @a. > > > > > > At first glance, the compiler could pull the write to "c" above the > > > conditional, but the "memory" constraint in smp_rmb() prevents this. > > > From a hardware viewpoint, the write to "c" does depend on the "if", > > > as the conditional branch does precede that write in execution order. > > > > > > But yes, this is using smp_rmb() in a very strange way, if that is > > > what you are getting at. > > > > Well, the CPU could decide that the store to C happens either way around > > the branch. I'm not sure I'd rely on CPUs not being _that_ clever. > > If I remember correctly, both Power and ARM guarantee that the CPU won't > be that clever. Not sure about Itanium. I wouldn't be so sure about ARM. On 32-bit, at least, we have conditional store instructions so if the compiler could somehow use one of those for the first WRITE_ONCE then there's very obviously no control dependency on the second WRITE_ONCE and they could be observed out of order. I note that smp_rmb() on ARM and arm64 actually orders against subsequent (in program order) writes, so this is still pretty theoretical for us. Will