From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() Date: Wed, 11 Oct 2017 12:28:57 -0700 Message-ID: <20171011192857.GR3521@linux.vnet.ibm.com> References: <20171011155948.GE3521@linux.vnet.ibm.com> <20171011161220.zqkdhynxerrcmvdd@hirez.programming.kicks-ass.net> <20171011162412.o6lmjiag7spwabge@hirez.programming.kicks-ass.net> <20171011164748.GK3521@linux.vnet.ibm.com> <20171011165405.xd4xfa2x3dcz4e57@hirez.programming.kicks-ass.net> <20171011170631.GN3521@linux.vnet.ibm.com> <20171011171137.bb2evetsnk2qtdhc@hirez.programming.kicks-ass.net> <20171011173423.GO3521@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:36058 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752465AbdJKT3D (ORCPT ); Wed, 11 Oct 2017 15:29:03 -0400 Received: from pps.filterd (m0098420.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9BJOkVL010465 for ; Wed, 11 Oct 2017 15:29:02 -0400 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0b-001b2d01.pphosted.com with ESMTP id 2dhpgs8cmw-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 11 Oct 2017 15:29:02 -0400 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Oct 2017 15:29:01 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Linus Torvalds Cc: Dmitry Vyukov , Peter Zijlstra , David Howells , Will Deacon , LKML , Ingo Molnar , Mark Rutland , "linux-arch@vger.kernel.org" , Jonathan Corbet , Alexander Kuleshov On Wed, Oct 11, 2017 at 11:56:54AM -0700, Linus Torvalds wrote: > On Wed, Oct 11, 2017 at 11:43 AM, Dmitry Vyukov wrote: > > > > A long thread and I lost track somewhat, but, yes, from KTSAN (data > > race detector) point of view we will need a way to understand when > > things are ordered due to data and control dependencies > > (i.e.effectively acquire but only wrt the dependent stuff). > > There is a logic level and physical level, it's perfectly fine if on > > physical level all these _DEP/_CTRL variants are no-op (the same as > > READ_ONCE, at least on todays archs with todays compilers). But on > > logical level they represent a well defined, meaningful thing. > > No, they do not. > > Do you believe in fairies and Santa Claus? > > Because quite frankly, the likelihood of either of those being true is > _way_ higher than the likelihood of any normal human ever getting > those things right. > > So asking a programmer to annotate whether two memory accesses have a > data dependency or a control dependency is completely inappropriate. > You won't get people understanding it to begin with, much less then > figure out subtle things like whether a control dependency is an > actual branch, or might be turned into a data dependency through > select, or whatever. > > We've had really smart people who wrote core code that couldn't get it > right, and that weren't sure if a control dependency was really > guaranteed or not. > > That is *exactly* the kinds of thing that _automation_ should handle. > Not some human. Figure the data/control dependencies out from the > code, not from some logical level. > > I saw the contortions that the ISO C people tried to go through just > to describe control and data dependencies. It was awful. It should > have never been described on that kind of level to begin with, when it > would have been much easier to just describe it to compiler people as > "this is a consume relationship". The same rules apply here. Don't > make it about some high-level thing and humans annontating things. > Make it about the actual generated code. Speaking as one of the ISO C people... What exactly do you have in mind when you say to just describe it to compiler people as "this is a consume relationship"? Thanx, Paul