From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753364AbaBZRlM (ORCPT ); Wed, 26 Feb 2014 12:41:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54011 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751248AbaBZRlK (ORCPT ); Wed, 26 Feb 2014 12:41:10 -0500 Subject: Re: [RFC][PATCH 0/5] arch: atomic rework From: Torvald Riegel To: paulmck@linux.vnet.ibm.com Cc: Michael Matz , Linus Torvalds , Richard Biener , Will Deacon , Peter Zijlstra , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "gcc@gcc.gnu.org" In-Reply-To: <20140224172856.GP8264@linux.vnet.ibm.com> References: <1393095223.28840.4914.camel@triegel.csb> <20140223003933.GQ4250@linux.vnet.ibm.com> <20140223063426.GT4250@linux.vnet.ibm.com> <20140224172856.GP8264@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Date: Wed, 26 Feb 2014 18:39:51 +0100 Message-ID: <1393436391.28840.8975.camel@triegel.csb> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2014-02-24 at 09:28 -0800, Paul E. McKenney wrote: > On Mon, Feb 24, 2014 at 05:55:50PM +0100, Michael Matz wrote: > > Hi, > > > > On Mon, 24 Feb 2014, Linus Torvalds wrote: > > > > > > To me that reads like > > > > > > > > int i; > > > > int *q = &i; > > > > int **p = &q; > > > > > > > > atomic_XXX (p, CONSUME); > > > > > > > > orders against accesses '*p', '**p', '*q' and 'i'. Thus it seems they > > > > want to say that it orders against aliased storage - but then go further > > > > and include "indirectly through a chain of pointers"?! Thus an > > > > atomic read of a int * orders against any 'int' memory operation but > > > > not against 'float' memory operations? > > > > > > No, it's not about type at all, and the "chain of pointers" can be > > > much more complex than that, since the "int *" can point to within an > > > object that contains other things than just that "int" (the "int" can > > > be part of a structure that then has pointers to other structures > > > etc). > > > > So, let me try to poke holes into your definition or increase my > > understanding :) . You said "chain of pointers"(dereferences I assume), > > e.g. if p is result of consume load, then access to > > p->here->there->next->prev->stuff is supposed to be ordered with that load > > (or only when that last load/store itself is also an atomic load or > > store?). > > > > So, what happens if the pointer deref chain is partly hidden in some > > functions: > > > > A * adjustptr (B *ptr) { return &ptr->here->there->next; } > > B * p = atomic_XXX (&somewhere, consume); > > adjustptr(p)->prev->stuff = bla; > > > > As far as I understood you, this whole ptrderef chain business would be > > only an optimization opportunity, right? So if the compiler can't be sure > > how p is actually used (as in my function-using case, assume adjustptr is > > defined in another unit), then the consume load would simply be > > transformed into an acquire (or whatever, with some barrier I mean)? Only > > _if_ the compiler sees all obvious uses of p (indirectly through pointer > > derefs) can it, yeah, do what with the consume load? > > Good point, I left that out of my list. Adding it: > > 13. By default, pointer chains do not propagate into or out of functions. > In implementations having attributes, a [[carries_dependency]] > may be used to mark a function argument or return as passing > a pointer chain into or out of that function. > > If a function does not contain memory_order_consume loads and > also does not contain [[carries_dependency]] attributes, then > that function may be compiled using any desired dependency-breaking > optimizations. > > The ordering effects are implementation defined when a given > pointer chain passes into or out of a function through a parameter > or return not marked with a [[carries_dependency]] attributed. > > Note that this last paragraph differs from the current standard, which > would require ordering regardless. I would prefer if we could get rid off [[carries_dependency]] as well; currently, it's a hint whose effectiveness really depends on how the particular implementation handles this attribute. If we still need something like it in the future, it would be good if it had a clearer use and performance effects.