Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752880AbaBXQz4 (ORCPT ); Mon, 24 Feb 2014 11:55:56 -0500 Received: from cantor2.suse.de ([195.135.220.15]:34835 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752572AbaBXQzw (ORCPT ); Mon, 24 Feb 2014 11:55:52 -0500 Date: Mon, 24 Feb 2014 17:55:50 +0100 (CET) From: Michael Matz To: Linus Torvalds cc: Richard Biener , Paul McKenney , Torvald Riegel , 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" Subject: Re: [RFC][PATCH 0/5] arch: atomic rework In-Reply-To: Message-ID: References: <20140220083032.GN4250@linux.vnet.ibm.com> <20140220181116.GT4250@linux.vnet.ibm.com> <1392922421.28840.36.camel@triegel.csb> <1393095223.28840.4914.camel@triegel.csb> <20140223003933.GQ4250@linux.vnet.ibm.com> <20140223063426.GT4250@linux.vnet.ibm.com> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Ciao, Michael. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/