Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964927AbaFQT4l (ORCPT ); Tue, 17 Jun 2014 15:56:41 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:52634 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964803AbaFQT4k (ORCPT ); Tue, 17 Jun 2014 15:56:40 -0400 Date: Tue, 17 Jun 2014 12:56:35 -0700 From: "Paul E. McKenney" To: Christoph Lameter Cc: Linus Torvalds , Rusty Russell , Andrew Morton , Tejun Heo , David Howells , linux-kernel@vger.kernel.org, Oleg Nesterov Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu accessors and operations Message-ID: <20140617195634.GQ4669@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140617144151.GD4669@linux.vnet.ibm.com> <20140617152752.GC31819@htj.dyndns.org> <20140617160040.GE31819@htj.dyndns.org> <20140617160508.GF31819@htj.dyndns.org> <20140617185510.GM4669@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14061719-0928-0000-0000-000002B6B7DF Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 17, 2014 at 02:39:46PM -0500, Christoph Lameter wrote: > On Tue, 17 Jun 2014, Paul E. McKenney wrote: > > > We are talking about one CPU initializing all CPUs' portions of > > dynamically allocated per-CPU memory, so there really is more than > > one CPU involved. > > Well that only occurs on initialization before the address of the > struct that contains the offset is available to other processors. Given runtime dynamic allocation of per-CPU memory, you still need proper synchronization. And yes, on non-Alpha CPUs, the dependency ordering through any pointer suffices on the use side. The thing doing allocation and initialization will still need memory barriers, of course. > During operation the percpu area functions like a single processor. And > its designed that way to avoid synchronization issues and take full > advantage of *no* synchronization for full speed. We compromise on that > for statistics but that is only read access. During operation that does not involve cross-CPU accesses, agreed. Thanx, Paul -- 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/