From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756857AbZBWUAh (ORCPT ); Mon, 23 Feb 2009 15:00:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753064AbZBWUA0 (ORCPT ); Mon, 23 Feb 2009 15:00:26 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:56817 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751719AbZBWUAZ (ORCPT ); Mon, 23 Feb 2009 15:00:25 -0500 Date: Mon, 23 Feb 2009 11:59:26 -0800 From: Andrew Morton To: paulmck@linux.vnet.ibm.com Cc: nickpiggin@yahoo.com.au, vegard.nossum@gmail.com, mingo@elte.hu, stable@kernel.org, npiggin@suse.de, penberg@cs.helsinki.fi, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error) Message-Id: <20090223115926.ed03e954.akpm@linux-foundation.org> In-Reply-To: <20090223193057.GL6751@linux.vnet.ibm.com> References: <20090220140157.GA12799@elte.hu> <20090222030030.GD6860@linux.vnet.ibm.com> <20090223051709.GA5990@linux.vnet.ibm.com> <200902240029.37815.nickpiggin@yahoo.com.au> <20090223161726.GD6751@linux.vnet.ibm.com> <20090223111009.d48ff81d.akpm@linux-foundation.org> <20090223193057.GL6751@linux.vnet.ibm.com> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 23 Feb 2009 11:30:57 -0800 "Paul E. McKenney" wrote: > On Mon, Feb 23, 2009 at 11:10:09AM -0800, Andrew Morton wrote: > > On Mon, 23 Feb 2009 08:17:26 -0800 "Paul E. McKenney" wrote: > > > > > On Tue, Feb 24, 2009 at 12:29:36AM +1100, Nick Piggin wrote: > > > > On Monday 23 February 2009 16:17:09 Paul E. McKenney wrote: > > > > > > > > > The boot CPU runs in the context of its idle thread during boot-up. > > > > > During this time, idle_cpu(0) will always return nonzero, which will > > > > > fool Classic and Hierarchical RCU into deciding that a large chunk of > > > > > the boot-up sequence is a big long quiescent state. This in turn causes > > > > > RCU to prematurely end grace periods during this time. > > > > > > > > > > This patch creates a new global variable that is set to 1 just before > > > > > the boot CPU first enters the scheduler, after which the idle task > > > > > really is idle. > > > > > > > > Nice work all (btw. if this patch goes in rather than using system_state, > > > > then please make the variable __read_mostly). > > > > > > Hmmm... I misread this and made system_state be __read_mostly. Let > > > me know if this is bad, easy to fix if needed. > > > > Please don't use system_state. The whole thing is just bad design. > > It's a global variable, breaks encapsulation, creates interactions etc. > > CS-101 stuff. > > OK. Would it help if I wrapped an accessor function around system_state? That doesn't help the core problem: system_state creates interactions between unrelated subsystems. And the number of interactions grows exponentially with the number of subsystems which use system_state. > I do need some sort of global state if I am to solve this problem. ;-) As I suggested before, please add a new state variable which is private to your subsystem. That way it remains isolated from other subsystems.