From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755727AbZBWUoT (ORCPT ); Mon, 23 Feb 2009 15:44:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751630AbZBWUoE (ORCPT ); Mon, 23 Feb 2009 15:44:04 -0500 Received: from e9.ny.us.ibm.com ([32.97.182.139]:36144 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751193AbZBWUoB (ORCPT ); Mon, 23 Feb 2009 15:44:01 -0500 Date: Mon, 23 Feb 2009 12:43:59 -0800 From: "Paul E. McKenney" To: Ingo Molnar Cc: Andrew Morton , Nick Piggin , Vegard Nossum , stable@kernel.org, Nick Piggin , Pekka Enberg , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error) Message-ID: <20090223204359.GT6751@linux.vnet.ibm.com> Reply-To: paulmck@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> <20090223193351.GA11921@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090223193351.GA11921@elte.hu> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 23, 2009 at 08:33:51PM +0100, Ingo Molnar wrote: > > * 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, i've removed the patch - Paul, would you mind to re-send > your original flag solution, with it marked __read_mostly and > with the extern declarations put into a suitable header file? > > Paul, incidentally, this very minute i tracked down that the > patch is also causing boot lockups in -tip testing. I havent yet > fully debugged it, but a question comes immediately: if there's > no grace periods during bootup, wont rcu_sync() & friends just > hang indefinitely? Ouch!!! Indeed they would. > More thought is needed. One fix would be to sprinkle calls to rcu_qsctr_inc() through the boot process. But a much better approach would be for me to make synchronize_rcu() check this same flag, and simply return if called during early boot. The rationale for this is that there is but a single CPU during early boot, so tinyrcu.c's optimization can be used. ;-) Out of both paranoia and self defense, I would check num_online_cpus() in my proposed call into RCU. ;-) Seem reasonable? And does synchronize_sched() also need the UP-only optimization? Thanx, Paul