From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753985Ab1HXTRt (ORCPT ); Wed, 24 Aug 2011 15:17:49 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:56267 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753798Ab1HXTRr (ORCPT ); Wed, 24 Aug 2011 15:17:47 -0400 Date: Wed, 24 Aug 2011 12:16:21 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Don Zickus , x86@kernel.org, Andi Kleen , Robert Richter , ying.huang@intel.com, LKML , jason.wessel@windriver.com Subject: Re: [RFC][PATCH 2/6] x86, nmi: create new NMI handler routines Message-ID: <20110824191621.GL2417@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1313786266-9585-1-git-send-email-dzickus@redhat.com> <1313786266-9585-3-git-send-email-dzickus@redhat.com> <20110824170411.GI2417@linux.vnet.ibm.com> <20110824174456.GN2067@redhat.com> <1314208261.6925.51.camel@twins> <20110824181643.GP2067@redhat.com> <1314209995.6925.59.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1314209995.6925.59.camel@twins> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 24, 2011 at 08:19:54PM +0200, Peter Zijlstra wrote: > On Wed, 2011-08-24 at 14:16 -0400, Don Zickus wrote: > > On Wed, Aug 24, 2011 at 07:51:00PM +0200, Peter Zijlstra wrote: > > > On Wed, 2011-08-24 at 13:44 -0400, Don Zickus wrote: > > > > > > + rcu_read_lock(); > > > > > > + a = rcu_dereference_raw(*ap); > > > > > > > > > > The reason for rcu_dereference_raw() is to prevent lockdep from choking > > > > > due to being called from an NMI handler, correct? If so, please add a > > > > > comment to this effect on this and similar uses. > > > > > > > > That sounds right. But honestly, I just copied what notifier_call_chain > > > > had. Regardless, I will make sure to document that in my next version. > > > > Thanks! > > > > > > Not quite right, nmi_enter() does lockdep_disable() and makes > > > lock_is_held() return always true. > > > > > > I think this (and the other sites) could do with rcu_dereference_check(, > > > lockdep_is_held(&desc->lock)); not that it wouldn't be anything but > > > documentation since the actual test isn't working from NMI context but I > > > do think its worth it for that alone. > > > > So you want me to remove the _raw part of the dereference? I can test > > that with lockdep enabled to verify things don't go splat. > > Ah, right, its never used from the desc->lock context and we always hold > rcu_read_lock(), so a simple rcu_dereference() should indeed suffice. Even better! ;-) Thanx, Paul