From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752249Ab2BPKRY (ORCPT ); Thu, 16 Feb 2012 05:17:24 -0500 Received: from s15943758.onlinehome-server.info ([217.160.130.188]:42979 "EHLO mail.x86-64.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751281Ab2BPKRV (ORCPT ); Thu, 16 Feb 2012 05:17:21 -0500 Date: Thu, 16 Feb 2012 11:17:14 +0100 From: Borislav Petkov To: "Paul E. McKenney" Cc: Borislav Petkov , "Srivatsa S. Bhat" , tony.luck@intel.com, tglx@linutronix.de, mingo@elte.hu, hpa@zytor.com, x86@kernel.org, linux-edac@vger.kernel.org, linux-kernel@vger.kernel.org, marcos.mage@gmail.com, prasad@linux.vnet.ibm.com Subject: Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer() Message-ID: <20120216101714.GA17412@aftab> References: <20120111142213.16610.85866.stgit@srivatsabhat.in.ibm.com> <20120111161153.GA2925@linux.vnet.ibm.com> <4F102353.1040109@linux.vnet.ibm.com> <20120113214435.GC2909@linux.vnet.ibm.com> <20120206161802.GC31237@aftab> <20120212174133.GL3737@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120212174133.GL3737@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Feb 12, 2012 at 09:41:33AM -0800, Paul E. McKenney wrote: > On Mon, Feb 06, 2012 at 05:18:02PM +0100, Borislav Petkov wrote: > > On Fri, Jan 13, 2012 at 01:44:35PM -0800, Paul E. McKenney wrote: > > > Looks good to me, but I do need to defer to people who know this code > > > better than do I. The key thing that (from what I can see) makes > > > rcu_dereference() unnecessary is that the smp_rmb() used in conjunction > > > with polling the .finished field takes care of ordering. > > > > Right, this was me trying hard not to screw up touching mcelog.next, > > thus trying to use the rcu_dereference_index_check() primitive without > > thinking it through too much. But you're right, I'm polling the > > ->finished field 4 times (totally arbitrary, btw) which should suffice > > while the mce_log() routine above writes those entries. > > > > Although, the question still remains, since mce_log() accesses > > mcelog.next through the rcu_dereference_index_check() primitive, > > shouldn't I do it the same way? > > I don't claim to be an mce_log() expert, but when I looked it over, > I didn't see a need for rcu_dereference_index_check(). Unless I am > confused (quite possible), the memory barriers are sufficient. > > The rcu_read_lock() and rcu_read_unlock() seem to be needed to avoid > premature freeing, though. Looka ere: commit f56e8a0765cc4374e02f4e3a79e2427b5096b075 Author: Paul E. McKenney Date: Fri Mar 5 15:03:27 2010 -0800 x86/mce: Fix RCU lockdep splats Create an rcu_dereference_check_mce() that checks for RCU-sched read side and mce_read_mutex being held on update side. Replace uses of rcu_dereference() in arch/x86/kernel/cpu/mcheck/mce.c with this new macro. ... +#define rcu_dereference_check_mce(p) \ + rcu_dereference_check((p), \ + rcu_read_lock_sched_held() || \ + lockdep_is_held(&mce_read_mutex)) + So I guess the check is to see that we're rather holding the mce_chrdev_read_mutex when accessing this from userspace (the mcelog userspace thing). I can't comment on the RCU-sched thing because I don't know it that well. It has to do with extended CPU idle times but why do we require to be in RCU read-side critical section when accessing the index? Hmmm, I think a guy called Paul should know :-) ... Thanks. -- Regards/Gruss, Boris. Advanced Micro Devices GmbH Einsteinring 24, 85609 Dornach GM: Alberto Bozzo Reg: Dornach, Landkreis Muenchen HRB Nr. 43632 WEEE Registernr: 129 19551