From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755617Ab2BLRlm (ORCPT ); Sun, 12 Feb 2012 12:41:42 -0500 Received: from e36.co.us.ibm.com ([32.97.110.154]:53622 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754677Ab2BLRlk (ORCPT ); Sun, 12 Feb 2012 12:41:40 -0500 Date: Sun, 12 Feb 2012 09:41:33 -0800 From: "Paul E. McKenney" To: Borislav Petkov Cc: "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: <20120212174133.GL3737@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120206161802.GC31237@aftab> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12021217-3352-0000-0000-00000291DE89 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul