From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760364Ab3JPJ0I (ORCPT ); Wed, 16 Oct 2013 05:26:08 -0400 Received: from mail-ee0-f47.google.com ([74.125.83.47]:39606 "EHLO mail-ee0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760179Ab3JPJ0H (ORCPT ); Wed, 16 Oct 2013 05:26:07 -0400 Date: Wed, 16 Oct 2013 11:26:01 +0200 From: Ingo Molnar To: Eric Dumazet Cc: Peter Zijlstra , Christoph Lameter , Tejun Heo , akpm@linuxfoundation.org, rostedt@goodmis.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Eric Dumazet , David Miller Subject: Re: [PATCH 1/6] net: ip4_datagram_connect: Use correct form of statistics update Message-ID: <20131016092601.GB23440@gmail.com> References: <20131015174722.615394057@linux.com> <20131015174745.197380080@linux.com> <20131016083545.GP10651@twins.programming.kicks-ass.net> <1381914869.2045.112.camel@edumazet-glaptop.roam.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1381914869.2045.112.camel@edumazet-glaptop.roam.corp.google.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 * Eric Dumazet wrote: > On Wed, 2013-10-16 at 10:35 +0200, Peter Zijlstra wrote: > > On Tue, Oct 15, 2013 at 12:47:23PM -0500, Christoph Lameter wrote: > > > ip4_datagram_connect is called with BH processing enabled. Therefore > > > we cannot use IP_INC_STATS_BH but must use IP_INC_STATS which disables > > > BH handling before incrementing the counter. > > > > > > The following trace is triggered without this patch: > > > > > > [ 9293.806634] __this_cpu_add operation in preemptible [00000000] code: ntpd/2150 > > > > You lost the BUG there; that really needs to be there: > > > > - BUG makes people pay attention > > - This was an actual BUG wasn't it? > > > > Sure there can be false positives, but in all cases people should > > amend the code. Sometimes with a comment explaining why the raw > > primitive should be used; sometimes to fix an actual bug, but a patch > > needs to be written. Therefore BUG! > > Yep this is a real BUG for linux 2.6.36+ on 32bit arches, > > The effect of this bug is that on 32bit arches, we might corrupt a > seqcount : Later, we can spin forever on it. Ouch, that's a pretty serious bug ... The patch title should reflect this fact. > In linux 2.6.36 we converted IP mib from 32 to 64 bits, therefore this > fix should be backported up to 2.6.36 > > Prior to 2.6.36, the bug was that some increments of SNMP stat could be > lost, because two cpus could access the same location, hardly a > problem... Acked-by: Ingo Molnar Thanks, Ingo