From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753079AbaFJPUN (ORCPT ); Tue, 10 Jun 2014 11:20:13 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:46034 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752748AbaFJPUK (ORCPT ); Tue, 10 Jun 2014 11:20:10 -0400 Date: Tue, 10 Jun 2014 08:20:06 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Oleg Nesterov , Linus Torvalds , Steven Rostedt , LKML , Thomas Gleixner , Andrew Morton , Ingo Molnar , Clark Williams Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc) Message-ID: <20140610152006.GA30219@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140603172632.GA27956@redhat.com> <20140603200125.GB1105@redhat.com> <20140606203350.GU4581@linux.vnet.ibm.com> <20140608130718.GA11129@redhat.com> <20140609162613.GE4581@linux.vnet.ibm.com> <20140610083726.GY3213@twins.programming.kicks-ass.net> <20140610125234.GI4581@linux.vnet.ibm.com> <20140610130138.GC3213@twins.programming.kicks-ass.net> <20140610143632.GM4581@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140610143632.GM4581@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14061015-6688-0000-0000-00000277FB57 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 10, 2014 at 07:36:32AM -0700, Paul E. McKenney wrote: > On Tue, Jun 10, 2014 at 03:01:38PM +0200, Peter Zijlstra wrote: > > On Tue, Jun 10, 2014 at 05:52:35AM -0700, Paul E. McKenney wrote: > > > On Tue, Jun 10, 2014 at 10:37:26AM +0200, Peter Zijlstra wrote: > > > > On Mon, Jun 09, 2014 at 09:26:13AM -0700, Paul E. McKenney wrote: > > > > > That would indeed be a bad thing, as it could potentially lead to > > > > > use-after-free bugs. Though one could argue that any code that resulted > > > > > in use-after-free would be quite aggressive. But still... > > > > > > > > Let me hijack this thread for yet another issue... So I had an RCU > > > > related use-after-free the other day, and while Sasha was able to > > > > trigger it quite easily, I had a multi-day struggle to reproduce. > > > > > > > > Once I figured out what the exact problem was it was also clear to me > > > > why it was so hard for me to reproduce. > > > > > > > > So normally its easier to trigger races on bigger machines, more cpus, > > > > more concurrency, more races, all good. > > > > > > > > _However_ with RCU the grace period machinery is slower the bigger the > > > > machine, so bigger machine, slower grace period, slower RCU free, less > > > > likely to hit use-after-free. > > > > > > > > So I was thinking, and I know you all will go kick me for this because > > > > the very last thing we need is what I'm about to propose: more RCU > > > > flavours :-). > > > > > > > > How about an rcu_read_unlock() reference counted RCU variant that's > > > > ultra aggressive in doing the callbacks in order to better trigger such > > > > issues? > > > > > > If you are using synchronize_rcu() for the update side, then I suggest > > > rcutorture.gp_exp=1 to force use expediting throughout. > > > > No such luck, this was regular kfree() from call_rcu(). And the callback > > execution was typically delayed long enough to never 'see' the > > use-after-free. > > Figures. ;-) > > Well, there is always the approach of booting your big systems with most > of the CPUs turned off. Another approach would be to set HZ=10000 or > some such, assuming the kernel can actually survive that kind of abuse. And yet another approach is to have a pair of low-priority processes per CPU that context-switch back and forth to each other if that CPU has nothing else to do. This should get rid of most of the increase in grace-period duration with increasing numbers of CPUs. Thanx, Paul