From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752433AbaFJMwn (ORCPT ); Tue, 10 Jun 2014 08:52:43 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:49873 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752390AbaFJMwk (ORCPT ); Tue, 10 Jun 2014 08:52:40 -0400 Date: Tue, 10 Jun 2014 05:52:35 -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: <20140610125234.GI4581@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140603130233.658a6a3c@gandalf.local.home> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140610083726.GY3213@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14061012-0928-0000-0000-000002983BBA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul