Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758160Ab2F0RO2 (ORCPT ); Wed, 27 Jun 2012 13:14:28 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:52528 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756880Ab2F0RO0 (ORCPT ); Wed, 27 Jun 2012 13:14:26 -0400 Date: Wed, 27 Jun 2012 10:10:44 -0700 From: "Paul E. McKenney" To: Mike Galbraith Cc: Alexey Vlasov , linux-kernel@vger.kernel.org Subject: Re: Attaching a process to cgroups Message-ID: <20120627171044.GF2418@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120619185856.GC31797@beaver> <1340195298.15707.3.camel@marge.simpson.net> <20120621075428.GE31797@beaver> <1340266982.29752.37.camel@marge.simpson.net> <20120626180620.GK2397@linux.vnet.ibm.com> <1340781811.10881.70.camel@marge.simpson.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1340781811.10881.70.camel@marge.simpson.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12062717-3534-0000-0000-000009DA947C Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2002 Lines: 45 On Wed, Jun 27, 2012 at 09:23:31AM +0200, Mike Galbraith wrote: > On Tue, 2012-06-26 at 11:06 -0700, Paul E. McKenney wrote: > > On Thu, Jun 21, 2012 at 10:23:02AM +0200, Mike Galbraith wrote: > > > On Thu, 2012-06-21 at 11:54 +0400, Alexey Vlasov wrote: > > > > On Wed, Jun 20, 2012 at 02:28:18PM +0200, Mike Galbraith wrote: > > > > > > > > > > kernel/cgroup.c::cgroup_attach_task() > > > > > { > > > > > ... > > > > > synchronize_rcu(); > > > > > ... > > > > > } > > > > > > > > So nothing can be done here? (I mean if only I knew how to fix it I > > > > wouldn't ask about it ;) > > > > > > Sure, kill the obnoxious thing, it's sitting right in the middle of the > > > userspace interface. > > > > > > I banged on it a while back (wrt explosive android patches), extracted > > > RCU from the userspace interface. It seemed to work great, much faster, > > > couldn't make it explode. I wouldn't bet anything I wasn't willing to > > > immediately part with that the result was really really safe though ;-) > > > > Or replace it with synchronize_rcu_expedited(). You can "get lucky" > > for quite some time removing synchronize_rcu() calls! > > s/remove/replace, but yup. A company that wanted to use the android > patches plus my tinkering showed a fix they needed on top to close a > race discovered in their testing. So yeah, even when all seems fine, > extracting synchronize_rcu() may expose evils you couldn't encounter > before, and didn't happen to encounter afterward. I really did mean "remove". Removing a synchronize_rcu() does result in a race, but often an extremely low-probability race. So you can remove a synchronize_rcu() and get lucky for a long time, but sooner or later, something will explode. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/