From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755276Ab0DZUmT (ORCPT ); Mon, 26 Apr 2010 16:42:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40509 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755263Ab0DZUmQ (ORCPT ); Mon, 26 Apr 2010 16:42:16 -0400 Date: Mon, 26 Apr 2010 16:42:08 -0400 From: Vivek Goyal To: "Paul E. McKenney" Cc: Li Zefan , linux kernel mailing list , Jens Axboe , Gui Jianfeng Subject: Re: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue() Message-ID: <20100426204208.GF3372@redhat.com> References: <20100422155452.GD3228@redhat.com> <20100422231556.GW2524@linux.vnet.ibm.com> <20100422235555.GA12004@redhat.com> <20100423001751.GX2524@linux.vnet.ibm.com> <20100423144138.GA5026@redhat.com> <20100423194649.GF2589@linux.vnet.ibm.com> <4BD4ED7A.5020205@cn.fujitsu.com> <20100426020631.GU2440@linux.vnet.ibm.com> <20100426133920.GA1559@redhat.com> <20100426144542.GA2529@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100426144542.GA2529@linux.vnet.ibm.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 26, 2010 at 07:45:42AM -0700, Paul E. McKenney wrote: > On Mon, Apr 26, 2010 at 09:39:20AM -0400, Vivek Goyal wrote: > > On Sun, Apr 25, 2010 at 07:06:31PM -0700, Paul E. McKenney wrote: > > > On Mon, Apr 26, 2010 at 09:33:46AM +0800, Li Zefan wrote: > > > > >>>>>> With RCU correctness on, We see following warning. This patch fixes it. > > > > >>>>> This is in initialization code, so that there cannot be any concurrent > > > > >>>>> updates, correct? If so, looks good. > > > > >>>>> > > > > >>>> I think theoritically two instances of cfq_init_queue() can be running > > > > >>>> in parallel (for two different devices), and they both can call > > > > >>>> blkiocg_add_blkio_group(). But then we use a spin lock to protect > > > > >>>> blkio_cgroup. > > > > >>>> > > > > >>>> spin_lock_irqsave(&blkcg->lock, flags); > > > > >>>> > > > > >>>> So I guess two parallel updates should be fine. > > > > >>> OK, in that case, would it be possible add this spinlock to the condition > > > > >>> checked by css_id()'s rcu_dereference_check()? > > > > >> Hi Paul, > > > > >> > > > > >> I think adding these spinlock to condition checked might become little > > > > >> messy. And the reason being that this lock is subsystem (controller) > > > > >> specific and maintained by controller. Now if any controller implements > > > > >> a lock and we add that lock in css_id() rcu_dereference_check(), it will > > > > >> look ugly. > > > > >> > > > > >> So probably a better way is to make sure that css_id() is always called > > > > >> under rcu read lock so that we don't hit this warning? > > > > > > > > > > As long as holding rcu_read_lock() prevents css_id() from the usual > > > > > problems such as access memory that was concurrently freed, yes. > > > > > > > > blkiocg_add_blkio_group() also calls cgroup_path(), which also needs to > > > > be called within rcu_read_lock, so I think Vivek's patch is better than > > > > the one you posted in another mail thread. > > > > > > My apologies, Vivek! I lost track of your patch. I have now replaced > > > my patch with yours. > > > > Thanks Paul. > > > > I sent this patch to Jens also, thinking he will apply to his tree. Looks > > like he has not applied it yet though. > > > > Jens, is it ok if this patch gets merged through paul's tree or should it > > go through blk tree? > > I am happy for it to go either way, so just let me know! I am also happy to go either way. I guess you can go ahead with pulling it in. Thanks Vivek