From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754316Ab0DZCGg (ORCPT ); Sun, 25 Apr 2010 22:06:36 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:60539 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753536Ab0DZCGf (ORCPT ); Sun, 25 Apr 2010 22:06:35 -0400 Date: Sun, 25 Apr 2010 19:06:31 -0700 From: "Paul E. McKenney" To: Li Zefan Cc: Vivek Goyal , linux kernel mailing list , Jens Axboe , Gui Jianfeng Subject: Re: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue() Message-ID: <20100426020631.GU2440@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4BD4ED7A.5020205@cn.fujitsu.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul