From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH] cgroup: Fix root_count when mount fails due to busy subsystem Date: Fri, 23 Jan 2009 19:36:53 +0100 Message-ID: <20090123183653.GC5984@elte.hu> References: <20090123004703.25103.29754.stgit@menage.corp.google.com> <20090123181056.GA31142@elte.hu> <6599ad830901231032h76a301ceica371145eca30388@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <6599ad830901231032h76a301ceica371145eca30388@mail.gmail.com> Sender: linux-kernel-owner@vger.kernel.org To: Paul Menage Cc: akpm@linux-foundation.org, serue@us.ibm.com, linux-kernel@vger.kernel.org, containers@lists.osdl.org List-Id: containers.vger.kernel.org * Paul Menage wrote: > On Fri, Jan 23, 2009 at 10:10 AM, Ingo Molnar wrote: > > > > * Paul Menage wrote: > > > >> cgroup: Fix root_count when mount fails due to busy subsystem > >> > >> root_count was being incremented in cgroup_get_sb() after all error > >> checking was complete, but decremented in cgroup_kill_sb(), which can be > >> called on a superblock that we gave up on due to an error. This patch > >> changes cgroup_kill_sb() to only decrement root_count if the root was > >> previously linked into the list of roots. > > > > i'm wondering, what happens in the buggy case: does cgroup_kill_sb() get > > called twice (if yes, why?), > > No. > > > or do we call cgroup_kill_sb() on a not yet > > added sb and hence root_count has not been elevated yet? > > Right. > > > (if yes, which > > codepath does this?) > > It's via the call to deactivate_super(). Which exact call chain is that? > The code could be restructured such that: > > - we don't set sb->s_fs_info until we've linked the new root into the root_list > - do any necessary cleanup for a failed root in cgroup_get_sb() > - have cgroup_kill_sb() handle either no root or a fully-initialized root > > But then you're replacing "only decrement root_count if root was linked > in to list" with "only do root cleanup if root was atached to sb" in > cgroup_kill_sb(). I don't see that one is much cleaner than the other. Agreed, that's not an improvement. > For 2.6.29, we should fix this by reverting the broken part of the patch > that made it into 2.6.29-rcX Agreed too - i withdraw my objection. Nevertheless my observation remains: kernel/cgroup.c has a complex looking error paths which should be cleaned up. (independently of this issue) Ingo