From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751879AbaAOIVE (ORCPT ); Wed, 15 Jan 2014 03:21:04 -0500 Received: from cantor2.suse.de ([195.135.220.15]:41294 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751132AbaAOIVB (ORCPT ); Wed, 15 Jan 2014 03:21:01 -0500 Date: Wed, 15 Jan 2014 09:20:58 +0100 From: Michal Hocko To: Tejun Heo Cc: Hugh Dickins , Johannes Weiner , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] mm/memcg: iteration skip memcgs not yet fully initialized Message-ID: <20140115082058.GA8782@dhcp22.suse.cz> References: <20140114133005.GC32227@dhcp22.suse.cz> <20140114142904.GA12131@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140114142904.GA12131@htj.dyndns.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 14-01-14 09:29:04, Tejun Heo wrote: > Hey, Michal. > > On Tue, Jan 14, 2014 at 02:30:05PM +0100, Michal Hocko wrote: > > On Mon 13-01-14 17:54:04, Hugh Dickins wrote: > > > It is surprising that the mem_cgroup iterator can return memcgs which > > > have not yet been fully initialized. By accident (or trial and error?) > > > this appears not to present an actual problem; but it may be better to > > > prevent such surprises, by skipping memcgs not yet online. > > > > My understanding was that !online cgroups are not visible for the > > iterator. it is css_online that has to be called before they are made > > visible. > > > > Tejun? > > From the comment above css_for_each_descendant_pre() > > * Walk @root's descendants. @root is included in the iteration and the > * first node to be visited. Must be called under rcu_read_lock(). A > * descendant css which hasn't finished ->css_online() or already has > * finished ->css_offline() may show up during traversal and it's each > * subsystem's responsibility to verify that each @pos is alive. /me slaps self. I was even reviewing patches which introduced that. But still I managed to convince myself that online means before online rather than right after again and again. Sorry about the confusion. > What it guarantees is that an online css would *always* show up in the > iteration. It's kinda difficult to guarantee both directions just > with RCU locking. You gotta make at least one end loose to make it > work with RCU. > > Thanks. > > -- > tejun -- Michal Hocko SUSE Labs