From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933200Ab3BLPnf (ORCPT ); Tue, 12 Feb 2013 10:43:35 -0500 Received: from cantor2.suse.de ([195.135.220.15]:57578 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758691Ab3BLPnd (ORCPT ); Tue, 12 Feb 2013 10:43:33 -0500 Date: Tue, 12 Feb 2013 16:43:30 +0100 From: Michal Hocko To: Johannes Weiner Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, KAMEZAWA Hiroyuki , Ying Han , Tejun Heo , Glauber Costa , Li Zefan Subject: Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators Message-ID: <20130212154330.GG4863@dhcp22.suse.cz> References: <1357235661-29564-5-git-send-email-mhocko@suse.cz> <20130208193318.GA15951@cmpxchg.org> <20130211151649.GD19922@dhcp22.suse.cz> <20130211175619.GC13218@cmpxchg.org> <20130211192929.GB29000@dhcp22.suse.cz> <20130211195824.GB15951@cmpxchg.org> <20130211212756.GC29000@dhcp22.suse.cz> <20130211223943.GC15951@cmpxchg.org> <20130212095419.GB4863@dhcp22.suse.cz> <20130212151002.GD15951@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130212151002.GD15951@cmpxchg.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 12-02-13 10:10:02, Johannes Weiner wrote: > On Tue, Feb 12, 2013 at 10:54:19AM +0100, Michal Hocko wrote: > > On Mon 11-02-13 17:39:43, Johannes Weiner wrote: > > > On Mon, Feb 11, 2013 at 10:27:56PM +0100, Michal Hocko wrote: > > > > On Mon 11-02-13 14:58:24, Johannes Weiner wrote: > > > > > That way, if the dead count gives the go-ahead, you KNOW that the > > > > > position cache is valid, because it has been updated first. > > > > > > > > OK, you are right. We can live without css_tryget because dead_count is > > > > either OK which means that css would be alive at least this rcu period > > > > (and RCU walk would be safe as well) or it is incremented which means > > > > that we have started css_offline already and then css is dead already. > > > > So css_tryget can be dropped. > > > > > > Not quite :) > > > > > > The dead_count check is for completed destructions, > > > > Not quite :P. dead_count is incremented in css_offline callback which is > > called before the cgroup core releases its last reference and unlinks > > the group from the siblinks. css_tryget would already fail at this stage > > because CSS_DEACT_BIAS is in place at that time but this doesn't break > > RCU walk. So I think we are safe even without css_get. > > But you drop the RCU lock before you return. > > dead_count IS incremented for every destruction, but it's not reliable > for concurrent ones, is what I meant. Again, if there is a dead_count > mismatch, your pointer might be dangling, easy case. However, even if > there is no mismatch, you could still race with a destruction that has > marked the object dead, and then frees it once you drop the RCU lock, > so you need try_get() to check if the object is dead, or you could > return a pointer to freed or soon to be freed memory. Wait a moment. But what prevents from the following race? rcu_read_lock() mem_cgroup_css_offline(memcg) root->dead_count++ iter->last_dead_count = root->dead_count iter->last_visited = memcg // final css_put(memcg); // last_visited is still valid rcu_read_unlock() [...] // next iteration rcu_read_lock() iter->last_dead_count == root->dead_count // KABOOM The race window between dead_count++ and css_put is quite big but that is not important because that css_put can happen anytime before we start the next iteration and take rcu_read_lock. -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx165.postini.com [74.125.245.165]) by kanga.kvack.org (Postfix) with SMTP id 28BD96B0005 for ; Tue, 12 Feb 2013 10:43:34 -0500 (EST) Date: Tue, 12 Feb 2013 16:43:30 +0100 From: Michal Hocko Subject: Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators Message-ID: <20130212154330.GG4863@dhcp22.suse.cz> References: <1357235661-29564-5-git-send-email-mhocko@suse.cz> <20130208193318.GA15951@cmpxchg.org> <20130211151649.GD19922@dhcp22.suse.cz> <20130211175619.GC13218@cmpxchg.org> <20130211192929.GB29000@dhcp22.suse.cz> <20130211195824.GB15951@cmpxchg.org> <20130211212756.GC29000@dhcp22.suse.cz> <20130211223943.GC15951@cmpxchg.org> <20130212095419.GB4863@dhcp22.suse.cz> <20130212151002.GD15951@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130212151002.GD15951@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, KAMEZAWA Hiroyuki , Ying Han , Tejun Heo , Glauber Costa , Li Zefan On Tue 12-02-13 10:10:02, Johannes Weiner wrote: > On Tue, Feb 12, 2013 at 10:54:19AM +0100, Michal Hocko wrote: > > On Mon 11-02-13 17:39:43, Johannes Weiner wrote: > > > On Mon, Feb 11, 2013 at 10:27:56PM +0100, Michal Hocko wrote: > > > > On Mon 11-02-13 14:58:24, Johannes Weiner wrote: > > > > > That way, if the dead count gives the go-ahead, you KNOW that the > > > > > position cache is valid, because it has been updated first. > > > > > > > > OK, you are right. We can live without css_tryget because dead_count is > > > > either OK which means that css would be alive at least this rcu period > > > > (and RCU walk would be safe as well) or it is incremented which means > > > > that we have started css_offline already and then css is dead already. > > > > So css_tryget can be dropped. > > > > > > Not quite :) > > > > > > The dead_count check is for completed destructions, > > > > Not quite :P. dead_count is incremented in css_offline callback which is > > called before the cgroup core releases its last reference and unlinks > > the group from the siblinks. css_tryget would already fail at this stage > > because CSS_DEACT_BIAS is in place at that time but this doesn't break > > RCU walk. So I think we are safe even without css_get. > > But you drop the RCU lock before you return. > > dead_count IS incremented for every destruction, but it's not reliable > for concurrent ones, is what I meant. Again, if there is a dead_count > mismatch, your pointer might be dangling, easy case. However, even if > there is no mismatch, you could still race with a destruction that has > marked the object dead, and then frees it once you drop the RCU lock, > so you need try_get() to check if the object is dead, or you could > return a pointer to freed or soon to be freed memory. Wait a moment. But what prevents from the following race? rcu_read_lock() mem_cgroup_css_offline(memcg) root->dead_count++ iter->last_dead_count = root->dead_count iter->last_visited = memcg // final css_put(memcg); // last_visited is still valid rcu_read_unlock() [...] // next iteration rcu_read_lock() iter->last_dead_count == root->dead_count // KABOOM The race window between dead_count++ and css_put is quite big but that is not important because that css_put can happen anytime before we start the next iteration and take rcu_read_lock. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org