Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932132Ab1ELOiz (ORCPT ); Thu, 12 May 2011 10:38:55 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:35100 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757822Ab1ELOix (ORCPT ); Thu, 12 May 2011 10:38:53 -0400 Date: Thu, 12 May 2011 07:38:44 -0700 From: "Paul E. McKenney" To: Minchan Kim Cc: KAMEZAWA Hiroyuki , KOSAKI Motohiro , CAI Qian , avagin@gmail.com, Andrey Vagin , Andrew Morton , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Rientjes , Hugh Dickins , Oleg Nesterov Subject: Re: [PATCH 2/4] oom: kill younger process first Message-ID: <20110512143844.GQ2258@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110509182110.167F.A69D9226@jp.fujitsu.com> <20110510171335.16A7.A69D9226@jp.fujitsu.com> <20110510171641.16AF.A69D9226@jp.fujitsu.com> <20110512095243.c57e3e83.kamezawa.hiroyu@jp.fujitsu.com> <20110512105351.a57970d7.kamezawa.hiroyu@jp.fujitsu.com> <20110512123942.4b641e2d.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 Content-Length: 3282 Lines: 86 On Thu, May 12, 2011 at 01:17:13PM +0900, Minchan Kim wrote: > On Thu, May 12, 2011 at 12:39 PM, KAMEZAWA Hiroyuki > wrote: > > On Thu, 12 May 2011 11:23:38 +0900 > > Minchan Kim wrote: > > > >> On Thu, May 12, 2011 at 10:53 AM, KAMEZAWA Hiroyuki > >> wrote: > >> > On Thu, 12 May 2011 10:30:45 +0900 > >> > Minchan Kim wrote: > > > >> > As above implies, (B)->prev pointer is invalid pointer after list_del(). > >> > So, there will be race with list modification and for_each_list_reverse under > >> > rcu_read__lock() > >> > > >> > So, when you need to take atomic lock (as tasklist lock is) is... > >> > > >> > ?1) You can't check 'entry' is valid or not... > >> > ? ?In above for_each_list_rcu(), you may visit an object which is under removing. > >> > ? ?You need some flag or check to see the object is valid or not. > >> > > >> > ?2) you want to use list_for_each_safe(). > >> > ? ?You can't do list_del() an object which is under removing... > >> > > >> > ?3) You want to walk the list in reverse. > >> > > >> > ?3) Some other reasons. For example, you'll access an object pointed by the > >> > ? ?'entry' and the object is not rcu safe. > >> > > >> > make sense ? > >> > >> Yes. Thanks, Kame. > >> It seems It is caused by prev poisoning of list_del_rcu. > >> If we remove it, isn't it possible to traverse reverse without atomic lock? > >> > > > > IIUC, it's possible (Fix me if I'm wrong) but I don't like that because of 2 reasons. > > > > 1. LIST_POISON is very important information at debug. > > Indeed. > But if we can get a better something although we lost debug facility, > I think it would be okay. > > > > > 2. If we don't clear prev pointer, ok, we'll allow 2 directional walk of list > > ? under RCU. > > ? But, in following case > > ? 1. you are now at (C). you'll visit (C)->next...(D) > > ? 2. you are now at (D). you want to go back to (C) via (D)->prev. > > ? 3. But (D)->prev points to (B) > > > > ?It's not a 2 directional list, something other or broken one. > > Yes. but it shouldn't be a problem in RCU semantics. > If you need such consistency, you should use lock. > > I recall old thread about it. > In http://lwn.net/Articles/262464/, mmutz and Paul already discussed > about it. :) > > > ?Then, the rculist is 1 directional list in nature, I think. > > Yes. But Why RCU become 1 directional list is we can't find a useful usecases. > > > > > So, without very very big reason, we should keep POISON. > > Agree. > I don't insist on it as it's not a useful usecase for persuading Paul. > That's because it's not a hot path. > > It's started from just out of curiosity. > Thanks for very much clarifying that, Kame! Indeed, we would need a large performance/scalability/simplicity advantage to put up with such a loss of debugging information. If it turns out that you really need this, please let me know, but please also provide data supporting your need. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/