Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751573Ab1ELERP (ORCPT ); Thu, 12 May 2011 00:17:15 -0400 Received: from mail-qy0-f181.google.com ([209.85.216.181]:35231 "EHLO mail-qy0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750845Ab1ELERO convert rfc822-to-8bit (ORCPT ); Thu, 12 May 2011 00:17:14 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=NXfVWcu8GYGk+/UqPVk+HI8sW69PFDmY/D5T+pbHzfwlR98UZjfQ7L+73vkCHfKgGK pU9f7zcJGYbocMr4GbqDBvUaaO/461+t4ofpCjkHVSuplcpu0Qg2U7M57b+SvZGGH30d dwNhlxnsOI8619fbI5BLiNelB4o1q8+qCdihM= MIME-Version: 1.0 In-Reply-To: <20110512123942.4b641e2d.kamezawa.hiroyu@jp.fujitsu.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> Date: Thu, 12 May 2011 13:17:13 +0900 Message-ID: Subject: Re: [PATCH 2/4] oom: kill younger process first From: Minchan Kim To: KAMEZAWA Hiroyuki Cc: 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 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2861 Lines: 82 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! -- Kind regards, Minchan Kim -- 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/