Date: Thu, 12 May 2011 12:39:42 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        CAI Qian <caiqian@redhat.com>, avagin@gmail.com,
        Andrey Vagin <avagin@openvz.org>,
        Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mel@csn.ul.ie>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        David Rientjes <rientjes@google.com>, Hugh Dickins <hughd@google.com>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 2/4] oom: kill younger process first
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On Thu, 12 May 2011 11:23:38 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> On Thu, May 12, 2011 at 10:53 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Thu, 12 May 2011 10:30:45 +0900
> > Minchan Kim <minchan.kim@gmail.com> 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.

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.
  Then, the rculist is 1 directional list in nature, I think. 

So, without very very big reason, we should keep POISON.

Thanks,
-Kame


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