On Thu, 12 May 2011 11:23:38 +0900 Minchan Kim <minchan.kim@xxxxxxxxx> wrote: > On Thu, May 12, 2011 at 10:53 AM, KAMEZAWA Hiroyuki > <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > > On Thu, 12 May 2011 10:30:45 +0900 > > Minchan Kim <minchan.kim@xxxxxxxxx> 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... > > > > Â1) You can't check 'entry' is valid or not... > > Â ÂIn above for_each_list_rcu(), you may visit an object which is under removing. > > Â ÂYou need some flag or check to see the object is valid or not. > > > > Â2) you want to use list_for_each_safe(). > > Â ÂYou can't do list_del() an object which is under removing... > > > > Â3) You want to walk the list in reverse. > > > > Â3) Some other reasons. For example, you'll access an object pointed by the > > Â Â'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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>