On Thu, May 12, 2011 at 01:17:13PM +0900, Minchan Kim wrote: > On Thu, May 12, 2011 at 12:39 PM, KAMEZAWA Hiroyuki > <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > > 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... > >> > > >> > 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, 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>