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... >> > >> > 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. 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 > Â under RCU. > Â But, in following case > Â 1. you are now at (C). you'll visit (C)->next...(D) > Â 2. you are now at (D). you want to go back to (C) via (D)->prev. > Â 3. But (D)->prev points to (B) > > ÂIt'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. :) > ÂThen, 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, 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