On Tue 12-02-13 17:13:32, Michal Hocko wrote: > On Tue 12-02-13 16:43:30, Michal Hocko wrote: > [...] > The example was not complete: > > > Wait a moment. But what prevents from the following race? > > > > rcu_read_lock() > > cgroup_next_descendant_pre > css_tryget(css); > memcg = mem_cgroup_from_css(css) atomic_add(CSS_DEACT_BIAS, &css->refcnt) > > > mem_cgroup_css_offline(memcg) > > We should be safe if we did synchronize_rcu() before root->dead_count++, > no? > Because then we would have a guarantee that if css_tryget(memcg) > suceeded then we wouldn't race with dead_count++ it triggered. > > > root->dead_count++ > > iter->last_dead_count = root->dead_count > > iter->last_visited = memcg > > // final > > css_put(memcg); > > // last_visited is still valid > > rcu_read_unlock() > > [...] > > // next iteration > > rcu_read_lock() > > iter->last_dead_count == root->dead_count > > // KABOOM Ohh I have missed that we took a reference on the current memcg which will be stored into last_visited. And then later, during the next iteration it will be still alive until we are done because previous patch moved css_put to the very end. So this race is not possible. I still need to think about parallel iteration and a race with removal. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>