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 > > The race window between dead_count++ and css_put is quite big but that > is not important because that css_put can happen anytime before we start > the next iteration and take rcu_read_lock. -- 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>