Michal Hocko <mho...@suse.cz> wrote:
>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. I thought the whole point was to not have a reference in last_visited because have the iterator might be unused indefinitely :-) We only store a pointer and validate it before use the next time around. So I think the race is still possible, but we can deal with it by not losing concurrent dead count changes, i.e. one atomic read in the iterator function. -- Sent from my Android phone with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
