On Tue 12-02-13 08:10:51, Paul E. McKenney wrote: > On Tue, Feb 12, 2013 at 04:43:30PM +0100, Michal Hocko wrote: > > On Tue 12-02-13 10:10:02, Johannes Weiner wrote: > > > On Tue, Feb 12, 2013 at 10:54:19AM +0100, Michal Hocko wrote: > > > > On Mon 11-02-13 17:39:43, Johannes Weiner wrote: > > > > > On Mon, Feb 11, 2013 at 10:27:56PM +0100, Michal Hocko wrote: > > > > > > On Mon 11-02-13 14:58:24, Johannes Weiner wrote: > > > > > > > That way, if the dead count gives the go-ahead, you KNOW that the > > > > > > > position cache is valid, because it has been updated first. > > > > > > > > > > > > OK, you are right. We can live without css_tryget because dead_count is > > > > > > either OK which means that css would be alive at least this rcu period > > > > > > (and RCU walk would be safe as well) or it is incremented which means > > > > > > that we have started css_offline already and then css is dead already. > > > > > > So css_tryget can be dropped. > > > > > > > > > > Not quite :) > > > > > > > > > > The dead_count check is for completed destructions, > > > > > > > > Not quite :P. dead_count is incremented in css_offline callback which is > > > > called before the cgroup core releases its last reference and unlinks > > > > the group from the siblinks. css_tryget would already fail at this stage > > > > because CSS_DEACT_BIAS is in place at that time but this doesn't break > > > > RCU walk. So I think we are safe even without css_get. > > > > > > But you drop the RCU lock before you return. > > > > > > dead_count IS incremented for every destruction, but it's not reliable > > > for concurrent ones, is what I meant. Again, if there is a dead_count > > > mismatch, your pointer might be dangling, easy case. However, even if > > > there is no mismatch, you could still race with a destruction that has > > > marked the object dead, and then frees it once you drop the RCU lock, > > > so you need try_get() to check if the object is dead, or you could > > > return a pointer to freed or soon to be freed memory. > > > > Wait a moment. But what prevents from the following race? > > > > rcu_read_lock() > > mem_cgroup_css_offline(memcg) > > 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. > > The usual approach is to make sure that there is a grace period (either > synchronize_rcu() or call_rcu()) between the time that the data is > made inaccessible to readers (this would be mem_cgroup_css_offline()?) > and the time it is freed (css_put(), correct?). Yes, that was my suggestion and I put it before dead_count is incremented down the mem_cgroup_css_offline road. Johannes still thinks we can do without it if we reduce the number of atomic_read(dead_count) which sounds like a way to go but I will rather think about it with a fresh head tomorrow. Anyway, thanks for jumping in. Earth is always a bit shaky when all the barriers and rcu mix together. -- 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>