On Tue, Feb 12, 2013 at 12:25:26PM -0500, Johannes Weiner wrote: > On Tue, Feb 12, 2013 at 08:10:51AM -0800, 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?). > > Absolutely! And there is a synchronize_rcu() in between those two > operations. > > However, we want to keep a weak reference to the cgroup after we drop > the rcu read-side lock, so rcu alone is not enough for us to guarantee > object life time. We still have to carefully detect any concurrent > offlinings in order to validate the weak reference next time around. That would make things more interesting. ;-) Exactly who or what holds the weak reference? And the idea is that if you attempt to use the weak reference beforehand, the css_put() does not actually free it, but if you attempt to use it afterwards, you get some sort of failure indication? Thanx, Paul -- 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>