Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators

[Michal Hocko <mhocko@suse.cz>]

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.
-- 
Michal Hocko
SUSE Labs