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

From: Michal Hocko
Date: Tue Feb 12 2013 - 11:24:47 EST


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.

--
Michal Hocko
SUSE Labs
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