Messages in this thread | | | Date | Tue, 12 Feb 2013 17:13:32 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators |
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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 > > 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
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