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

[Michal Hocko <mhocko@suse.cz>]

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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