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

[Johannes Weiner <hannes@cmpxchg.org>]

Michal Hocko <mhocko@suse.cz> wrote:

>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.

I thought the whole point was to not have a reference in last_visited because have the iterator might be unused indefinitely :-)

We only store a pointer and validate it before use the next time around.  So I think the race is still possible, but we can deal with it by not losing concurrent dead count changes, i.e. one atomic read in the iterator function.

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