Re: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators

[Ying Han]

On Tue, Dec 11, 2012 at 7:50 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> On Sun 09-12-12 08:59:54, Ying Han wrote:
>> On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> [...]
>> > +               /*
>> > +                * Even if we found a group we have to make sure it is alive.
>> > +                * css && !memcg means that the groups should be skipped and
>> > +                * we should continue the tree walk.
>> > +                * last_visited css is safe to use because it is protected by
>> > +                * css_get and the tree walk is rcu safe.
>> > +                */
>> > +               if (css == &root->css || (css && css_tryget(css)))
>> > +                       memcg = mem_cgroup_from_css(css);
>> >
>> >                 if (reclaim) {
>> > -                       iter->position = id;
>> > +                       struct mem_cgroup *curr = memcg;
>> > +
>> > +                       if (last_visited)
>> > +                               css_put(&last_visited->css);
>> > +
>> > +                       if (css && !memcg)
>> > +                               curr = mem_cgroup_from_css(css);
>>
>> In this case, the css_tryget() failed which implies the css is on the
>> way to be removed. (refcnt ==0) If so, why it is safe to call
>> css_get() directly on it below? It seems not preventing the css to be
>> removed by doing so.
>
> Well, I do not remember exactly but I guess the code is meant to say
> that we need to store a half-dead memcg because the loop has to be
> retried. As we are under RCU hood it is just half dead.
> Now that you brought this up I think this is not safe as well because
> another thread could have seen the cached value while we tried to retry
> and his RCU is not protecting the group anymore. The follow up patch
> fixes this by retrying within the loop. I will bring that part into
> this patch already and then leave only css clean up in the other patch.
>
> Thanks for spotting this Ying!

I understand the intention here where we want to move on to the next
css if the css_tryget() failed. But css_get() won't hold on it in that
case.

I fixed that on my local branch which do the retry after css_tryget()
failed, just like what you talked about. And I will wait for you fix
on that.

--Ying

>
>>
>> > +                       /* make sure that the cached memcg is not removed */
>> > +                       if (curr)
>> > +                               css_get(&curr->css);
>>
>> --Ying
> --
> Michal Hocko
> SUSE Labs
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