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

[Michal Hocko]

On Tue 11-12-12 16:50:25, Michal Hocko 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.

Hmm, thinking about it some more, it _is_ be safe in the end.

We are safe because we are under RCU. And even if somebody else looked
at the half-dead memcg from iter->last_visited it cannot disappear
because the current one will retry without dropping RCU so the grace
period couldn't have been finished.

		CPU0					CPU1
rcu_read_lock()						rcu_read_lock()
while(!memcg) {						while(!memcg)
[...]
spin_lock(&iter->iter_lock)
[...]
if (css == &root->css ||
		(css && css_tryget(css)))
	memcg = mem_cgroup_from_css(css)
[...]
if (css && !memcg)
	curr = mem_cgroup_from_css(css)
if (curr)
	css_get(curr);
spin_unlock(&iter->iter_lock)
							spin_lock(&iter->iter_lock)
							/* sees the half dead memcg but its cgroup is still valid */ 
							[...]
							spin_unlock(&iter->iter_lock)
/* we do retry */
}
rcu_read_unlock()

so the css_get will just helps to prevent from further code obfuscation.

Makes sense? The code gets much simplified later in the series,
fortunately.
-- 
Michal Hocko
SUSE Labs
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