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SubjectRe: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators
On Tue 11-12-12 17:15:59, Michal Hocko wrote:
> 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@suse.cz> 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 I've just realized that one sentence vanished while I was writing
this.

So either we retry (while(!memcg)) and see the half-dead memcg with a
valid cgroup because we are under rcu so cgroup iterator will find a
next one. Or we race with somebody else on the iterator and that is
described bellow.

> 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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--
Michal Hocko
SUSE Labs


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