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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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