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    SubjectRe: [PATCH 1/4] cgroup: add CSS ID
    Hi.

    Sorry for very late reply.

    It looks good in general.
    Just a few comments.

    > +/**
    > + * css_lookup - lookup css by id
    > + * @ss: cgroup subsys to be looked into.
    > + * @id: the id
    > + *
    > + * Returns pointer to cgroup_subsys_state if there is valid one with id.
    > + * NULL if not. Should be called under rcu_read_lock()
    > + */
    > +struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
    > +{
    > + struct css_id *cssid = NULL;
    > +
    > + BUG_ON(!ss->use_id);
    > + cssid = idr_find(&ss->idr, id);
    > +
    > + if (unlikely(!cssid))
    > + return NULL;
    > +
    > + return rcu_dereference(cssid->css);
    > +}
    > +
    Just for clarification, is there any user of this function ?
    (I agree it's natulal to define 'lookup' function, though.)

    > +/**
    > + * css_get_next - lookup next cgroup under specified hierarchy.
    > + * @ss: pointer to subsystem
    > + * @id: current position of iteration.
    > + * @root: pointer to css. search tree under this.
    > + * @foundid: position of found object.
    > + *
    > + * Search next css under the specified hierarchy of rootid. Calling under
    > + * rcu_read_lock() is necessary. Returns NULL if it reaches the end.
    > + */
    > +struct cgroup_subsys_state *
    > +css_get_next(struct cgroup_subsys *ss, int id,
    > + struct cgroup_subsys_state *root, int *foundid)
    > +{
    > + struct cgroup_subsys_state *ret = NULL;
    > + struct css_id *tmp;
    > + int tmpid;
    > + int rootid = css_id(root);
    > + int depth = css_depth(root);
    > +
    I think it's safe here, but isn't it better to call css_id/css_depth
    under rcu_read_lock(they call rcu_dereference) ?

    > + if (!rootid)
    > + return NULL;
    > +
    > + BUG_ON(!ss->use_id);
    > + rcu_read_lock();
    > + /* fill start point for scan */
    > + tmpid = id;
    > + while (1) {
    > + /*
    > + * scan next entry from bitmap(tree), tmpid is updated after
    > + * idr_get_next().
    > + */
    > + spin_lock(&ss->id_lock);
    > + tmp = idr_get_next(&ss->idr, &tmpid);
    > + spin_unlock(&ss->id_lock);
    > +
    > + if (!tmp)
    > + break;
    > + if (tmp->depth >= depth && tmp->stack[depth] == rootid) {
    Can't be css_is_ancestor used here ?
    I think it would be more easy to understand.

    > + ret = rcu_dereference(tmp->css);
    > + if (ret) {
    > + *foundid = tmpid;
    > + break;
    > + }
    > + }
    > + /* continue to scan from next id */
    > + tmpid = tmpid + 1;
    > + }
    > +
    > + rcu_read_unlock();
    > + return ret;
    > +}
    > +


    Thanks,
    Daisuke Nishimura.


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