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    SubjectRe: [PATCH 1/4] cgroup: add CSS ID
    On Thu, 22 Jan 2009 11:37:29 +0900
    Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

    > 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.)
    >
    A user in my plan is swap_cgroup.



    > > +/**
    > > + * 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) ?
    >
    As commented, this css_get_next() call should be called under rcu_read_lock().


    > > + 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.
    >
    Hmm, it requires

    css_is_ancestor(tmp->css, root);

    and adds memory barriers to acsess tmp->css and tmp->css->id, root->id
    (compiler will not optimize these accesses because of memory barrier.)

    So, I think bare code is better here.

    Thank you for review.

    -Kame



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