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    SubjectRe: [RFC][PATCH 1/6] memcg: fix pre_destory handler
    > +static bool memcg_is_obsolete(struct mem_cgroup *mem)
    > +{

    Will this function be called with mem->css.refcnt == 0? If yes, then
    this function is racy.

    cg = mem->css.cgroup
    cgroup_diput()
    mem_cgroup_destroy()
    mem->css.cgroup = NULL;
    kfree(cg);
    if (!cg || cgroup_is_removed(cg)...)

    (accessing invalid cg)

    > + struct cgroup *cg = mem->css.cgroup;
    > + /*
    > + * "Being Removed" means pre_destroy() handler is called.
    > + * After "pre_destroy" handler is called, memcg should not
    > + * have any additional charges.
    > + * This means there are small races for mis-accounting. But this
    > + * mis-accounting should happen only under swap-in opration.
    > + * (Attachin new task will fail if cgroup is under rmdir()).
    > + */
    > +
    > + if (!cg || cgroup_is_removed(cg) || cgroup_is_being_removed(cg))
    > + return true;
    > + return false;
    > +}
    > +

    ...

    > static void mem_cgroup_destroy(struct cgroup_subsys *ss,
    > struct cgroup *cont)
    > {
    > - mem_cgroup_free(mem_cgroup_from_cont(cont));
    > + struct mem_cgroup *mem = mem_cgroup_from_cont(cont):
    > + mem_cgroup_free(mem);
    > + /* forget */
    > + mem->css.cgroup = NULL;

    mem might already be destroyed by mem_cgroup_free(mem).

    > }


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