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    SubjectRe: Possible race between cgroup_attach_proc and de_thread, and questionable code in de_thread.
    On 07/28, NeilBrown wrote:
    >
    > +/* Thread group leader can change, so stop loop when we see one
    > + * even if it isn't 'g' */
    > #define while_each_thread(g, t) \
    > - while ((t = next_thread(t)) != g)
    > + while ((t = next_thread(t)) != g && !thread_group_leader(t))

    This assumes that while_each_thread(g, t) always uses g == group_leader.

    > @@ -70,8 +70,13 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
    > list_del_rcu(&p->tasks);
    > list_del_init(&p->sibling);
    > __this_cpu_dec(process_counts);
    > - }
    > - list_del_rcu(&p->thread_group);
    > + } else
    > + /* only remove members from the thread group.
    > + * The thread group leader must stay so that
    > + * while_each_thread() uses can see the end of
    > + * the list and stop.
    > + */
    > + list_del_rcu(&p->thread_group);

    Hmm... this looks obvously wrong. This leaves the dead thread on
    the list.

    Oleg.



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