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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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