On 01/19, Pavel Tikhomirov wrote:
>
> Having these two
> differently behaving groups can lead to confusion. Also it is
> a problem for CRIU, as when we restore process tree we need to
> somehow determine which descendants belong to which group and
> much harder - to put them exactly to these group.
Hmm. could you explain how this change helps CRIU? I mean, why
restorer can't do prctl(CHILD_SUBREAPER) before the first fork?
Anyway, afaics the patch is sub-optimal and not correct...
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1715,6 +1715,8 @@ struct task_struct {
> struct signal_struct *signal;
> struct sighand_struct *sighand;
>
> + struct list_head csr_descendant;
> +
You don't need this new member and descendants_lock. task_struct has
the ->real_parent pointer so you can work the tree without recursion.
> +static void prctl_set_child_subreaper(struct task_struct *reaper, bool arg2)
> +{
> + LIST_HEAD(descendants);
> +
> + reaper->signal->is_child_subreaper = arg2;
> + if (!arg2)
> + return;
> +
> + spin_lock(&descendants_lock);
> + read_lock(&tasklist_lock);
> +
> + list_add(&reaper->csr_descendant, &descendants);
> +
> + while (!list_empty(&descendants)) {
> + struct task_struct *tsk;
> + struct task_struct *p;
> +
> + tsk = list_first_entry(&descendants, struct task_struct,
> + csr_descendant);
> +
> + list_for_each_entry(p, &tsk->children, sibling) {
This is not enough. Every thread has its own ->children list, you need
to walk the sub-threads as well.
> + * If we've found child_reaper - skip descendants in
> + * it's subtree as they will never get out pidns
> + */
> + if (is_child_reaper(task_pid(p)))
> + continue;
Again, a child reaper can be multi-threaded, this check can be false
negative.
Probably is_child_reaper() should be renamed somehow and a new helper
makes sense... something like
bool task_is_child_reaper(struct task_struct *p)
{
return same_thread_group(p,
task_active_pid_ns(p)->child_reaper);
}
Oleg.
