On 08/19, Oleg Nesterov wrote: > > On 08/19, Kay Sievers wrote: > > > > version 4: > > Looks correct... But I don't trust myself. Especially after I missed > the problem with klthreads ;) And I missed another problem. So. We do need to check that father != init_task even if we check has_child_subreaper. Suppose that a kernel thread execs the usermode task T which does prctl(REAPER). Suppose that its grandchild C exits, it should be reparented to T. If T is alive - everything fine, the lookup finds T with ->is_sub_reaper set. If T has exited - everything fine again, C->parent was already reparented to pid_ns->child_reaper (or another sub-reaper). But! If T exits, there is a window between setting PF_EXITING and forget_original_parent() which should re-parent C->parent. If C exits in this window, it will see PF_EXITING and continue the lookup, but it will never reach pid_ns->child_reaper. (if we could check ->exit_state instead of PF_EXITING, everything would be fine). And cough... there is another, not that subtle problem ;) That task T can _clear_ ->is_child_subreaper after forking the child. But since this obviously can't clear C->has_child_subreaper, we can't trust it. So. please add this check back. I insisted you should remove it, but I was wrong. Otherwise looks correct. Damn. And why do we check PF_EXITING but not exit_state? this is because we have to drop tasklist for exit_task_namespaces(), see 762a24beed3f3ab93224bd447710e6c36fcf1968. However, there were a lot of changes since then. Afaics we can change do_notify_parent() to use task_active_pid_ns(tsk->parent) and then we can call exit_task_namespaces() before exit_notify(). In this case we can change exit_notify/forget_original_parent to reparent and set exit_state under tasklist, this also saves unlock+lock. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
