On 02/09/2011 09:44 PM, Sukadev Bhattiprolu wrote: > Oren Laadan [orenl@xxxxxxxxxxxxxxx] wrote: > | > | > | > But if that is true, I need to investigate why Louis Rilling was getting > | > the crash in Jun 2010 - which he tried to fix here: > | > > | > http://lkml.org/lkml/2010/6/16/295 > | > | I see. So basically there is a kerenl bug that can be potentially > | exposed by the c/r code. Therefore, we need to fix the kernel bug... > | (and until such a fix makes it to mainline, we'll add it as part of > | the linux-cr patchset). > > Yes, but there is more than one problem (at least in our C/R kernel). > > There is the bug that Louis Rilling reported and Eric has a fix for. > Even if we apply Eric's fix to the C/R kernel, we still will have > another problem if do_ghost_task() sets ->exit_signal to -1. > > Consider this in 2.6.37: > > Container-init: Ghost child of container-init > > do_ghost_task() > zap_pid_ns..() > Send SIGKILL > > do_wait() > - adds self to ->wait_chldexit queue > - goes through do_wait_thread() - finds that > it has at least one child (on tsk->children), > but it has not yet exited > - so waits for the child to exit > wakes up for SIGKILL > ->exit_signal = -1 > do_exit() > > Note that exit_notify() does not notify parent when the ghost process > exits, because ->exit_signal is -1. I see.. nice catch :) To address this, initially I thought that we could make ghosts take the tasklist_lock (write) when they change their ->exit_signal. But that's not enough because the parent may already be blocked in wait (so it's too late). Therefore, we also need to have ghosts wake-up their parent through __wake_up_parent(). so something like: void ghost_auto_reapable() { write_lock(&tasklist_lock); current->exit_signal = -1; __wake_up_sync_key(current, current->parent); write_unlock(&tasklist_lock); } If the parent wasn't at all waiting for us, no harm done... > > So you may ask how did the container-init have a ghost child. That was > due to a bug in usercr :-). You don't need a bug: the ghost flag is used for both ghost and dead tasks (the former used to instantiate specific pids, the latter to make other tasks orphans). So restarting a container that had orphan tasks is guaranteed to do this. Oren. > > But my point is such a userspace bug can leave the above container init > unkillable. > > Note that this does not happen with normal threads which set ->exit_signal > to -1 . That is because of the following two pieces of code in copy_process(): > > /* ok, now we should be set up.. */ > p->exit_signal = (clone_flags & CLONE_THREAD) ? -1 : (clone_flags & CSIGNAL); > > and > > /* CLONE_PARENT re-uses the old parent */ > if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) { > p->real_parent = current->real_parent; > p->parent_exec_id = current->parent_exec_id; > > With this our container-init above will not have any children to wait > for in do_wait_thread(). > > Sukadev > _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
