[Devel] Re: [PATCH][usercr]: Ghost tasks must be detached

[Oren Laadan]

On 02/10/2011 01:17 AM, Sukadev Bhattiprolu wrote:
> Oren Laadan [orenl at cs.columbia.edu] wrote:
> | 
> | 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);
> 
> You meant __wake_up_parent() here I guess.

Yes...

> 
> But if we do this in do_ghost_task(), the parent could wakeup, find
> that it still has a live child (this ghost) and go back to sleep before
> the ghost becomes EXIT_DEAD right ?

You are right again...

> 
> If so, we would still have the problem ?
> 
> i.e we must stop being a chld a of the cinit for it to not wait for us.
> Or we might need to detect that the the pidns is going away, so signalling
> the parent won't cause any harm.  But that is racy too :-(

So instead, we can call __wake_up_parent() from exit_checkpoint()
if indeed we are already reaped there:

exit_checkpoint()
{
	...
	if (current->flags & PF_RESTARTING) {
		...
		/* either zombie or reaped ghost/dead */
		if (current->exit_state = EXIT_DEAD)
			__wake_up_parent(...);	  /* probably need lock */
		...
	}
	...
}

and to avoid userspace misuse, disallow non-thread-group-leader ghosts.

?

Oren.

> 
> | }
> | 
> | 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.
> 
> Ah, thats a good point.
> 
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