Re: [PATCH UPDATED 03/10] threadgroup: extend threadgroup_lock() to cover exit and exec

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

Hello, Frederic.

On Fri, Dec 02, 2011 at 05:28:03PM +0100, Frederic Weisbecker wrote:
> But I don't think it's very useful to protect against irq_exit_thread(),
> what happens there is purely of internal irq interest.
> 
> Then right after, PF_EXITING is set before any interesting change.
> Isn't it possible to simply lock this flag setting? IIRC, as soon
> as the PF_EXITING flag is set, you ignore the task for attachment.

I think that's technically possible but it does introduce another
class of tasks - the dying ones.  e.g. If a task has PF_EXITING set
and the containing process is migrating, we'll have to migrate all
tasks but the dying one and cgroup ->exit callbacks can be called on
the lonely task after the migration is complete.  It's kinda messy and
if someone makes a wrong assumption there, the bug is gonna be even
more difficult to reproduce / track down than now.  Yes, smaller scope
locking is nicer but I would like to avoid api weirdities like that.

Thanks.

-- 
tejun