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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Sun, Nov 27, 2011 at 11:30:01AM -0800, Tejun Heo wrote:
> Hello, Frederic.
> 
> On Fri, Nov 25, 2011 at 03:01:39PM +0100, Frederic Weisbecker wrote:
> > > +	/*
> > > +	 * @tsk's threadgroup is going through changes - lock out users
> > > +	 * which expect stable threadgroup.  Do this before actually
> > > +	 * starting tearing down @tsk so that locked threadgroup has either
> > > +	 * alive or dead tasks, not something inbetween.
> > > +	 */
> > > +	threadgroup_change_begin(tsk);
> > > +
> > 
> > I still wonder why there is a so big coverage of this lock. I mean
> > why is it called right before exit_irq_thread() and released so late.
> > All we want is to lock cgroup_exit() I think, after which tasks can't be
> > migrated.
> 
> That way, cgroup plugins never see tasks in the process of
> deconstruction.  Its ->mm, ->sighand, ->signal and so on either don't
> exist or continue to exist across all cgroup callbacks.  It's simpler
> and safer (especially as bugs in this area would only be visible when
> migration and exit race), and if we're gonna synchronize exit path at
> all, there isn't anything to lose by excluding the whole thing.

Fine.

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.