Re: [PATCH 3/4] threadgroup: extend threadgroup_lock() to cover exit and exec

[Tejun Heo <htejun@gmail.com>]

Hello, Oleg.

Sorry about the very long delay.  I moved cross atlantic and had a
pretty long vacation while doing it.  Hope you can still remember some
of this one. :)

On Sun, Sep 18, 2011 at 07:37:23PM +0200, Oleg Nesterov wrote:
> > With this change, threadgroup_lock() guarantees that the target
> > threadgroup will remain stable - no new task will be added, no new
> > PF_EXITING will be set and exec won't happen.
> 
> To me, this is the only "contradictory" change,

What do you mean "contradictory"?  Can you please elaborate?

> > +	/*
> > +	 * Release threadgroup and make sure we are holding no locks.
> > +	 */
> > +	threadgroup_change_done(tsk);
> 
> I am wondering, can't we narrow the scope of threadgroup_change_begin/done
> in do_exit() path?
> 
> The code after 4/4 still has to check PF_EXITING, this is correct. And yes,
> with this patch PF_EXITING becomes stable under ->group_rwsem. But, it seems,
> we do not really need this?
> 
> I mean, can't we change cgroup_exit() to do threadgroup_change_begin/done
> instead? We do not really care about PF_EXITING, we only need to ensure that
> we can't race with cgroup_exit(), right?

If we confine our usage to cgroup, excluding just against
cgroup_exit() might work although this is still a bit nasty.  ie. some
callbacks might not expect half torn-down tasks in methods other than
the exit callback.

Also, it makes the mechanism unnecessarily cgroup-specific without
gaining much if anything.  It's per-threadgroup rwsem so contention
isn't a problem and narrowing critical section isn't likely to be
beneficial (maybe slightly increase the chance of the cacheline for
the lock to be hot?).

Thank you.

-- 
tejun