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

[Oleg Nesterov <oleg@redhat.com>]

Hi,

On 10/10, Tejun Heo wrote:
>
> Hope you can still remember some
> of this one. :)

I am not sure ;)

> 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?

Because, iirc, with this patch do_exit() does (almost) everything
under rw_sem. OK, down_read() should be cheap, but still.

See also below.

> > > +	/*
> > > +	 * 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.

Oh, sorry, I don't understand... I already forgot the details.

> Also, it makes the mechanism unnecessarily cgroup-specific without
> gaining much if anything.

Yes! And _personally_ I think it should be cgroup-specific, that is
why I dislike the very fact do_exit() uses it directly. To me it would
be cleaner to shift it into cgroup hooks. Yes, sure, this is subjective.

In fact I still hope we can kill this sem altogether, but so far I have
no idea how we can do this. We do need the new per-process lock to
protect (in particular) ->thread_group. It is quite possible that it
should be rw_semaphore. But in this case we down_write(), not _read
in exit/fork paths, and its scope should be small.

I do not think the current lock should have more users. Of course I
can be wrong. And what exactly it protects? I mean copy_process().
Almost everything, but this simply connects to cgroup fork hooks.

Just my opinion, I am not going to insist.

Oleg.