cgroup_attach_task && while_each_thread (Was: cgroup attach task - slogging cpu)

[Oleg Nesterov <oleg@redhat.com>]

And I am starting to think that this change should also fix the
while_each_thread() problems in this particular case.

In generak the code like

	rcu_read_lock();
	task = find_get_task(...);
	rcu_read_unlock();

	rcu_read_lock();
	t = task;
	do {
		...
	} while_each_thread (task, t);
	rcu_read_unlock();

is wrong even if while_each_thread() was correct (and we have a lot
of examples of this pattern). A GP can pass before the 2nd rcu-lock,
and we simply can't trust ->thread_group.next.

But I didn't notice that cgroup_attach_task(tsk, threadgroup) can only
be called with threadgroup == T when a) tsk is ->group_leader and b)
we hold threadgroup_lock() which blocks de_thread(). IOW, in this case
"tsk" can't be removed from ->thread_group list before other threads.

If next_thread() sees thread_group.next != leader, we know that the
that .next thread didn't do __unhash_process() yet, and since we
know that in this case "leader" didn't do this too we are safe.

In short: __unhash_process(leader) (in this) case can never change
->thread_group.next of another thread, because leader->thread_group
should be already list_empty().

On 10/09, Oleg Nesterov wrote:
>
> On 10/09, Oleg Nesterov wrote:
> >
> > On 10/09, Li Zefan wrote:
> > >
> > > Anjana, could you revise the patch and send it out with proper changelog
> > > and Signed-off-by? And please add "Cc: <stable@vger.kernel.org> # 3.9+"
> >
> > Yes, Anjana, please!
>
> Please note also that the PF_EXITING check has the same problem, it also
> needs "goto next".
>
> > > > check in the main loop. So Anjana was right (sorry again!), and we
> > > > should probably do
> > > >
> > > > 		ent.cgrp = task_cgroup_from_root(...);
> > > > 		if (ent.cgrp != cgrp) {
> > > > 			retval = flex_array_put(...);
> > > > 			...
> > > > 		}
> > > >
> > > > 		if (!threadgroup)
> > > > 			break;
> > > >
> > >
> > > Or
> > >
> > > do {
> > > 	...
> > > 	if (ent.cgrp == cgrp)
> > > 		goto next;
> >
> > Or this, agreed.
> >
> > > > Or I am wrong again?
> > >
> > > No, you are not! :)
> >
> > Thanks ;)
> >
> > Oleg.