Oleg Nesterov (oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org) wrote:

On 01/12, Mandeep Singh Baines wrote:

quoted

Oleg Nesterov (oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org) wrote:

quoted

Still can't understand... Lets look at this trivial example again.

We start from the main thread M, it is ->group_leader. There is
another thread T in this thread group. We are doing

	OLD = M;

	t = M;
	do {
		do_smth(t);
	}
	while (t->group_leader == OLD && ((t = next_thread(t)) != M);

The first iteration does do_smth(M).

T calls de_thread() and, in particular, it does M->group_leader = T
(see "leader->group_leader = tsk" in de_thread).

after that t->group_leader == OLD fails. t == M, its group_leader == T.
do_smth(T) won't be called.

No?

I think we can handle this by removing the assignment. So in de_thread():

-		leader->group_leader = tsk;

Ah, so that was you plan. I was confused by the 3rd argument, why
it is needed?

Good question. On second thought, I don't think its needed as shown
in you're solution below.

Yes, I thought about this too. Suppose we remove this assignment,
then we can simply do

	#define while_each_thread(g, t) \
		while (t->group_leader == g->group_leader && (t = next_thread(t)) != g)

with the same effect. (to remind, currently I ignore the barriers/etc).

Nice! I think this works.

But this can _only_ help if we start at the group leader!

But I don't think this solution requires we start at the group leader.

My thinking:

Case 1: g is the exec thread

The only condition under which g->group_leader would change is if g is the
exec thread. If you are the exec the only requirement is that you visit the
exec thread. Visiting any other threads is optional. Since g is the exec
thread, you've already visited it and can safely stop once
g->group_leader is re-assigned to g.

Case 2: g is the group leader

If g is the group leader and a subthread execs, you'll terminate just
after visiting the exec thread.

Case 3: g is some other thread

In this case, g MUST be current so you don't really need to worry
about de_thread() since current can't be de_threaded.

May be we should enforce this rule (for the lockless case), I dunno...
In that case I'd prefer to add the new while_each_thread_rcu() helper.
But! in this case we do not need to change de_thread(), we can simply do

	#define while_each_thread_rcu(t) \
		while (({ t = next_thread(t); !thread_group_leader(t); }))

Won't this terminate just before visiting the exec thread?

The definition above was one of the possibilities I considered, but
I wasn't able to convince myself this is the best option.

See? Or do you think I missed something?

Just in case... note that while_each_thread_rcu() doesn't use 'g'
at all. May be it makes sense to keep the old "t != g &&", but this
is minor.

Oleg.

Regards,
Mandeep