Hi Oleg,

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

On 01/06, Mandeep Singh Baines wrote:

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

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in particular, http://marc.info/?l=linux-kernel&m=127714242731448
I think this should work, but then we should do something with the
users like zap_threads().

With that patch, won't you potentially miss the exec thread if an exec
occurs while you're iterating over the list? Is that OK?

Of course it is not OK ;) Note the "we should do something with" above.

So requirements should be something like this:

(I assume, you mean the lockless case)

Correct.

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* Any task alive for the duration of the iteration MUST be visited
* No task should be visited more than once
* Any task born or exiting after starting the iteration MAY be skipped
* You can start at any task in the thread group

Well yes, but it is not easy to exactly define what after/before
means in this case.

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Would something like this work:

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

Where o should have the value of g->group_leader.

I don't understand how this helps... and how this can work even
ignoring the barriers.

OK, we have the main thream M and the sub-thread T, we are doing

	do {
		do_something(t);
	} while_each_thread(M, t, M);

why we can't miss T if it does exec?

So for:

struct task *M; /* assuming this is passed in to us */
struct task *L = M->group_leader;
struct task *I = M;

do {
	do_something(T);
} while_each_thread(M, T, L);

Here is my thinking.

If some thread K does exec, you won't miss it because:

1) Ignoring the group_leader check, you'll visit K just by following
   next_thread(). That's the case today and is what you except
   when iterating over an rcu_list.
2) (t->group_leader == o) will fail iff t is the exec thread.
   Since we test t->group_leader before re-assigning it (t=next_thread()),
   the test will fail only after visiting the exec thread. So you'll
   visit the exec thread and then terminate the loop.

I realize its a klutzy interface (requires 3 variables) but it seems
correct (ignoring barriers) and meets all the requirements. I'm hoping
it inspires a solution which is less klutzy and meet its all the
requirements.

Regards,
Mandeep

Oleg.