On Wed, Dec 21, 2011 at 02:08:48PM +0100, Oleg Nesterov wrote:

On 12/21, Frederic Weisbecker wrote:

quoted

- Is the check for use_task_css_set_links in cgroup_post_fork() safe? given
it is checked outside css_set_lock?

Imagine this:

CPU 0                                                        CPU 1
----                                                         -----

cgroup_enable_task_cg() {
	uset_tasks_css_set_links = 1
	for_each_thread() {
		add tasks in the list
	}
}
                                                           do_fork() {
                                                               cgroup_post_fork() {
                                                                     use_tasks_css_set_links appears
                                                                     to be equal to 0 due to write/read
                                                                      not flushed. New task won't
                                                                      appear to the list.

Yes, I was thinking about this too.

Or (I think) they can race "contrariwise". CPU_1 creates the new child,
then CPU_0 sets uset_tasks_css_set_links = 1. But afaics there is no any
guarantee that CPU_0 sees the result of list_add_tail_rcu().

Exactly! In fact even if RCU was safe with while_each_thread() it wouldn't
be enough for us because of that.

I fear we need the read_lock(tasklist_lock) here, with a pair of smp
barriers to ensure use_task_css_set_links update is visible as
expected.

I'll try to cook something.