On Wed, Dec 21, 2011 at 02:08:48PM +0100, Oleg Nesterov wrote:
> On 12/21, Frederic Weisbecker wrote:
> > - 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.
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