On 12/21, Frederic Weisbecker wrote:
> Hi,
>
> Starring at some parts of cgroups, I have a few questions:
>
> - Is cgroup_enable_task_cg_list()'s while_each_thread() safe
> against concurrent exec()? The leader may change in de_thread()
> and invalidate the test done in while_each_thread().
Yes. Oh, we need to do something with while_each_thread.
> - do_each_thread() also needs RCU and cgroup_enable_task_cg_list()
> seems to remind it. But it seems there is at least one caller that
> doesn't call rcu_read_lock(): update_cpu_mask() -> update_tasks_cpumask() -> cgroup_scan_tasks()
I don't see any caller which takes rcu_read_lock...
> - By the time we call cgroup_post_fork(), it is ready to be woken up
> and usable by the scheduler.
No, the new child can't run until do_fork()->wake_up_new_task().
> - 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().
Oleg.
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