Re: cgroup_attach_task && while_each_thread (Was: cgroup attach task - slogging cpu)

[Li Zefan <lizefan@huawei.com>]

On 2013/10/10 0:54, Oleg Nesterov wrote:
> And I am starting to think that this change should also fix the
> while_each_thread() problems in this particular case.
> 
> In generak the code like
> 
> 	rcu_read_lock();
> 	task = find_get_task(...);
> 	rcu_read_unlock();
> 
> 	rcu_read_lock();
> 	t = task;
> 	do {
> 		...
> 	} while_each_thread (task, t);
> 	rcu_read_unlock();
> 
> is wrong even if while_each_thread() was correct (and we have a lot
> of examples of this pattern). A GP can pass before the 2nd rcu-lock,
> and we simply can't trust ->thread_group.next.
> 
> But I didn't notice that cgroup_attach_task(tsk, threadgroup) can only
> be called with threadgroup == T when a) tsk is ->group_leader and b)
> we hold threadgroup_lock() which blocks de_thread(). IOW, in this case
> "tsk" can't be removed from ->thread_group list before other threads.
> 
> If next_thread() sees thread_group.next != leader, we know that the
> that .next thread didn't do __unhash_process() yet, and since we
> know that in this case "leader" didn't do this too we are safe.
> 
> In short: __unhash_process(leader) (in this) case can never change
> ->thread_group.next of another thread, because leader->thread_group
> should be already list_empty().
> 

If threadgroup == false, and if the tsk is existing or is already in
the targeted cgroup, we won't break the loop due to the bug but do
this:

  while_each_thread(task, t)

If @task isn't the leader, we might got stuck in the loop?