Q: cgroup: Questions about possible issues in cgroup locking

[Mandeep Singh Baines]

Oleg Nesterov (oleg at redhat.com) wrote:
> OK, finally we should do something with this problem ;)
> 
> On 01/19, Oleg Nesterov wrote:
> >
> > I'll try to investigate if we can remove
> >
> > 	leader->group_leader = tsk;
> >
> > from de_thread(). In fact I already thought about this change a long
> > ago without any connection to while_each_thread(). This assignment
> > looks "assymetrical" compared to other threads we kill. But we did
> > have a reason (reasons?). Hopefully, the only really important reason
> > was already removed by 087806b1.
> 
> On the second thought, I think we should not do this.
> 
> For example, do_prlimit() assumes that tsk->group_leader is correct
> under tasklist_lock.
> 
> OK, lets return to the thread_group_leader() check. To ensure we do
> not visit the same thread twice we can check 'g', not 't'.
> 
> This is what I am going to send, after I re-check once again...
> 
> I have the problem with the changelog ;) Somehow it should explain
> that while_each_thread_rcu(g, t) can't race with do_group_exit().
> I think it can't, list_del_rcu(leader->thread_group) happens when
> this entry is already "empty", it should be the last thread in group.
> If the non-leader thread goes away from the least, we still have
> the "path" to reach the leader. But this is not easy to explain.
> 
> As for the barrier. If de_thread() changes the leader it drops
> and re-acquires tasklist_lock (this implies mb) after it changes
> old_leader->exit_signal (used in thread_group_leader) and before
> __unhash_process() which does list_del_rcu().
> 
> This means that if while_each_thread() sees the result of
> list_del_rcu(old_leader) it must also see that
> thread_group_leader(old_leader) != T.
> 
> What do you think? Do you see any problems?
> 
> Oleg.
> ---
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 7d379a6..f169bfd 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2323,9 +2323,24 @@ extern bool current_is_single_threaded(void);
>  #define do_each_thread(g, t) \
>  	for (g = t = &init_task ; (g = t = next_task(g)) != &init_task ; ) do
>  
> +/*
> + * needs tasklist_lock or ->siglock, or rcu if the caller ensures
> + * that 'g' can't exit or exec.
> + */
>  #define while_each_thread(g, t) \
>  	while ((t = next_thread(t)) != g)
>  
> +/*
> + * rcu-safe, but should start at ->group_leader.
> + * thread_group_leader(g) protects against the race with exec which
> + * removes the leader from list.
> + * smp_rmb() pairs with implicit mb() implied by unlock + lock in
> + * de_thread()->release_task() path.
> + */
> +#define while_each_thread_rcu(g, t)				\
> +	while ((t = next_thread(t)) != g &&			\
> +		({ smp_rmb(); thread_group_leader(g); }))
> +

Couldn't you miss the exec_thread if:

t = exec_thread && !thread_group_leader(g)

i.e. if you just passed (leader->group_leader = tsk;) in de_thread().

Could we change do_prlimit()? Especially since its slow path.

I really like you're earlier solution (ignoring barrier):

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

Regards,
Mandeep

>  static inline int get_nr_threads(struct task_struct *tsk)
>  {
>  	return tsk->signal->nr_threads;
>