Re: Possible race between cgroup_attach_proc and de_thread, and questionable code in de_thread.

[Oleg Nesterov <oleg@redhat.com>]

Sorry for delay, just noticed this thread...

On 07/27, NeilBrown wrote:
>
>  The race as I understand it is with this code:
>
>
> 		list_replace_rcu(&leader->tasks, &tsk->tasks);
> 		list_replace_init(&leader->sibling, &tsk->sibling);
>
> 		tsk->group_leader = tsk;
> 		leader->group_leader = tsk;
>
>
>  which seems to be called with only tasklist_lock held, which doesn't seem to
>  be held in the cgroup code.
>
>  If the "thread_group_leader(leader)" call in cgroup_attach_proc() runs before
>  this chunk is run with the same value for 'leader', but the
>  while_each_thread is run after, then the while_read_thread() might loop
>  forever.  rcu_read_lock doesn't prevent this from happening.

Yes. This was already discussed. See http://marc.info/?t=127688987300002

Damn. I forgot about this completely.

>  The code in de_thread() is actually questionable by itself.
>  "list_replace_rcu" cannot really be used on the head of a list - it is only
>  meant to be used on a member of a list.
>  To move a list from one head to another you should be using
>  list_splice_init_rcu().

Hmm... can't understand this part.

And just in case... list_replace_rcu() looks fine afaics. The real problem
is release_task(old_leader) which does list_del_rcu(old_leader->thread_group),
this is what breaks while_each_thread().

>  The ->tasks list doesn't seem to have a clearly distinguished 'head'

Exactly. This is the problem.

But: you seem to confused ->tasks and ->thread_group ;)

Oleg.