Re: [PATCH] sys_getppid oopses on debug kernel (v2)

[Oleg Nesterov <oleg@tv-sign.ru>]

On 08/09, Kirill Korotaev wrote:
>
> >Why do we need to use ->group_leader? All threads should have the same
> >->real_parent.
> I'm not sure this is true for old LinuxThreads...

Yes, from the user-space pov it may be not true, but ->group_leader->real_parent
should be equal ->real_parent anyway.

> >Why do we need tasklist_lock? I think rcu_read_lock() is enough.
> >
> >In other words, do you see any problems with this code
> >
> >	smlinkage long sys_getppid(void)
> >	{
> >		int pid;
> >
> >		rcu_read_lock();
> >		pid = rcu_dereference(current->real_parent)->tgid;
> >		rcu_read_unlock();
> >
> >		return pid;
> >	}
> >
> >? Yes, we may read a stale value for ->real_parent, but the memory
> >can't be freed while we are under rcu_read_lock(). And in this case
> >the returned value is ok because the task could be reparented just
> >after return anyway.
> Your patch doesn't cure the problem.
> rcu_read_lock just disables preemtion and rcu_dereference
> introduces memory barrier. _None_ of this _prevents_
> another CPU from freeing old real_parent in parallel with your dereference.

How so? Note that release_task() doesn't call put_task_struct(), it does
call_rcu(&p->rcu, delayed_put_task_struct) instead. When delayed_put_task_struct()
is called, all CPUs must see the new value of ->real_parent (otherwise
RCU is just broken). If CPU sees the old value of ->real_parent, rcu_read_lock()
protects us from delayed_put_task_struct() on another CPU.

Ok, I think this is the same "classic" pattern as:

	old = global_ptr;
	global_ptr = new;
	call_rcu(..free_old...);
vs
	rcu_read_lock();
	use(global_ptr);
	rcu_read_unlock();

Do you agree?

Oleg.