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

[Kirill Korotaev <dev@sw.ru>]

>>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?
Agree :)
Sorry, I didn't notice that task structs are freed now with RCU :)))
In this case your patch really helps the problem. Care to prepare it?

Thanks,
Kirill