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

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Date:	Wed, 09 Aug 2006 17:02:55 +0400
From:	Kirill Korotaev <dev@...ru>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	Andrew Morton <akpm@...l.org>, Dave Hansen <haveblue@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sys_getppid oopses on debug kernel (v2)

>>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
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