linux-kernel - Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa

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Date:	Sun, 19 Oct 2014 12:20:59 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>
Subject: Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

On 18.10.2014 23:36, Peter Zijlstra wrote:
> On Sat, Oct 18, 2014 at 12:33:27PM +0400, Kirill Tkhai wrote:
>> How about this?
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index b78280c..d46427e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1165,7 +1165,21 @@ static void task_numa_compare(struct task_numa_env *env,
>>  
>>  	rcu_read_lock();
>>  	cur = ACCESS_ONCE(dst_rq->curr);
>> -	if (cur->pid == 0) /* idle */
>> +	/*
>> +	 * No need to move the exiting task, and this ensures that ->curr
>> +	 * wasn't reaped and thus get_task_struct() in task_numa_assign()
>> +	 * is safe; note that rcu_read_lock() can't protect from the final
>> +	 * put_task_struct() after the last schedule().
>> +	 */
>> +	if (is_idle_task(cur) || (cur->flags & PF_EXITING))
>> +		cur = NULL;
>> +	/*
>> +	 * Check once again to be sure curr is still on dst_rq. Even if
>> +	 * it points on a new task, which is using the memory of freed
>> +	 * cur, it's OK, because we've locked RCU before
>> +	 * delayed_put_task_struct() callback is called to put its struct.
>> +	 */
>> +	if (cur != ACCESS_ONCE(dst_rq->curr))
>>  		cur = NULL;
>>  
>>  	/*
> 
> So you worry about the refcount doing 0->1 ? In which case the above is
> still wrong and we should be using atomic_inc_not_zero() in order to
> acquire the reference count.
> 

We can't use atomic_inc_not_zero(). The problem is that cur is pointing
to a memory, which may be not a task_struct even. No guarantees at all.
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