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Date:	Sat, 18 Oct 2014 21:36:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
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 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.
--
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