linux-kernel - Re: [BUG] TASK_DEAD task is able to be woken up in special condition

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Date:	Mon, 16 Jan 2012 14:38:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yasunori Goto <y-goto@...fujitsu.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Hiroyuki KAMEZAWA <kamezawa.hiroyu@...fujitsu.com>,
	Motohiro Kosaki <kosaki.motohiro@...fujitsu.com>,
	Linux Kernel ML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] TASK_DEAD task is able to be woken up in special condition

On Mon, 2012-01-16 at 20:51 +0900, Yasunori Goto wrote:

> try_to_wake_up() has a problem which may change status from TASK_DEAD to
> TASK_RUNNING in race condition with SMI or guest environment of virtual
> machine. (See: https://lkml.org/lkml/2011/12/21/523)
> As a result, exited task is scheduled() again and panic occurs.
> 
> By this patch, do_exit() waits for releasing task->pi_lock which is used
> in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after
> waking up.


This Changelog isn't very good. Please spell out the problem instead of
referring to it so that people using git-blame and the like don't then
have to go look up some (possibly non-existent) web-resource.

> Signed-off-by: Yasunori Goto <y-goto@...fujitsu.com>
> 
> ---
>  kernel/exit.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> Index: linux-3.2/kernel/exit.c
> ===================================================================
> --- linux-3.2.orig/kernel/exit.c
> +++ linux-3.2/kernel/exit.c
> @@ -1038,6 +1038,14 @@ NORET_TYPE void do_exit(long code)
>  
>  	preempt_disable();
>  	exit_rcu();
> +
> +	/*
> +	 * try_to_wake_up() might be waking me up due to race condition.
> +	 * Make sure it is finished.
> +	 */

That comment is waaaay too terse. What race and what must be finished?

> +	smp_mb();
> +	raw_spin_unlock_wait(&tsk->pi_lock);
> +
>  	/* causes final put_task_struct in finish_task_switch(). */
>  	tsk->state = TASK_DEAD;
>  	schedule();
> 

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