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 20:51:40 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	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


> Ok. Thansk for your agreement.
> I'll test this patch.

Hi.

I tested this patch. I've not found any problems so far.
Please merge it.


-----
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.


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

-- 
Yasunori Goto 


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