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

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Message-Id: <20120106210108.AB18.E1E9C6FF@jp.fujitsu.com>
Date:	Fri, 06 Jan 2012 21:01:08 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Fri, 2012-01-06 at 19:22 +0900, Yasunori Goto wrote:
> > I just confirmed booting up on my box, and I would like to get rough agreement
> > about this way to solve this issue at first. 
> 
> I really don't like it. It makes the ttwu path more complex and more
> expensive. ttwu is one of the hottest and more complex paths in the
> scheduler, it needs neither more overhead nor more complexity.

Hmmmmm.
Ok.
> 
> I'd really much rather put another raw_spin_unlocked_wait() in do_exit()
> before we set TASK_DEAD. It probably needs an smp memory barrier too.

Do you mean the following patch?

---

Signed-off-by: Yasunori Goto <y-goto@...fujitsu.com>

---
 kernel/exit.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux-3.2-rc7/kernel/exit.c
===================================================================
--- linux-3.2-rc7.orig/kernel/exit.c
+++ linux-3.2-rc7/kernel/exit.c
@@ -1038,6 +1038,10 @@ NORET_TYPE void do_exit(long code)
 
 	preempt_disable();
 	exit_rcu();
+
+	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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