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

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Date:	Wed, 25 Jan 2012 17:51:30 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Yasunori Goto <y-goto@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 Wed, 2012-01-25 at 16:45 +0100, Oleg Nesterov wrote:
> On 01/24, Peter Zijlstra wrote:
> >
> > On Tue, 2012-01-24 at 11:19 +0100, Peter Zijlstra wrote:
> > > On Wed, 2012-01-18 at 15:20 +0100, Oleg Nesterov wrote:
> > > > do_exit() is different because it can not handle the spurious wakeup.
> > > > Well, may be we can? we can simply do
> > > >
> > > >                 for (;;) {
> > > >                         tsk->state = TASK_DEAD;
> > > >                         schedule();
> > > >                 }
> > > >
> > > > __schedule() can't race with ttwu() once it takes rq->lock. If the
> > > > exiting task is deactivated, finish_task_switch() will see EXIT_DEAD.
> > >
> > > TASK_DEAD, right?
> 
> Yes, but... I simply can't understand what I was thinking about.
> And probably I missed something again, but I think this can't work.

Oh man, total confusion. :-) Every time I look at this bug I see
different shadows on the wall.

> Afaics, this can only help to prevent the race with ttwu_remote()
> doing ttwu_do_wakeup() under rq->lock.

ttwu_do_wakeup() must always be called with rq->lock held.

> But we still can race with the !p->on_rq case which sets TASK_WAKING.
> It can do this after finish_task_switch() observes TASK_DEAD and does
> put_task_struct().

<random scribbling that got erased>

No, see below !p->on_rq isn't possible and thus pi_lock is indeed
sufficient.

> > I think Yasunori-San's patch isn't
> > sufficient, note how the p->state = TASK_RUNNING in ttwu_do_wakeup() can
> > happen outside of p->pi_lock when the task gets queued on a remote cpu.
> 
> Hmm, really? I am not sure, but I do not trust myself.
> 
> To simplify, you mean that
> 
> 	mb();
> 	unlock_wait(pi_lock);
> 
> 	tsk->state = TASK_DEAD;
> 
> can change ->state from TASK_WAKING to TASK_DEAD, right? Is this really
> possible? ttwu() ensures p->on_rq == F in this case.

Ahhh.. hold on, p->on_rq must be true, since we disabled preemption
before setting TASK_DEAD, so the thing cannot be scheduled out.

Does this mean that both Yasunori-San's solution and yours work again?

/me goes in search of a fresh mind.. shees!




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