linux-kernel - Re: [BUG] long freezes on thinkpad t60

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Date:	Thu, 21 Jun 2007 13:09:09 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>, cebbert@...hat.com,
	chris@...ee.ca, linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: [BUG] long freezes on thinkpad t60

On Thu, Jun 21, 2007 at 10:39:31AM +0200, Ingo Molnar wrote:
> 
> * Jarek Poplawski <jarkao2@...pl> wrote:
> 
> > BTW, I've looked a bit at these NMI watchdog traces, and now I'm not 
> > even sure it's necessarily the spinlock's problem (but I don't exclude 
> > this possibility yet). It seems both processors use task_rq_lock(), so 
> > there could be also a problem with that loop. The way the correctness 
> > of the taken lock is verified is racy: there is a small probability 
> > that if we have taken the wrong lock the check inside the loop is done 
> > just before the value is beeing changed elsewhere under the right 
> > lock. Another possible problem could be a result of some wrong 
> > optimization or wrong propagation of change of this task_rq(p) value.
> 
> ok, could you elaborate this in a bit more detail? You say it's racy - 
> any correctness bug in task_rq_lock() will cause the kernel to blow up 
> in spectacular ways. It's a fairly straightforward loop:
> 
>  static inline struct rq *__task_rq_lock(struct task_struct *p)
>          __acquires(rq->lock)
>  {
>          struct rq *rq;
> 
>  repeat_lock_task:
>          rq = task_rq(p);
>          spin_lock(&rq->lock);
>          if (unlikely(rq != task_rq(p))) {
>                  spin_unlock(&rq->lock);
>                  goto repeat_lock_task;
>          }
>          return rq;
>  }
> 
> the result of task_rq() depends on p->thread_info->cpu wich will only 
> change if a task has migrated over to another CPU. That is a 
> fundamentally 'slow' operation, but even if a task does it intentionally 
> in a high frequency way (for example via repeated calls to 
> sched_setaffinity) there's no way it could be faster than the spinlock 
> code here. So ... what problems can you see with it?

OK, you are right - I withdraw this "idea". Sorry!

Jarek P.
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