Re: [PATCH][RT] futex: protect against pi_blocked_on corruption duringrequeue PI -V2

[Darren Hart]

On 07/13/2010 04:52 AM, Thomas Gleixner wrote:
> On Tue, 13 Jul 2010, Thomas Gleixner wrote:
> > This code causes braindamage. I really wonder whether we need to
> > remove it according to the "United Nations Convention against Torture
> > and Other Cruel, Inhuman or Degrading Treatment or Punishment".
>
> Ok, finally managed to twist my brain around it. Mike, can you give it
> a test ride ?

Since Mike is out I built this version and ran it a few times. I saw a 
100% reproduction rate previously. I haven't seen any errors in a 
handful of runs (~10) with this patch.

I do still see the hrtimer latency message on the first run, so this is 
likely unrelated to the issue we're addressing:

Jul 13 14:47:59 elm9m94 kernel: hrtimer: interrupt took 123924 ns

As for Thomas's changes, only a couple nitpics below:


> @@ -2255,18 +2265,51 @@ static int futex_wait_requeue_pi(u32 __u
>   	/* Queue the futex_q, drop the hb lock, wait for wakeup. */
>   	futex_wait_queue_me(hb,&q, to);
>
> -	spin_lock(&hb->lock);
> -	ret = handle_early_requeue_pi_wakeup(hb,&q,&key2, to);
> -	spin_unlock(&hb->lock);
> -	if (ret)
> -		goto out_put_keys;
> +	/*
> +	 * Avoid races with requeue and trying to block on two mutexes
> +	 * (hb->lock and uaddr2's rtmutex) by serializing access to
> +	 * pi_blocked_on with pi_lock.
> +	 */
> +	raw_spin_lock_irq(&current->pi_lock);
> +	if (current->pi_blocked_on) {
> +		/* Requeue happened already */

This comment isn't quite accurate. The requeue may be in progress, which 
means the q.lock_ptr is not trustworthy as noted below. Consider:

		/*
		 * We have been requeued, or are in the process
		 * of being requeued.
		 */

> +		raw_spin_unlock_irq(&current->pi_lock);
> +	} else {
> +		/*
> +		 * Setting pi_blocked_on to PI_WAKEUP_INPROGRESS
> +		 * prevents a concurrent requeue from enqueuein us on

s/enqueuein/enqueueing/  not that my dictionary thinks either one of 
them are words ;-)

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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