Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753816Ab0GLUk6 (ORCPT <rfc822;w@1wt.eu>);
	Mon, 12 Jul 2010 16:40:58 -0400
Received: from www.tglx.de ([62.245.132.106]:46508 "EHLO www.tglx.de"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1752421Ab0GLUk5 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 12 Jul 2010 16:40:57 -0400
Date: Mon, 12 Jul 2010 22:40:18 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Darren Hart <dvhltc@us.ibm.com>
cc: Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        Eric Dumazet <eric.dumazet@gmail.com>, John Kacur <jkacur@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
In-Reply-To: <4C3B68B9.5060404@us.ibm.com>
Message-ID: <alpine.LFD.2.00.1007122226230.3321@localhost.localdomain>
References: <1278714780-788-1-git-send-email-dvhltc@us.ibm.com> <1278714780-788-5-git-send-email-dvhltc@us.ibm.com> <1278790882.7352.101.camel@marge.simson.net> <4C3B68B9.5060404@us.ibm.com>
User-Agent: Alpine 2.00 (LFD 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2980
Lines: 94

On Mon, 12 Jul 2010, Darren Hart wrote:
> On 07/10/2010 12:41 PM, Mike Galbraith wrote:
> > On Fri, 2010-07-09 at 15:33 -0700, Darren Hart wrote:
> > > > Out of curiosity, what's wrong with holding his pi_lock across the
> > > > wakeup?  He can _try_ to block, but can't until pi state is stable.
> > > > 
> > > > I presume there's a big fat gotcha that's just not obvious to futex
> > > > locking newbie :)
> 
> Nor to some of us that have been engrossed in futexes for the last couple
> years! I discussed the pi_lock across the wakeup issue with Thomas. While this
> fixes the problem for this particular failure case, it doesn't protect
> against:
> 
> <tglx> assume the following:
> <tglx> t1 is on the condvar
> <tglx> t2 does the requeue dance and t1 is now blocked on the outer futex
> <tglx> t3 takes hb->lock for a futex in the same bucket
> <tglx> t2 wakes due to signal/timeout
> <tglx> t2 blocks on hb->lock
> 
> You are likely to have not hit the above scenario because you only had one
> condvar, so the hash_buckets were not heavily shared and you weren't likely to
> hit:
> 
> <tglx> t3 takes hb->lock for a futex in the same bucket
> 
> 
> I'm going to roll up a patchset with your (Mike) spin_trylock patch and run it
> through some tests. I'd still prefer a way to detect early wakeup without
> having to grab the hb->lock(), but I haven't found it yet.
> 
> +	while(!spin_trylock(&hb->lock))
> +		cpu_relax();
>  	ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
>  	spin_unlock(&hb->lock);

And this is nasty as it will create unbound priority inversion :(

We discussed another solution on IRC in meantime:

in futex_wait_requeue_pi()

   futex_wait_queue_me(hb, &q, to);

   raw_spin_lock(current->pi_lock);
   if (current->pi_blocked_on) {
      /*
       * We know that we can only be blocked on the outer futex
       * so we can skip the early wakeup check
       */
       raw_spin_unlock(current->pi_lock);
       ret = 0;
   } else {
      current->pi_blocked_on = PI_WAKEUP_INPROGRESS;
      raw_spin_unlock(current->pi_lock);

      spin_lock(&hb->lock);
      ret = handle_early_requeue_pi_wakeup();
      ....
      spin_lock(&hb->lock);
   }

Now in the rtmutex magic we need in task_blocks_on_rt_mutex():

   raw_spin_lock(task->pi_lock);

   /*
    * Add big fat comment why this is only relevant to futex
    * requeue_pi
    */

   if (task != current && task->pi_blocked_on == PI_WAKEUP_INPROGRESS) {
      raw_spin_lock(task->pi_lock);

      /*
       * Returning 0 here is fine. the requeue code is just going to
       * move the futex_q to the other bucket, but that'll be fixed
       * up in handle_early_requeue_pi_wakeup()
       */

      return 0;
   }

Thanks,

	tglx

    
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/