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From: Steven Rostedt <rostedt@goodmis.org>
To: paulmck@linux.vnet.ibm.com
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dipankar Sarma <dipankar@in.ibm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: INFO: possible circular locking dependency detected
Date: Thu, 14 Jul 2011 13:32:22 -0400	[thread overview]
Message-ID: <1310664742.27864.45.camel@gandalf.stny.rr.com> (raw)
In-Reply-To: <20110714170540.GE2349@linux.vnet.ibm.com>

On Thu, 2011-07-14 at 10:05 -0700, Paul E. McKenney wrote:
> On Thu, Jul 14, 2011 at 01:02:09PM -0400, Steven Rostedt wrote:
> > On Thu, 2011-07-14 at 12:58 -0400, Steven Rostedt wrote:
> > 
> > > void __rcu_read_unlock(void)
> > > {
> > > 	struct task_struct *t = current;
> > > 
> > > 	barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
> > > 	--t->rcu_read_lock_nesting;
> > > 	barrier();  /* decrement before load of ->rcu_read_unlock_special */
> > > 	if (t->rcu_read_lock_nesting == 0 &&
> > > 	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> > > 		rcu_read_unlock_special(t);
> > > 
> > > Thus the question is, how did we get rcu_read_unlock_special set here?
> > 
> > Looks like another process could set this with:
> > 
> > static int rcu_boost(struct rcu_node *rnp)
> > {
> > 	[...]
> > 	t = container_of(tb, struct task_struct, rcu_node_entry);
> > 	rt_mutex_init_proxy_locked(&mtx, t);
> > 	t->rcu_boost_mutex = &mtx;
> > 	t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;
> 
> But only if that task was preempted while in the RCU read-side critical
> section that resulted in the call to rcu_read_unlock_special(), which
> should not happen if the task has irqs disabled for the duration of that
> RCU read-side critical section, right?
> 

static void rcu_read_unlock_special(struct task_struct *t)
{
	[...]
	special = t->rcu_read_unlock_special;   (A)
	[...]

		for (;;) {			(B)
			rnp = t->rcu_blocked_node;
			raw_spin_lock(&rnp->lock);  /* irqs already disabled. */
			if (rnp == t->rcu_blocked_node)
				break;
			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
		}
	[...]
		list_del_init(&t->rcu_node_entry);
	[...]
		if (empty)
			raw_spin_unlock_irqrestore(&rnp->lock, flags);
		else
			rcu_report_unblock_qs_rnp(rnp, flags);

		/* Unboost if we were boosted. */
		if (special & RCU_READ_UNLOCK_BOOSTED) {
			t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BOOSTED;
			rt_mutex_unlock(t->rcu_boost_mutex);
			t->rcu_boost_mutex = NULL;
		}


Now what happens if between (A) and (B) the kthread wakes up and calls
rc_boost()?

static int rcu_boost(struct rcu_node *rnp)
{
	[...]
	raw_spin_lock_irqsave(&rnp->lock, flags);
	[...]
	t = container_of(tb, struct task_struct, rcu_node_entry);
	[...]
	t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;
	raw_spin_unlock_irqrestore(&rnp->lock, flags);


Seems that we could have RCU_READ_UNLOCK_BOOSTED set, and never get
cleared, because rcu_read_unlock_special() doesn't look at the flags
directly, but at a local variable. The next rcu_read_unlock() will now
see this flag set!

-- Steve



  reply	other threads:[~2011-07-14 17:32 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-07-14 14:49 Sergey Senozhatsky
2011-07-14 16:41 ` Peter Zijlstra
2011-07-14 16:57   ` Paul E. McKenney
2011-07-14 19:16     ` Sergey Senozhatsky
2011-07-14 19:15   ` Sergey Senozhatsky
2011-07-14 19:34     ` Paul E. McKenney
2011-07-14 19:38       ` Dave Jones
2011-07-14 20:33         ` Paul E. McKenney
2011-07-14 19:38       ` Sergey Senozhatsky
2011-07-14 16:58 ` Steven Rostedt
2011-07-14 17:02   ` Steven Rostedt
2011-07-14 17:05     ` Paul E. McKenney
2011-07-14 17:32       ` Steven Rostedt [this message]
2011-07-14 17:46         ` Steven Rostedt
2011-07-14 19:18           ` Paul E. McKenney
2011-07-14 19:41             ` Steven Rostedt
2011-07-14 20:33               ` Paul E. McKenney
2011-07-15 11:05             ` Ed Tomlinson
2011-07-15 11:29               ` Peter Zijlstra
2011-07-15 11:35                 ` Ed Tomlinson
2011-07-15 11:39                 ` Peter Zijlstra
2011-07-15 18:11                   ` Paul E. McKenney
2011-07-15 12:42                 ` Paul E. McKenney
2011-07-15 13:07                   ` Peter Zijlstra
2011-07-15 14:36                     ` Paul E. McKenney
2011-07-15 15:04                       ` Peter Zijlstra
2011-07-15 15:59                         ` Paul E. McKenney
2011-07-15 16:11                           ` Peter Zijlstra
2011-07-15 16:56                             ` Paul E. McKenney
2011-07-15 21:48                               ` Ed Tomlinson
2011-07-15 22:04                                 ` Paul E. McKenney
2011-07-16 19:42                                   ` Ed Tomlinson
2011-07-17  0:02                                     ` Paul E. McKenney
2011-07-17  1:56                                       ` Ed Tomlinson
2011-07-17 14:28                                         ` Paul E. McKenney
2011-07-18 15:15                                           ` Paul E. McKenney
2011-07-18  9:29                                     ` Peter Zijlstra
2011-07-18 15:29                                       ` Paul E. McKenney
2011-07-15 16:55                     ` Steven Rostedt
2011-07-15 17:03                       ` Paul E. McKenney
2011-07-15 17:16                         ` Steven Rostedt
2011-07-15 17:24                           ` Paul E. McKenney
2011-07-15 17:42                             ` Steven Rostedt
2011-07-15 18:33                               ` Paul E. McKenney
  -- strict thread matches above, loose matches on Subject: below --
2011-08-07 16:22 Justin P. Mattock
2011-08-11 20:57 ` Justin P. Mattock
2009-12-06 10:11 Richard Zidlicky
2009-10-10 23:09 John Kacur
2007-02-08 15:03 Pedro M. López
2006-10-16 14:05 alpha @ steudten Engineering
2006-10-16 14:32 ` Nick Piggin
2006-10-16 15:42   ` Randy Dunlap
2006-10-16 15:46     ` Nick Piggin
2006-10-19  6:02   ` Andrew Morton
2006-10-19  6:30     ` Nick Piggin

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