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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Ed Tomlinson <edt@aei.ca>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	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: Fri, 15 Jul 2011 10:03:04 -0700	[thread overview]
Message-ID: <20110715170304.GD2327@linux.vnet.ibm.com> (raw)
In-Reply-To: <1310748957.27864.62.camel@gandalf.stny.rr.com>

On Fri, Jul 15, 2011 at 12:55:57PM -0400, Steven Rostedt wrote:
> On Fri, 2011-07-15 at 15:07 +0200, Peter Zijlstra wrote:
> 
> > OK, so the latter case cannot happen (rcu_preempt_check_callbacks only
> > sets NEED_QS when rcu_read_lock_nesting), we need two interrupts for
> > this to happen.
> > 
> > rcu_read_lock()
> > 
> >  <IRQ>
> >    |= RCU_READ_UNLOCK_NEED_QS
> > 
> > rcu_read_unlock()
> >   __rcu_read_unlock()
> >    --rcu_read_lock_nesting;
> >      <IRQ>
> > 	ttwu()
> >           rcu_read_lock()
> > 	  rcu_read_unlock()
> > 	    rcu_read_unlock_special()
> > 	      *BANG*
> >    rcu_read_unlock_special()
> > 
> 
> What about this patch? Not even compiled tested.

This runs afoul of the restriction that ->rcu_read_unlock_special must
be updated with irqs disabled, please see below.

I am also missing what the goal is -- I don't immediatly see how this
prevents the scenario that Ed ran into, for example.

								Thanx, Paul

> -- Steve
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 14dc7dd..e3545fa 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -284,18 +284,17 @@ static struct list_head *rcu_next_node_entry(struct task_struct *t,
>   * notify RCU core processing or task having blocked during the RCU
>   * read-side critical section.
>   */
> -static void rcu_read_unlock_special(struct task_struct *t)
> +static int rcu_read_unlock_special(struct task_struct *t, int special)
>  {
>  	int empty;
>  	int empty_exp;
>  	unsigned long flags;
>  	struct list_head *np;
>  	struct rcu_node *rnp;
> -	int special;
> 
>  	/* NMI handlers cannot block and cannot safely manipulate state. */
>  	if (in_nmi())
> -		return;
> +		return special;
> 
>  	local_irq_save(flags);
> 
> @@ -303,7 +302,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  	 * If RCU core is waiting for this CPU to exit critical section,
>  	 * let it know that we have done so.
>  	 */
> -	special = t->rcu_read_unlock_special;
>  	if (special & RCU_READ_UNLOCK_NEED_QS) {
>  		rcu_preempt_qs(smp_processor_id());
>  	}
> @@ -311,7 +309,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  	/* Hardware IRQ handlers cannot block. */
>  	if (in_irq()) {
>  		local_irq_restore(flags);
> -		return;
> +		return special;
>  	}
> 
>  	/* Clean up if blocked during RCU read-side critical section. */
> @@ -373,6 +371,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  	} else {
>  		local_irq_restore(flags);
>  	}
> +	return special;
>  }
> 
>  /*
> @@ -385,13 +384,21 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  void __rcu_read_unlock(void)
>  {
>  	struct task_struct *t = current;
> +	int special;
> 
> +	special = ACCESS_ONCE(t->rcu_read_unlock_special);
> +	/*
> +	 * Clear special here to prevent interrupts from seeing it
> +	 * enabled after decrementing lock_nesting and calling
> +	 * rcu_read_unlock_special().
> +	 */

Any change to ->rcu_read_unlock_special from an irq handler that happens
here is lost.  Changes to ->rcu_read_unlock_special must be done with
irqs disabled.  And I hope to avoid irq disabling on the rcu_read_unlock()
fastpath.

> +	t->rcu_read_unlock_special = 0;
>  	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);
> +	if (t->rcu_read_lock_nesting == 0 && special)
> +		special = rcu_read_unlock_special(t, special);

And changes to ->rcu_read_unlock_special from an irq handler that happens
here are also lost.

> +	t->rcu_read_unlock_special = special;
>  #ifdef CONFIG_PROVE_LOCKING
>  	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
>  #endif /* #ifdef CONFIG_PROVE_LOCKING */
> 
> 

  reply	other threads:[~2011-07-15 17:03 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
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 [this message]
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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