Re: INFO: possible circular locking dependency detected

["Paul E. McKenney" <paulmck@linux.vnet.ibm.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 */
> 
>