Re: INFO: possible circular locking dependency detected

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jul 14, 2011 at 03:41:42PM -0400, Steven Rostedt wrote:
> On Thu, 2011-07-14 at 12:18 -0700, Paul E. McKenney wrote:
> 
> > I believe that this affects only TREE_PREEMPT_RCU kernels with RCU_BOOST
> > set: interrupt disabling takes care of TINY_PREEMPT_RCU.  I think, anyway.
> 
> I agree that this doesn't affect TINY, but that doesn't mean you
> shouldn't change it to be like TREE. You still have the rcu_boost
> variable in the task struct wasting space, and having the them closer to
> the same algorithm the better (less learning curve).
> 
> 
> > 
> > Please see below for a patch that I believe fixes this problem.
> > It relies on the fact that RCU_READ_UNLOCK_BOOSTED cannot be set unless
> > RCU_READ_UNLOCK_BLOCKED is also set, which allows the two to be in
> > separate variables.  The original ->rcu_read_unlock_special is handled
> > only by the corresponding thread, while the new ->rcu_boosted is accessed
> > and updated only with the rcu_node structure's ->lock held.
> > 
> > Thoughts?
> > 
> 
> Looks good!
> 
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

Thank you!

							Thanx, Paul

> -- Steve
> 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 496770a..2a88747 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1254,6 +1254,9 @@ struct task_struct {
> >  #ifdef CONFIG_PREEMPT_RCU
> >  	int rcu_read_lock_nesting;
> >  	char rcu_read_unlock_special;
> > +#ifdef CONFIG_RCU_BOOST
> > +	int rcu_boosted;
> > +#endif /* #ifdef CONFIG_RCU_BOOST */
> >  	struct list_head rcu_node_entry;
> >  #endif /* #ifdef CONFIG_PREEMPT_RCU */
> >  #ifdef CONFIG_TREE_PREEMPT_RCU
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index 75113cb..8d38a98 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -342,6 +342,11 @@ static void rcu_read_unlock_special(struct task_struct *t)
> >  #ifdef CONFIG_RCU_BOOST
> >  		if (&t->rcu_node_entry == rnp->boost_tasks)
> >  			rnp->boost_tasks = np;
> > +		/* Snapshot and clear ->rcu_boosted with rcu_node lock held. */
> > +		if (t->rcu_boosted) {
> > +			special |= RCU_READ_UNLOCK_BOOSTED;
> > +			t->rcu_boosted = 0;
> > +		}
> >  #endif /* #ifdef CONFIG_RCU_BOOST */
> >  		t->rcu_blocked_node = NULL;
> >  
> > @@ -358,7 +363,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
> >  #ifdef CONFIG_RCU_BOOST
> >  		/* 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;
> >  		}
> > @@ -1174,7 +1178,7 @@ 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;
> > +	t->rcu_boosted = 1;
> >  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >  	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
> >  	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
> 
>