Re: INFO: possible circular locking dependency detected

[Steven Rostedt]

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@xxxxxxxxxxx>

-- 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. */


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