Re: INFO: possible circular locking dependency detected

[Paul E. McKenney]

On Fri, Jul 15, 2011 at 01:42:31PM -0400, Steven Rostedt wrote:
> On Fri, 2011-07-15 at 10:24 -0700, Paul E. McKenney wrote:
> 
> > But the rcu_read_unlock() called from within the irq handler would
> > take a second snapshot of ->special.  It could then enter
> > rcu_read_unlock_special().
> 
> You agree that an interrupt preempting the rcu_read_unlock() is causing
> the issues correct? But it is also contained within rcu_read_unlock().
> That is, we just don't want interrupts or softirqs from calling the
> special function when it preempted rcu_read_unlock().
> 
> How about this patch? (again totally untested and not even compiled)

I really dislike the added overhead, especially the implied
preempt_disable() and preempt_enable() calls.  I am actually trying to
-reduce- its overhead, for example, by removing the function call...

But as a short-term hack-around, it could be OK.  It does seem to
cover all the possible conditions, at least all the ones I can see at
the moment.

Longer term, enclosing the rq/pi lock critical sections with
rcu_read_lock() and rcu_read_unlock() seems more reasonable.

Hmmm...  Does just setting CONFIG_IRQ_FORCED_THREADING suffice to test
this stuff?  Or is "threadirqs" also required on the kernel command line?

							Thanx, Paul

> -- Steve
> 
> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> index 7784bd2..0bdf0ea 100644
> --- a/kernel/rcupdate.c
> +++ b/kernel/rcupdate.c
> @@ -46,6 +46,8 @@
>  #include <linux/module.h>
>  #include <linux/hardirq.h>
> 
> +DEFINE_PER_CPU(int, in_rcu_read_unlock);
> +
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  static struct lock_class_key rcu_lock_key;
>  struct lockdep_map rcu_lock_map =
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 14dc7dd..a4adbb7 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -375,6 +375,8 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  	}
>  }
> 
> +DECLARE_PER_CPU(int, in_rcu_read_unlock);
> +
>  /*
>   * Tree-preemptible RCU implementation for rcu_read_unlock().
>   * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
> @@ -386,12 +388,16 @@ void __rcu_read_unlock(void)
>  {
>  	struct task_struct *t = current;
> 
> +	get_cpu_var(in_rcu_read_unlock)++;
>  	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 &&
> +	    __get_cpu_var(in_rcu_read_unlock) == 1 &&
>  	    unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
>  		rcu_read_unlock_special(t);
> +	__get_cpu_var(in_rcu_read_unlock)--;
> +	put_cpu_var(in_rcu_read_unlock);
>  #ifdef CONFIG_PROVE_LOCKING
>  	WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
>  #endif /* #ifdef CONFIG_PROVE_LOCKING */
> 
> 
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