Re: INFO: possible circular locking dependency detected

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

On Fri, Jul 15, 2011 at 01:39:20PM +0200, Peter Zijlstra wrote:
> On Fri, 2011-07-15 at 13:29 +0200, Peter Zijlstra wrote:
> > 
> > Paul, what appears to be happening here is that some rcu_read_unlock()
> > gets interrupted, possibly before calling rcu_read_unlock_special(),
> > possibly not if the interrupt is itself the timer interrupt.
> > 
> > Supposing ->rcu_read_unlock_special is set before, any wakeup happening
> > from an interrupt hitting __rcu_read_unlock():
> 
> Hmm, ok not any wakeup from interrupt context because you have that
> in_irq() test in there, but if that IRQ doesn't happen to use RCU and
> does trigger softirqs and one of that softirq does a wakeup we're still
> in the same boat.

Agreed.  All fallout from adding rcu_read_unlock() while holding
rq/pi locks without the needed adjustments.  :-(

							Thanx, Paul

> > void __rcu_read_unlock(void)
> > {
> >         struct task_struct *t = current;
> >                 
> >         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);
> > #ifdef CONFIG_PROVE_LOCKING
> >         WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
> > #endif /* #ifdef CONFIG_PROVE_LOCKING */
> > }
> > 
> > After --t->rcu_read_lock_nesting, but before calling
> > rcu_read_unlock_special(), will trigger this lock inversion.
> > 
> > The alternative case, ->rcu_read_unlock_special is not set yet, it can
> > be set if the interrupt hitting in that same spot above, is the timer
> > interrupt, and the wakeup happens either from the softirq ran from the
> > hard IRQ tail, or as I suspect here happens, the wakeup of ksoftirqd/#.
> > 
> >