Messages in this thread |  | | Date | Fri, 15 Jul 2011 11:11:47 -0700 | From | "Paul E. McKenney" <> | Subject | Re: INFO: possible circular locking dependency detected |
| |
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/#. > > > >
|  |