Re: lockdep circular locking error (rcu_node_level_0 vs rq->lock)

Paul E. McKenney
2011-07-14T05:26:28Z

On Wed, Jul 13, 2011 at 09:29:32PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 13, 2011 at 09:33:09AM -0700, Paul E. McKenney wrote:

[ . . . ]

> Hmmm... Something is confused. I get boot time hangs with the
> occasional stack overflow, whether or not I add my patch on top.
> Left to myself, I would try applying your patch incrementally, and
> also disabling irqs before rcu_read_lock() and enabling them after
> rcu_read_unlock(), as this prevents rcu_read_unlock() from ever getting
> to the rcu_read_unlock_special() slowpath.

And the real question... Why did the rcu_read_unlock() called from
cpuacct_charge() enter rcu_read_unlock_special() in the first place?
If the runqueue lock is held, irqs should be disabled, so how did the
task get blocked or interrupted?

I need to get some sleep then figure out what is going on here.

Thanx, Paul