Re: lockdep splat in CPU hotplug

[Jiri Kosina]

On Tue, 21 Oct 2014, Paul E. McKenney wrote:

> > Looks like this indeed is something that lockdep *should* report (*), 
> > although I would be suprised that stack unwinder would be so confused 
> > by this -- there is no way for synchronize_sched_expedited() to be 
> > inlined all the way to cpuidle_pause().
> 
> I think that if synchronize_sched_expedited() was in fact called, it
> had already returned by the time we hit this problem.  But I must confess
> that I am not seeing how cpuidle_uninstall_idle_handler() gets to
> synchronize_rcu().

Umm, it directly calls it? :-)

	void cpuidle_uninstall_idle_handler(void)
	{
		if (enabled_devices) {
			initialized = 0;
			wake_up_all_idle_cpus();
		}

		/*
		 * Make sure external observers (such as the scheduler)
		 * are done looking at pointed idle states.
		 */
		synchronize_rcu();
	}


> > (*) there are multiple places where cpu_hotplug.lock -> cpuidle_lock lock 
> >     dependency is assumed. The patch that Dave pointed out adds 
> >     cpuidle_lock -> cpu_hotplug.lock dependency.
> > 
> > Still not clear whether this is what's happening here ... anyway, adding 
> > Paul to CC.
> 
> Hmmm...
> 
> Both cpuidle_pause() and cpuidle_pause_and_lock() acquire cpuidle_lock,
> and are at the top of both stacks.  Which was the original confusion.  ;-)

Yup, they are, but lockdep is complaining about cpuidle_pause() acquiring 
cpu_hotplug.lock ...

Thanks,

-- 
Jiri Kosina
SUSE Labs
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