On Tue, Oct 21, 2014 at 06:04:52PM +0200, Jiri Kosina wrote:
> 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();
> }
Ah, it would help if I did "git checkout linus/master" after updating,
wouldn't it now?
> > > (*) 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 ...
If it was attempting to acquire it via synchronize_sched_expedited(),
the attempt would fail and synchronize_sched_expedited() would fall
back to synchronize_sched()'s normal grace-period mechanism. (Not to
synchronize_sched() itself, of course, as that would be infinite
recursion.)
So I believe that something else is going on here.
Thanx, Paul
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