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SubjectRe: lockdep splat in CPU hotplug
On Tue, 21 Oct 2014, Dave Jones wrote:

> > I am seeing the lockdep report below when resuming from suspend-to-disk
> > with current Linus' tree (c2661b80609).
> >
> > The reason for CCing Ingo and Peter is that I can't make any sense of one
> > of the stacktraces lockdep is providing.
> >
> > Please have a look at the very first stacktrace in the dump, where lockdep
> > is trying to explain where cpu_hotplug.lock#2 has been acquired. It seems
> > to imply that cpuidle_pause() is taking cpu_hotplug.lock, but that's not
> > the case at all.
> Could inlining be confusing the trace here ?
> You can get from cpuidle_pause to cpuidle_uninstall_idle_handler -> synchronize_rcu
> -> synchronize_sched -> synchronize_sched_expedited which
> does a try_get_online_cpus which will take the cpu_hotplug.lock

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().

(*) 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.

Jiri Kosina

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