linux-kernel - Re: lockdep splat in CPU hotplug

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Date:	Tue, 21 Oct 2014 17:21:21 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Dave Jones <davej@...hat.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: 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
SUSE Labs
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