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 for <linuxppc-dev@lists.ozlabs.org>; Wed, 26 Jul 2017 23:21:03 +1000 (AEST)
Date: Wed, 26 Jul 2017 14:44:04 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Michael Ellerman <mpe@ellerman.id.au>
cc: linuxppc-dev@lists.ozlabs.org
Subject: Re: Possible circular locking dependency detected between
 cpu_hotplug_lock.rw_sem and wfc.work
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On Wed, 26 Jul 2017, Michael Ellerman wrote:

> Hi Thomas,
> 
> I'm seeing the lockdep barf below on some bare metal Power8 machines.
> 
> This seems to be caused by our smp_cpus_done(), which does:
> 
>   void __init smp_cpus_done(unsigned int max_cpus)
>   {
>   	/*
>   	 * We want the setup_cpu() here to be called on the boot CPU, but
>   	 * init might run on any CPU, so make sure it's invoked on the boot
>   	 * CPU.
>   	 */
>   	if (smp_ops && smp_ops->setup_cpu)
>   		work_on_cpu_safe(boot_cpuid, smp_setup_cpu_workfn, NULL);
> 
> 
> I don't think CPU hotplug can happen at this point, so I don't think
> there's really a bug.
> 
> But it looks like the work_on_cpu_safe() call could just go away, since
> you pinned init to the boot CPU in 8fb12156b8db ("init: Pin init task to
> the boot CPU, initially"). Though I can't see where init is unpinned, so
> maybe we do still need to do it?

It's undone in sched_init_smp(). So it looks safe. The call order is:

     smp_init()
	...
	smp_cpus_done()

     sched_init_smp()

Thanks,

	tglx