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Date: Tue, 3 Oct 2017 09:04:03 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Michael Ellerman <mpe@ellerman.id.au>
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Subject: Re: [patch V2 22/29] lockup_detector: Make watchdog_nmi_reconfigure()
 two stage
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On Tue, 3 Oct 2017, Thomas Gleixner wrote:
> On Tue, 3 Oct 2017, Michael Ellerman wrote:
> > Hi Thomas,
> > Unfortunately this is hitting the WARN_ON in start_wd_cpu() on powerpc
> > because we're calling it multiple times for the boot CPU.
> > 
> > The first call is via:
> > 
> >   start_wd_on_cpu+0x80/0x2f0
> >   watchdog_nmi_reconfigure+0x124/0x170
> >   softlockup_reconfigure_threads+0x110/0x130
> >   lockup_detector_init+0xbc/0xe0
> >   kernel_init_freeable+0x18c/0x37c
> >   kernel_init+0x2c/0x160
> >   ret_from_kernel_thread+0x5c/0xbc
> > 
> > And then again via the CPU hotplug registration:
> > 
> >   start_wd_on_cpu+0x80/0x2f0
> >   cpuhp_invoke_callback+0x194/0x620
> >   cpuhp_thread_fun+0x7c/0x1b0
> >   smpboot_thread_fn+0x290/0x2a0
> >   kthread+0x168/0x1b0
> >   ret_from_kernel_thread+0x5c/0xbc
> > 
> > 
> > The first call is new because previously watchdog_nmi_reconfigure()
> > wasn't called from softlockup_reconfigure_threads().
> 
> Hmm, don't you have the same problem with CPU hotplug or do you just get
> lucky because the hotplug callback in your code is ordered vs. the
> softlockup thread hotplug callback in a way that this does not hit?

Which leads me to the question why you need the hotplug state at all if the
softlockup detector is enabled. Wouldn't it make more sense to only
register the state if softlockup detector is turned off in Kconfig and
actually move it to the core code?

Thanks,

	tglx