Re: [patch V2 22/29] lockup_detector: Make watchdog_nmi_reconfigure() two stage

Thomas Gleixner [off-list ref] writes:

On Tue, 3 Oct 2017, Michael Ellerman wrote:

quoted

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?

I don't see it with CPU hotplug.

AFAICS that's because softlockup_reconfigure_threads() isn't called for
CPU hotplug. Unless there's a path I'm missing?

quoted

I'm not sure what the easiest fix is. One option would be to just drop
the WARN_ON, it's just there for paranoia AFAICS.

The straight forward way is to make use of the new probe function. Patch
below.

Thanks.

Hmm, I tried that patch, it makes the warning go away. But then I
triggered a deliberate hard lockup and got nothing.

Then I went back to the existing code (in linux-next), and I still get
no warning from a deliberate hard lockup.

So seems there may be some more gremlins. Will test more in the morning.

cheers