Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208

[Fengguang Wu]

On Fri, Sep 15, 2017 at 06:24:20PM +0200, Thomas Gleixner wrote:
> On Fri, 15 Sep 2017, Thomas Gleixner wrote:
>
> > On Fri, 15 Sep 2017, Thomas Gleixner wrote:
> >
> > > On Fri, 15 Sep 2017, kernel test robot wrote:
> > > > [    0.035023] CPU: Intel Common KVM processor (family: 0xf, model: 0x6, stepping: 0x1)
> > > > [    0.042302] Performance Events: unsupported Netburst CPU model 6 no PMU driver, software events only.
> > >
> > > Cute. So there is no supported PMU, but for some unknown reason the lockup
> > > detector can create an event, otherwise the perf availaibility check in
> > > lockup_detector_init() would fail ....
> > >
> > > Peter???
> >
> > In my VM the corresponding dmesg is:
> >
> > [    0.038086] Performance Events: unsupported p6 CPU model 61 no PMU driver, software events only.

What's your host CPU? I can reproduce it in Nehalem, Haswell and Sandy
Bridge machines with the attached script.

> > [    0.041031] Hierarchical SRCU implementation.
> > [    0.046210] NMI watchdog: Perf event create on CPU 0 failed with -2
> > [    0.046980] NMI watchdog: Perf NMI watchdog permanetely disabled
> >
> > Confused
>
> I still can't reproduce. Can you please apply the debug patch below and
> provide the output?

OK. I'll try and report back tomorrow.

Thanks,
Fengguang

> 8<-----------------
>
> diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
> index b2931154b5f2..e6c9ca516945 100644
> --- a/kernel/watchdog_hld.c
> +++ b/kernel/watchdog_hld.c
> @@ -171,6 +171,7 @@ static int hardlockup_detector_event_create(void)
> 	/* Try to register using hardware perf events */
> 	evt = perf_event_create_kernel_counter(wd_attr, cpu, NULL,
> 					       watchdog_overflow_callback, NULL);
> +	pr_info("EVT create on CPU %u returned %p\n", cpu, evt);
> 	if (IS_ERR(evt)) {
> 		pr_info("Perf event create on CPU %d failed with %ld\n", cpu,
> 			PTR_ERR(evt));
> @@ -221,7 +222,10 @@ void hardlockup_detector_perf_cleanup(void)
> 		struct perf_event *event = per_cpu(watchdog_ev, cpu);
>
> 		per_cpu(watchdog_ev, cpu) = NULL;
> -		perf_event_release_kernel(event);
> +		pr_info("EVT on CPU %u in dead mask: %p\n",  cpu, event);
> +		if (event)
> +			perf_event_release_kernel(event);
> +
> 	}
> 	cpumask_clear(&dead_events_mask);
> }
#!/bin/bash

kernel=$1

kvm=(
	qemu-system-x86_64
	-enable-kvm
	-cpu kvm64
	-kernel $kernel
	-m 399
	-smp 2
	-device e1000,netdev=net0
	-netdev user,id=net0
	-boot order=nc
	-no-reboot
	-watchdog i6300esb
	-watchdog-action debug
	-rtc base=localtime
	-serial stdio
	-display none
	-monitor null
)

append=(
	root=/dev/ram0
	hung_task_panic=1
	debug
	apic=debug
	sysrq_always_enabled
	rcupdate.rcu_cpu_stall_timeout=100
	net.ifnames=0
	printk.devkmsg=on
	panic=-1
	softlockup_panic=1
	nmi_watchdog=panic
	oops=panic
	load_ramdisk=2
	prompt_ramdisk=0
	drbd.minor_count=8
	systemd.log_level=err
	ignore_loglevel
	console=tty0
	earlyprintk=ttyS0,115200
	console=ttyS0,115200
	vga=normal
	rw
	drbd.minor_count=8
)

"${kvm[@]}" -append "${append[*]}"