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

Thanks,

tglx

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);
}
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