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

[Thomas Gleixner]

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);
 }