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

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Message-ID: <alpine.DEB.2.20.1709151820250.1833@nanos>
Date:   Fri, 15 Sep 2017 18:24:20 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     kernel test robot <fengguang.wu@...el.com>
cc:     LKP <lkp@...org>, LKML <linux-kernel@...r.kernel.org>,
        Don Zickus <dzickus@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, wfg@...ux.intel.com,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: 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);
 }