Messages in this thread Patch in this message | | | Date | Sat, 16 Sep 2017 19:35:10 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: d57108d4f6 ("watchdog/core: Get rid of the thread .."): BUG: unable to handle kernel NULL pointer dereference at 0000000000000208 |
| |
On Sat, 16 Sep 2017, Fengguang Wu wrote: > > > [ 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.
My bad. I booted the wrong config ....
> > > [ 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.
Don't bother. I found it already. On UP we have:
#define for_each_cpu(cpu, mask) \ for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
which is a total fail as it breaks any code which uses for_each_cpu() or any of the other variants on UP by assuming that all cpumask have bit 0 set.
That means any code which does not have conditional code for some of the cpumask functions is potentially broken. Sigh.
The simple cure for the watchdog is below.
Thanks,
tglx 8<------------------
diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c index b2931154b5f2..d4c0f75b189e 100644 --- a/kernel/watchdog_hld.c +++ b/kernel/watchdog_hld.c @@ -221,7 +221,12 @@ 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); + /* + * Check the event, because on UP for_each_cpu() assumes + * idiotically that all masks handed in have bit 0 set. + */ + if (event) + perf_event_release_kernel(event); } cpumask_clear(&dead_events_mask); }
| |