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SubjectRe: [PATCH] fix BUG using smp_processor_id() in touch_nmi_watchdog and touch_softlockup_watchdog
On Thu, Aug 19, 2010 at 08:42:56PM -0700, Andrew Morton wrote:
> On Thu, 19 Aug 2010 22:57:49 -0400 Don Zickus <dzickus@redhat.com> wrote:
>
> > On Wed, Aug 18, 2010 at 01:01:56PM -0700, Andrew Morton wrote:
> > @@ -430,6 +437,9 @@ static int watchdog_enable(int cpu)
> > wake_up_process(p);
> > }
> >
> > + /* if any cpu succeeds, watchdog is considered enabled for the system */
> > + watchdog_enabled = 1;
> > +
> > return 0;
> > }
> >
> > @@ -452,9 +462,6 @@ static void watchdog_disable(int cpu)
> > per_cpu(softlockup_watchdog, cpu) = NULL;
> > kthread_stop(p);
> > }
> > -
> > - /* if any cpu succeeds, watchdog is considered enabled for the system */
> > - watchdog_enabled = 1;
> > }
> >
> > static void watchdog_enable_all_cpus(void)
>
> hm, the code seems a bit screwy. Maybe it was always thus.

No, watchdog_enabled was something newly created for the lockup dectector.

>
> watchdog_enabled gets set in the per-cpu function but it gets cleared
> in the all-cpus function. Asymmetric.

Yes it is by design. I was using watchdog_enabled as a global state
variable. As soon as one cpu was enabled, I would set the bit. But only
if all the cpus disabled the watchdog would I clear the bit.

>
> Also afacit the action of cpu-hotunplug+cpu-hotplug will reenable the
> watchdog on a CPU which was supposed to have it disabled. Perhaps you
> could recheck that and make sure it all makes sense - perhaps we need a
> separate state variable which is purely "current setting of
> /proc/sys/kernel/nmi_watchdog" and doesn't get altered internally.

I wasn't tracking it on a per cpu basis. I didn't see a need to. The
watchdog should globally be on/off across the system. If a system comes
up and one of the cpus could not bring the watchdog online for some
reason, then that is a problem. If a cpu-hotunplug+cpu-hotplug fixes it,
all the better. :-)

Also, if I wanted to track it per cpu, there is a bunch of status bits in
per-cpu variables that could let the code know whether a particular cpu
watchdog is on/off for either hardlockup or softlockup.

Cheers,
Don


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