On Thu, 19 Aug 2010 22:57:49 -0400 Don Zickus <dzickus@xxxxxxxxxx> wrote:
> On Wed, Aug 18, 2010 at 01:01:56PM -0700, Andrew Morton wrote:
> > The surprise new requirement that touch_nmi_watchdog() be called from
> > non-preemptible code does seem to make sense IMO. It's hard to see why
> > anyone would be touching the watchdog unless he's spinning in irqs-off
> > code. Except, of course, when we have a utility function which can be
> > called from wither irqs-on or irqs-off: acpi_os_stall().
> >
> > That being said, it's not good to introduce new API requirements by
> > accident! An audit of all callers should first be performed, at least.
> >
> >
> > The surprise new requirement that touch_softlockup_watchdog() be called
> > from non-preemptible code doesn't make sense IMO. If I have a piece of
> > code in the kernel which I expect to sit in TASK_UNINTERRUPTIBLE state
> > for three minutes waiting for my egg to boil, I should be able to do
> > that and I should be able to touch the softlockup detector without
> > needing to go non-preemptible.
>
> Ok, so here is my patch that syncs the touch_*_watchdog back in line with
> the old semantics. Hopefully this will undo any harm I caused.
>
> ------------cut -->---------------------------
>
> >From b372e821c804982438db090db6b4a2f753c78091 Mon Sep 17 00:00:00 2001
> From: Don Zickus <dzickus@xxxxxxxxxx>
> Date: Thu, 19 Aug 2010 22:48:26 -0400
> Subject: [PATCH] [lockup detector] sync touch_*_watchdog back to old semantics
>
> During my rewrite, the semantics of touch_nmi_watchdog and
> touch_softlockup_watchdog changed enough to break some drivers
> (mostly over preemptable regions).
>
> This change brings those touch_*_watchdog functions back in line
> to how they used to work.
>
> Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx>
> ---
> kernel/watchdog.c | 17 ++++++++++++-----
> 1 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 613bc1f..99e35a2 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -122,7 +122,7 @@ static void __touch_watchdog(void)
>
> void touch_softlockup_watchdog(void)
> {
> - __get_cpu_var(watchdog_touch_ts) = 0;
> + __raw_get_cpu_var(watchdog_touch_ts) = 0;
> }
> EXPORT_SYMBOL(touch_softlockup_watchdog);
>
> @@ -142,7 +142,14 @@ void touch_all_softlockup_watchdogs(void)
> #ifdef CONFIG_HARDLOCKUP_DETECTOR
> void touch_nmi_watchdog(void)
> {
> - __get_cpu_var(watchdog_nmi_touch) = true;
> + if (watchdog_enabled) {
> + unsigned cpu;
> +
> + for_each_present_cpu(cpu) {
> + if (per_cpu(watchdog_nmi_touch, cpu) != true)
> + per_cpu(watchdog_nmi_touch, cpu) = true;
> + }
> + }
> touch_softlockup_watchdog();
> }
> EXPORT_SYMBOL(touch_nmi_watchdog);
> @@ -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.
watchdog_enabled gets set in the per-cpu function but it gets cleared
in the all-cpus function. Asymmetric.
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.
Anyway, I'll be disappearing for a few days so perhaps Frederic or hpa
can help get this all fixed/merged up?
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