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SubjectRe: [PATCH 6/9] watchdog: implement error handling for failure to set up hardware perf events
On Thu,  5 Feb 2015 15:40:22 -0500 Don Zickus <dzickus@redhat.com> wrote:

> From: Ulrich Obergfell <uobergfe@redhat.com>
>
> If watchdog_nmi_enable() fails to set up the hardware perf event
> of one CPU, the entire hard lockup detector is deemed unreliable.
> Hence, disable the hard lockup detector and shut down the hardware
> perf events on all CPUs.
>
> Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> ---
> kernel/watchdog.c | 18 ++++++++++++++++++
> 1 files changed, 18 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 26002ed..7ad8949 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -502,6 +502,15 @@ static void watchdog(unsigned int cpu)
> __this_cpu_write(soft_lockup_hrtimer_cnt,
> __this_cpu_read(hrtimer_interrupts));
> __touch_watchdog();
> +
> + /*
> + * watchdog_nmi_enable() clears the NMI_WATCHDOG_ENABLED bit in the
> + * failure path. Check for failures that can occur asynchronously -
> + * for example, when CPUs are on-lined - and shut down the hardware
> + * perf event on each CPU accordingly.
> + */
> + if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
> + watchdog_nmi_disable(cpu);

Silently disabling the hardware watchdog. Wouldn't it be better to
emit a printk to alert the operator about this event?

>
> #ifdef CONFIG_HARDLOCKUP_DETECTOR
> @@ -552,6 +561,15 @@ handle_err:
> goto out_save;
> }
>
> + /*
> + * Disable the hard lockup detector if _any_ CPU fails to set up
> + * set up the hardware perf event. The watchdog() function checks
> + * the NMI_WATCHDOG_ENABLED bit periodically.
> + */
> + smp_mb__before_atomic();
> + clear_bit(NMI_WATCHDOG_ENABLED_BIT, &watchdog_enabled);
> + smp_mb__after_atomic();

Please send along a patch which adds comments explaining what these
barriers are for.

What are these barriers for? ;)

> /* skip displaying the same error again */
> if (cpu > 0 && (PTR_ERR(event) == cpu0_err))
> return PTR_ERR(event);



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