linux-kernel - Re: [PATCH 6/9] watchdog: implement error handling for failure to set up hardware perf events

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Message-Id: <20150223131734.61ee63b5f4064e656f0da762@linux-foundation.org>
Date:	Mon, 23 Feb 2015 13:17:34 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [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@...hat.com> wrote:

> From: Ulrich Obergfell <uobergfe@...hat.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@...hat.com>
> Signed-off-by: Don Zickus <dzickus@...hat.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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