linux-kernel - Re: [tip:timers/urgent] clocksource: Prevent potential kgdb dead lock

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Date:	Tue, 26 Jan 2010 12:14:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	jason.wessel@...driver.com, johnstul@...ibm.com,
	schwidefsky@...ibm.com, tglx@...utronix.de
Cc:	tip-bot for Thomas Gleixner <tglx@...utronix.de>,
	linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, mingo@...hat.com, johnstul@...ibm.com,
	jason.wessel@...driver.com, schwidefsky@...ibm.com
Subject: Re: [tip:timers/urgent] clocksource: Prevent potential kgdb dead
 lock

On Tue, 26 Jan 2010 14:09:45 GMT tip-bot for Thomas Gleixner <tglx@...utronix.de> wrote:

> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -343,7 +343,19 @@ static void clocksource_resume_watchdog(void)
>  {
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&watchdog_lock, flags);
> +	/*
> +	 * We use trylock here to avoid a potential dead lock when
> +	 * kgdb calls this code after the kernel has been stopped with
> +	 * watchdog_lock held. When watchdog_lock is held we just
> +	 * return and accept, that the watchdog might trigger and mark
> +	 * the monitored clock source (usually TSC) unstable.
> +	 *
> +	 * This does not affect the other caller clocksource_resume()
> +	 * because at this point the kernel is UP, interrupts are
> +	 * disabled and nothing can hold watchdog_lock.
> +	 */
> +	if (!spin_trylock_irqsave(&watchdog_lock, flags))
> +		return;
>  	clocksource_reset_watchdog();
>  	spin_unlock_irqrestore(&watchdog_lock, flags);
>  }
>
> @@ -458,8 +470,8 @@ void clocksource_resume(void)
>   * clocksource_touch_watchdog - Update watchdog
>   *
>   * Update the watchdog after exception contexts such as kgdb so as not
> - * to incorrectly trip the watchdog.
> - *
> + * to incorrectly trip the watchdog. This might fail when the kernel
> + * was stopped in code which holds watchdog_lock.
>   */
>  void clocksource_touch_watchdog(void)
>  {

It would be neater and a shade more reliable to add a separate
clocksource_try_touch_watchdog() for kgdb's use.  Which could be inside
#ifdef CONFIG_KGDB.

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