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SubjectRe: [tip:timers/urgent] clocksource: Prevent potential kgdb dead lock
On Tue, 26 Jan 2010 14:09:45 GMT tip-bot for Thomas Gleixner <tglx@linutronix.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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