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Commit 9fb60336 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

clocksource: Make watchdog reset lockless 



KGDB needs to trylock watchdog_lock when trying to reset the
clocksource watchdog after the system has been stopped to avoid a
potential deadlock. When the trylock fails TSC usually becomes
unstable.

We can be more clever by using an atomic counter and checking it in
the clocksource_watchdog callback. We restart the watchdog whenever
the counter is > 0 and only decrement the counter when we ran through
a full update cycle.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <johnstul@us.ibm.com>
Acked-by: default avatarJason Wessel <jason.wessel@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1109121326280.2723@ionos


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent e8abccb7

kernel/time/clocksource.c

+18 −20
Original line number Diff line number Diff line
@@ -186,6 +186,7 @@ static struct timer_list watchdog_timer; 
static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);

static DEFINE_SPINLOCK(watchdog_lock);

static int watchdog_running;

static atomic_t watchdog_reset_pending;



static int clocksource_watchdog_kthread(void *data);

static void __clocksource_change_rating(struct clocksource *cs, int rating);
@@ -247,12 +248,14 @@ static void clocksource_watchdog(unsigned long data) 
	struct clocksource *cs;

	cycle_t csnow, wdnow;

	int64_t wd_nsec, cs_nsec;

	int next_cpu;

	int next_cpu, reset_pending;



	spin_lock(&watchdog_lock);

	if (!watchdog_running)

		goto out;



	reset_pending = atomic_read(&watchdog_reset_pending);



	list_for_each_entry(cs, &watchdog_list, wd_list) {



		/* Clocksource already marked unstable? */
@@ -268,7 +271,8 @@ static void clocksource_watchdog(unsigned long data) 
		local_irq_enable();



		/* Clocksource initialized ? */

		if (!(cs->flags & CLOCK_SOURCE_WATCHDOG)) {

		if (!(cs->flags & CLOCK_SOURCE_WATCHDOG) ||

		    atomic_read(&watchdog_reset_pending)) {

			cs->flags |= CLOCK_SOURCE_WATCHDOG;

			cs->wd_last = wdnow;

			cs->cs_last = csnow;
@@ -283,8 +287,11 @@ static void clocksource_watchdog(unsigned long data) 
		cs->cs_last = csnow;

		cs->wd_last = wdnow;



		if (atomic_read(&watchdog_reset_pending))

			continue;



		/* Check the deviation from the watchdog clocksource. */

		if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {

		if ((abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD)) {

			clocksource_unstable(cs, cs_nsec - wd_nsec);

			continue;

		}
@@ -302,6 +309,13 @@ static void clocksource_watchdog(unsigned long data) 
		}

	}



	/*

	 * We only clear the watchdog_reset_pending, when we did a

	 * full cycle through all clocksources.

	 */

	if (reset_pending)

		atomic_dec(&watchdog_reset_pending);



	/*

	 * Cycle through CPUs to check if the CPUs stay synchronized

	 * to each other.
@@ -344,23 +358,7 @@ static inline void clocksource_reset_watchdog(void) 


static void clocksource_resume_watchdog(void)

{

	unsigned long 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);

	atomic_inc(&watchdog_reset_pending);

}



static void clocksource_enqueue_watchdog(struct clocksource *cs)