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Subject[PATCH] 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: Thomas Gleixner <tglx@linutronix.de>
---
kernel/time/clocksource.c | 38 ++++++++++++++++++--------------------
1 file changed, 18 insertions(+), 20 deletions(-)

Index: linux-2.6/kernel/time/clocksource.c
===================================================================
--- linux-2.6.orig/kernel/time/clocksource.c
+++ linux-2.6/kernel/time/clocksource.c
@@ -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(unsigne
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(unsigne
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(unsigne
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;
}
@@ -303,6 +310,13 @@ static void clocksource_watchdog(unsigne
}

/*
+ * 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_wat

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)

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