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>
Cc: John Stultz <johnstul@us.ibm.com>
Acked-by: Jason 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: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 18 insertions, 20 deletions

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index e0980f0d9a0..cf52fda2e09 100644
--- a/kernel/time/clocksource.c
+++ b/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(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;
                 }
@@ -303,6 +310,13 @@ 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)