clocksource: Make watchdog robust vs. interruption

The clocksource watchdog code is interruptible and it has been
observed that this can trigger false positives which disable the TSC.

The reason is that an interrupt storm or a long running interrupt
handler between the read of the watchdog source and the read of the
TSC brings the two far enough apart that the delta is larger than the
unstable treshold. Move both reads into a short interrupt disabled
region to avoid that.

Reported-and-tested-by: Vernon Mauery <vernux@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@kernel.org

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index d4646b48dc4a7b0074dc7491e7f66186f398e70e..18a1baf31f2d531493526a48132dc38a5f2f0f03 100644 (file)
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -188,6 +188,7 @@ struct clocksource {
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
        /* Watchdog related data, used by the framework */
        struct list_head wd_list;
+       cycle_t cs_last;
        cycle_t wd_last;
 #endif
 } ____cacheline_aligned;

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 1c95fd677328e4fe3d0e582fc62284d9ec150a68..e0980f0d9a0ad2d559b98c12f26317b55044cad1 100644 (file)
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -185,7 +185,6 @@ static struct clocksource *watchdog;
 static struct timer_list watchdog_timer;
 static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
 static DEFINE_SPINLOCK(watchdog_lock);
-static cycle_t watchdog_last;
 static int watchdog_running;
 
 static int clocksource_watchdog_kthread(void *data);
@@ -254,11 +253,6 @@ static void clocksource_watchdog(unsigned long data)
        if (!watchdog_running)
                goto out;
 
-       wdnow = watchdog->read(watchdog);
-       wd_nsec = clocksource_cyc2ns((wdnow - watchdog_last) & watchdog->mask,
-                                    watchdog->mult, watchdog->shift);
-       watchdog_last = wdnow;
-
        list_for_each_entry(cs, &watchdog_list, wd_list) {
 
                /* Clocksource already marked unstable? */
@@ -268,19 +262,28 @@ static void clocksource_watchdog(unsigned long data)
                        continue;
                }
 
+               local_irq_disable();
                csnow = cs->read(cs);
+               wdnow = watchdog->read(watchdog);
+               local_irq_enable();
 
                /* Clocksource initialized ? */
                if (!(cs->flags & CLOCK_SOURCE_WATCHDOG)) {
                        cs->flags |= CLOCK_SOURCE_WATCHDOG;
-                       cs->wd_last = csnow;
+                       cs->wd_last = wdnow;
+                       cs->cs_last = csnow;
                        continue;
                }
 
-               /* Check the deviation from the watchdog clocksource. */
-               cs_nsec = clocksource_cyc2ns((csnow - cs->wd_last) &
+               wd_nsec = clocksource_cyc2ns((wdnow - cs->wd_last) & watchdog->mask,
+                                            watchdog->mult, watchdog->shift);
+
+               cs_nsec = clocksource_cyc2ns((csnow - cs->cs_last) &
                                             cs->mask, cs->mult, cs->shift);
-               cs->wd_last = csnow;
+               cs->cs_last = csnow;
+               cs->wd_last = wdnow;
+
+               /* Check the deviation from the watchdog clocksource. */
                if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
                        clocksource_unstable(cs, cs_nsec - wd_nsec);
                        continue;
@@ -318,7 +321,6 @@ static inline void clocksource_start_watchdog(void)
                return;
        init_timer(&watchdog_timer);
        watchdog_timer.function = clocksource_watchdog;
-       watchdog_last = watchdog->read(watchdog);
        watchdog_timer.expires = jiffies + WATCHDOG_INTERVAL;
        add_timer_on(&watchdog_timer, cpumask_first(cpu_online_mask));
        watchdog_running = 1;