[patch 11/29] lockup_detector: Remove park_in_progress hackery

[Thomas Gleixner]

b94f51183b06 ("kernel/watchdog: prevent false hardlockup on overloaded
system") tries to fix the following issue:

 watchdog_stop()
	hrtimer_cancel()
	perf_nmi_event_stop()

If the task gets preempted after canceling the hrtimer or the VM gets
scheduled out long enough, then there is a chance that the next NMI will
see a stale hrtimer interrupt count and trigger a false positive hard
lockup splat.

That commit added a complete abstrusity with a atomic variable telling the
NMI watchdog function that stop is in progress. This is so stupid, that
it's not funny anymore.

Of course the patch missed that the same issue can happen in start:

 watchdog_start()
	perf_nmi_event_start()
	hrtimer_start()

The same effect can be achieved by reversing the start/stop order:

 watchdog_stop()
	perf_nmi_event_stop()
	hrtimer_cancel()

 watchdog_start()
	hrtimer_start()
	perf_nmi_event_start()

Get rid of the nonsense.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 include/linux/nmi.h   |    1 -
 kernel/watchdog.c     |   39 ++++++++++++++++++---------------------
 kernel/watchdog_hld.c |    7 ++-----
 3 files changed, 20 insertions(+), 27 deletions(-)

--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -27,7 +27,6 @@ extern void touch_softlockup_watchdog_sy
 extern void touch_all_softlockup_watchdogs(void);
 extern unsigned int  softlockup_panic;
 extern int soft_watchdog_enabled;
-extern atomic_t watchdog_park_in_progress;
 #else
 static inline void touch_softlockup_watchdog_sched(void)
 {
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -134,8 +134,6 @@ void __weak watchdog_nmi_reconfigure(voi
 #define for_each_watchdog_cpu(cpu) \
 	for_each_cpu_and((cpu), cpu_online_mask, &watchdog_cpumask)
 
-atomic_t watchdog_park_in_progress = ATOMIC_INIT(0);
-
 static u64 __read_mostly sample_period;
 
 static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
@@ -320,8 +318,7 @@ static enum hrtimer_restart watchdog_tim
 	int duration;
 	int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;
 
-	if (!watchdog_enabled ||
-	    atomic_read(&watchdog_park_in_progress) != 0)
+	if (!watchdog_enabled)
 		return HRTIMER_NORESTART;
 
 	/* kick the hardlockup detector */
@@ -435,33 +432,38 @@ static void watchdog_set_prio(unsigned i
 
 static void watchdog_enable(unsigned int cpu)
 {
-	struct hrtimer *hrtimer = raw_cpu_ptr(&watchdog_hrtimer);
+	struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);
 
-	/* kick off the timer for the hardlockup detector */
+	/*
+	 * Start the timer first to prevent the NMI watchdog triggering
+	 * before the timer has a chance to fire.
+	 */
 	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	hrtimer->function = watchdog_timer_fn;
+	hrtimer_start(hrtimer, ns_to_ktime(sample_period),
+		      HRTIMER_MODE_REL_PINNED);
 
+	/* Initialize timestamp */
+	__touch_watchdog();
 	/* Enable the perf event */
 	watchdog_nmi_enable(cpu);
 
-	/* done here because hrtimer_start can only pin to smp_processor_id() */
-	hrtimer_start(hrtimer, ns_to_ktime(sample_period),
-		      HRTIMER_MODE_REL_PINNED);
-
-	/* initialize timestamp */
 	watchdog_set_prio(SCHED_FIFO, MAX_RT_PRIO - 1);
-	__touch_watchdog();
 }
 
 static void watchdog_disable(unsigned int cpu)
 {
-	struct hrtimer *hrtimer = raw_cpu_ptr(&watchdog_hrtimer);
+	struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);
 
 	watchdog_set_prio(SCHED_NORMAL, 0);
-	hrtimer_cancel(hrtimer);
-	/* disable the perf event */
-	watchdog_nmi_disable(cpu);
+	/*
+	 * Disable the perf event first. That prevents that a large delay
+	 * between disabling the timer and disabling the perf event causes
+	 * the perf NMI to detect a false positive.
+	 */
 	hardlockup_detector_perf_disable();
+	watchdog_nmi_disable(cpu);
+	hrtimer_cancel(hrtimer);
 }
 
 static void watchdog_cleanup(unsigned int cpu, bool online)
@@ -517,16 +519,11 @@ static int watchdog_park_threads(void)
 {
 	int cpu, ret = 0;
 
-	atomic_set(&watchdog_park_in_progress, 1);
-
 	for_each_watchdog_cpu(cpu) {
 		ret = kthread_park(per_cpu(softlockup_watchdog, cpu));
 		if (ret)
 			break;
 	}
-
-	atomic_set(&watchdog_park_in_progress, 0);
-
 	return ret;
 }
 
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -106,15 +106,12 @@ static struct perf_event_attr wd_hw_attr
 
 /* Callback function for perf event subsystem */
 static void watchdog_overflow_callback(struct perf_event *event,
-		 struct perf_sample_data *data,
-		 struct pt_regs *regs)
+				       struct perf_sample_data *data,
+				       struct pt_regs *regs)
 {
 	/* Ensure the watchdog never gets throttled */
 	event->hw.interrupts = 0;
 
-	if (atomic_read(&watchdog_park_in_progress) != 0)
-		return;
-
 	if (__this_cpu_read(watchdog_nmi_touch) == true) {
 		__this_cpu_write(watchdog_nmi_touch, false);
 		return;