watchdog/hardlockup/perf: Use atomics to track in-use cpu counter

Guenter reported:
  There is still a problem. When running 
    echo 6 > /proc/sys/kernel/watchdog_thresh
    echo 5 > /proc/sys/kernel/watchdog_thresh
  repeatedly, the message
 
   NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.
 
  stops after a while (after ~10-30 iterations, with fluctuations).
  Maybe watchdog_cpus needs to be atomic ?

That's correct as this again is affected by the asynchronous nature of the
smpboot thread unpark mechanism.

CPU 0				CPU1			CPU2
write(watchdog_thresh, 6)	
  stop()
    park()
  update()
  start()
    unpark()
				thread->unpark()
				  cnt++;
write(watchdog_thresh, 5)				thread->unpark()
  stop()
    park()			thread->park()
				   cnt--;		  cnt++;
  update()
  start()
    unpark()

That's not a functional problem, it just affects the informational message.

Convert watchdog_cpus to atomic_t to prevent the problem
Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20171101181126.j727fqjmdthjz4xk@redhat.com

kernel/watchdog_hld.c

@@ -12,6 +12,7 @@
 #define pr_fmt(fmt) "NMI watchdog: " fmt
 
 #include <linux/nmi.h>
+#include <linux/atomic.h>
 #include <linux/module.h>
 #include <linux/sched/debug.h>
 
@@ -25,7 +26,7 @@ static DEFINE_PER_CPU(struct perf_event *, dead_event);
 static struct cpumask dead_events_mask;
 
 static unsigned long hardlockup_allcpu_dumped;
-static unsigned int watchdog_cpus;
+static atomic_t watchdog_cpus = ATOMIC_INIT(0);
 
 void arch_touch_nmi_watchdog(void)
 {
@@ -189,7 +190,8 @@ void hardlockup_detector_perf_enable(void)
 	if (hardlockup_detector_event_create())
 		return;
 
-	if (!watchdog_cpus++)
+	/* use original value for check */
+	if (!atomic_fetch_inc(&watchdog_cpus))
 		pr_info("Enabled. Permanently consumes one hw-PMU counter.\n");
 
 	perf_event_enable(this_cpu_read(watchdog_ev));
@@ -207,7 +209,7 @@ void hardlockup_detector_perf_disable(void)
 		this_cpu_write(watchdog_ev, NULL);
 		this_cpu_write(dead_event, event);
 		cpumask_set_cpu(smp_processor_id(), &dead_events_mask);
-		watchdog_cpus--;
+		atomic_dec(&watchdog_cpus);
 	}
 }