watchdog/core, powerpc: Lock cpus across reconfiguration

Instead of dropping the cpu hotplug lock after stopping NMI watchdog and
threads and reaquiring for restart, the code and the protection rules
become more obvious when holding cpu hotplug lock across the full
reconfiguration.

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1710022105570.2114@nanos

arch/powerpc/kernel/watchdog.c

@@ -359,21 +359,17 @@ void watchdog_nmi_stop(void)
 {
 	int cpu;
 
-	cpus_read_lock();
 	for_each_cpu(cpu, &wd_cpus_enabled)
 		stop_wd_on_cpu(cpu);
-	cpus_read_unlock();
 }
 
 void watchdog_nmi_start(void)
 {
 	int cpu;
 
-	cpus_read_lock();
 	watchdog_calc_timeouts();
 	for_each_cpu_and(cpu, cpu_online_mask, &watchdog_cpumask)
 		start_wd_on_cpu(cpu);
-	cpus_read_unlock();
 }
 
 /*

kernel/smpboot.c

@@ -351,7 +351,7 @@ void smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread
 	static struct cpumask tmp;
 	unsigned int cpu;
 
-	get_online_cpus();
+	lockdep_assert_cpus_held();
 	mutex_lock(&smpboot_threads_lock);
 
 	/* Park threads that were exclusively enabled on the old mask. */
@@ -367,7 +367,6 @@ void smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread
 	cpumask_copy(old, new);
 
 	mutex_unlock(&smpboot_threads_lock);
-	put_online_cpus();
 }
 
 static DEFINE_PER_CPU(atomic_t, cpu_hotplug_state) = ATOMIC_INIT(CPU_POST_DEAD);

kernel/watchdog.c

@@ -535,7 +535,6 @@ static void softlockup_update_smpboot_threads(void)
 
 	smpboot_update_cpumask_percpu_thread(&watchdog_threads,
 					     &watchdog_allowed_mask);
-	__lockup_detector_cleanup();
 }
 
 /* Temporarily park all watchdog threads */
@@ -554,6 +553,7 @@ static void softlockup_unpark_threads(void)
 
 static void softlockup_reconfigure_threads(void)
 {
+	cpus_read_lock();
 	watchdog_nmi_stop();
 	softlockup_park_all_threads();
 	set_sample_period();
@@ -561,6 +561,12 @@ static void softlockup_reconfigure_threads(void)
 	if (watchdog_enabled && watchdog_thresh)
 		softlockup_unpark_threads();
 	watchdog_nmi_start();
+	cpus_read_unlock();
+	/*
+	 * Must be called outside the cpus locked section to prevent
+	 * recursive locking in the perf code.
+	 */
+	__lockup_detector_cleanup();
 }
 
 /*
@@ -605,9 +611,11 @@ static inline void watchdog_disable_all_cpus(void) { }
 static inline void softlockup_init_threads(void) { }
 static void softlockup_reconfigure_threads(void)
 {
+	cpus_read_lock();
 	watchdog_nmi_stop();
 	lockup_detector_update_enable();
 	watchdog_nmi_start();
+	cpus_read_unlock();
 }
 #endif /* !CONFIG_SOFTLOCKUP_DETECTOR */