watchdog/hardlockup: Clean up hotplug locking mess

All watchdog thread related functions are delegated to the smpboot thread
infrastructure, which handles serialization against CPU hotplug correctly.

The sysctl interface is completely decoupled from anything which requires
CPU hotplug protection.

No need to protect the sysctl writes against cpu hotplug anymore. Remove it
and add the now required protection to the powerpc arch_nmi_watchdog
implementation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Don Zickus <dzickus@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/20170912194148.418497420@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/powerpc/kernel/watchdog.c

@@ -359,6 +359,7 @@ void watchdog_nmi_reconfigure(bool run)
 {
 	int cpu;
 
+	cpus_read_lock();
 	if (!run) {
 		for_each_cpu(cpu, &wd_cpus_enabled)
 			stop_wd_on_cpu(cpu);
@@ -367,6 +368,7 @@ void watchdog_nmi_reconfigure(bool run)
 		for_each_cpu_and(cpu, cpu_online_mask, &watchdog_cpumask)
 			start_wd_on_cpu(cpu);
 	}
+	cpus_read_unlock();
 }
 
 /*

kernel/watchdog.c

@@ -664,7 +664,6 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 {
 	int err, old, *param = table->data;
 
-	cpu_hotplug_disable();
 	mutex_lock(&watchdog_mutex);
 
 	if (!write) {
@@ -681,7 +680,6 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 			proc_watchdog_update();
 	}
 	mutex_unlock(&watchdog_mutex);
-	cpu_hotplug_enable();
 	return err;
 }
 
@@ -725,7 +723,6 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 {
 	int err, old;
 
-	cpu_hotplug_disable();
 	mutex_lock(&watchdog_mutex);
 
 	old = READ_ONCE(watchdog_thresh);
@@ -735,7 +732,6 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 		proc_watchdog_update();
 
 	mutex_unlock(&watchdog_mutex);
-	cpu_hotplug_enable();
 	return err;
 }
 
@@ -750,7 +746,6 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
 {
 	int err;
 
-	cpu_hotplug_disable();
 	mutex_lock(&watchdog_mutex);
 
 	err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
@@ -758,7 +753,6 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
 		proc_watchdog_update();
 
 	mutex_unlock(&watchdog_mutex);
-	cpu_hotplug_enable();
 	return err;
 }
 #endif /* CONFIG_SYSCTL */