The watchdog proc interface causes extensive recursive locking of the cpu
hotplug percpu rwsem, which is deadlock prone.

Replace the get/put_online_cpus() pairs with cpu_hotplug_disable()/enable()
calls for now. Later patches will remove that requirement completely.

Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20170831073053.437818037@linutronix.de

---
 kernel/watchdog.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -703,7 +703,7 @@ static int proc_watchdog_common(int whic
 	int err, old, new;
 	int *watchdog_param = (int *)table->data;
 
-	get_online_cpus();
+	cpu_hotplug_disable();
 	mutex_lock(&watchdog_proc_mutex);
 
 	/*
@@ -752,7 +752,7 @@ static int proc_watchdog_common(int whic
 	}
 out:
 	mutex_unlock(&watchdog_proc_mutex);
-	put_online_cpus();
+	cpu_hotplug_enable();
 	return err;
 }
 
@@ -794,7 +794,7 @@ int proc_watchdog_thresh(struct ctl_tabl
 {
 	int err, old, new;
 
-	get_online_cpus();
+	cpu_hotplug_disable();
 	mutex_lock(&watchdog_proc_mutex);
 
 	old = ACCESS_ONCE(watchdog_thresh);
@@ -818,7 +818,7 @@ int proc_watchdog_thresh(struct ctl_tabl
 	}
 out:
 	mutex_unlock(&watchdog_proc_mutex);
-	put_online_cpus();
+	cpu_hotplug_enable();
 	return err;
 }
 
@@ -833,7 +833,7 @@ int proc_watchdog_cpumask(struct ctl_tab
 {
 	int err;
 
-	get_online_cpus();
+	cpu_hotplug_disable();
 	mutex_lock(&watchdog_proc_mutex);
 
 	err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
@@ -856,7 +856,7 @@ int proc_watchdog_cpumask(struct ctl_tab
 	}
 
 	mutex_unlock(&watchdog_proc_mutex);
-	put_online_cpus();
+	cpu_hotplug_enable();
 	return err;
 }