watchdog: Fix merge 'conflict'

Two watchdog changes that came through different trees had a non
conflicting conflict, that is, one changed the semantics of a variable
but no actual code conflict happened. So the merge appeared fine, but
the resulting code did not behave as expected.

Commit 195daf66 ("watchdog: enable the new user interface of the
watchdog mechanism") changes the semantics of watchdog_user_enabled,
which thereafter is only used by the functions introduced by
b3738d29 ("watchdog: Add watchdog enable/disable all functions").

There further appears to be a distinct lack of serialization between
setting and using watchdog_enabled, so perhaps we should wrap the
{en,dis}able_all() things in watchdog_proc_mutex.

This patch fixes a s2r failure reported by Michal; which I cannot
readily explain. But this does make the code internally consistent
again.
Reported-and-tested-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/watchdog.c

@@ -41,6 +41,8 @@
 #define NMI_WATCHDOG_ENABLED      (1 << NMI_WATCHDOG_ENABLED_BIT)
 #define SOFT_WATCHDOG_ENABLED     (1 << SOFT_WATCHDOG_ENABLED_BIT)
 
+static DEFINE_MUTEX(watchdog_proc_mutex);
+
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 static unsigned long __read_mostly watchdog_enabled = SOFT_WATCHDOG_ENABLED|NMI_WATCHDOG_ENABLED;
 #else
@@ -608,26 +610,36 @@ void watchdog_nmi_enable_all(void)
 {
 	int cpu;
 
-	if (!watchdog_user_enabled)
-		return;
+	mutex_lock(&watchdog_proc_mutex);
+
+	if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
+		goto unlock;
 
 	get_online_cpus();
 	for_each_online_cpu(cpu)
 		watchdog_nmi_enable(cpu);
 	put_online_cpus();
+
+unlock:
+	mutex_lock(&watchdog_proc_mutex);
 }
 
 void watchdog_nmi_disable_all(void)
 {
 	int cpu;
 
+	mutex_lock(&watchdog_proc_mutex);
+
 	if (!watchdog_running)
-		return;
+		goto unlock;
 
 	get_online_cpus();
 	for_each_online_cpu(cpu)
 		watchdog_nmi_disable(cpu);
 	put_online_cpus();
+
+unlock:
+	mutex_unlock(&watchdog_proc_mutex);
 }
 #else
 static int watchdog_nmi_enable(unsigned int cpu) { return 0; }
@@ -744,8 +756,6 @@ static int proc_watchdog_update(void)
 
 }
 
-static DEFINE_MUTEX(watchdog_proc_mutex);
-
 /*
  * common function for watchdog, nmi_watchdog and soft_watchdog parameter
  *