jump label: Add a WARN() if jump label key count goes negative

The count on a jump label key should never go negative. Add a
WARN() to check for this condition.

Signed-off-by: Jason Baron <jbaron@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: davem@davemloft.net
Cc: ddaney.cavm@gmail.com
Cc: a.p.zijlstra@chello.nl
Link: http://lkml.kernel.org/r/3c68556121be4d1920417a3fe367da1ec38246b4.1329851692.git.jbaron@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/jump_label.c

@@ -76,8 +76,11 @@ EXPORT_SYMBOL_GPL(jump_label_inc);
 static void __jump_label_dec(struct jump_label_key *key,
 		unsigned long rate_limit, struct delayed_work *work)
 {
-	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex))
+	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
+		WARN(atomic_read(&key->enabled) < 0,
+		     "jump label: negative count!\n");
 		return;
+	}
 
 	if (rate_limit) {
 		atomic_inc(&key->enabled);