lockdep debugging: give stacktrace for init_error

When I started adding support for lockdep to 64-bit powerpc, I got a
lockdep_init_error and with this patch was able to pinpoint why and where
to put lockdep_init().  Let's support this generally for others adding
lockdep support to their architecture.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/lockdep.c

@@ -375,6 +375,11 @@ unsigned int max_recursion_depth;
  * about it later on, in lockdep_info().
  */
 static int lockdep_init_error;
+static unsigned long lockdep_init_trace_data[20];
+static struct stack_trace lockdep_init_trace = {
+	.max_entries = ARRAY_SIZE(lockdep_init_trace_data),
+	.entries = lockdep_init_trace_data,
+};
 
 /*
  * Various lockdep statistics:
@@ -662,6 +667,7 @@ look_up_lock_class(struct lockdep_map *lock, unsigned int subclass)
 	if (unlikely(!lockdep_initialized)) {
 		lockdep_init();
 		lockdep_init_error = 1;
+		save_stack_trace(&lockdep_init_trace);
 	}
 #endif
 
@@ -3040,8 +3046,11 @@ void __init lockdep_info(void)
 		sizeof(struct held_lock) * MAX_LOCK_DEPTH);
 
 #ifdef CONFIG_DEBUG_LOCKDEP
-	if (lockdep_init_error)
-		printk("WARNING: lockdep init error! Arch code didnt call lockdep_init() early enough?\n");
+	if (lockdep_init_error) {
+		printk("WARNING: lockdep init error! Arch code didn't call lockdep_init() early enough?\n");
+		printk("Call stack leading to lockdep invocation was:\n");
+		print_stack_trace(&lockdep_init_trace, 0);
+	}
 #endif
 }