[PATCH] lockdep debugging: give stacktrace for init_error

[Johannes Berg <johannes@sipsolutions.net>]

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>

---
 kernel/lockdep.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- linux-2.6-git.orig/kernel/lockdep.c	2007-07-05 15:48:17.390241118 +0200
+++ linux-2.6-git/kernel/lockdep.c	2007-07-05 15:58:20.334168396 +0200
@@ -291,6 +291,11 @@ 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:
@@ -1170,6 +1175,7 @@ look_up_lock_class(struct lockdep_map *l
 	if (unlikely(!lockdep_initialized)) {
 		lockdep_init();
 		lockdep_init_error = 1;
+		save_stack_trace(&lockdep_init_trace);
 	}
 #endif
 
@@ -2636,8 +2642,11 @@ 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
 }