lockdep: fix potential NULL pointer dereferences

Hello,

In general we can get the symbol name of a particular lock and print it out if something went wrong regarding lock/unlock stuff.

However, I observed the following info(3.5.0-rc4) when running quotacheck(1) yesterday. It has occurred only once, I can not reproduced it afterwards.

[15800.938071] quotacheck/11934 is trying to release lock (
[15800.940040] BUG: unable to handle kernel NULL pointer dereference at 0000007c
[15800.940040] IP: [<c10f9b2c>] print_lockdep_cache+0x15/0x86
[15800.940040] *pde = 00000000
[15800.940040] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC

Maybe something wrong at other areas, but I also noticed there is no defense for __get_key_name() failure at both __print_lock_name() and print_lockdep_cache().

How about to trigger BUG_ON() to indicate it a bit more explicit once such situation is detected?

Thanks,
-Jeff

---
 kernel/lockdep.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

index ea9ee45..dfbcd86 100644
@@ -508,6 +508,7 @@ static void __print_lock_name(struct lock_class *class)
 	name = class->name;
 	if (!name) {
 		name = __get_key_name(class->key, str);
+		BUG_ON(!name);
 		printk("%s", name);
 	} else {
 		printk("%s", name);

@@ -538,6 +539,7 @@ static void print_lockdep_cache(struct lockdep_map *lock)
 	if (!name)
 		name = __get_key_name(lock->key->subkeys, str);
 
+	BUG_ON(!name);
 	printk("%s", name);
 }
 

1.7.9