[PATCH] lockdep: Check if nested lock is actually held

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

It is considered good form to lock the lock you claim to be nested in.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>

---
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index ea9ee45..7175447 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2998,6 +2998,43 @@ EXPORT_SYMBOL_GPL(lockdep_init_map);
 
 struct lock_class_key __lockdep_no_validate__;
 
+static int
+print_lock_nested_lock_not_held(struct task_struct *curr,
+				struct held_lock *lock,
+				struct lockdep_map *nest,
+				unsigned long ip)
+{
+	if (!debug_locks_off())
+		return 0;
+	if (debug_locks_silent)
+		return 0;
+
+	printk("\n");
+	printk("==================================\n");
+	printk("[ BUG: Nested lock was not taken ]\n");
+	print_kernel_ident();
+	printk("----------------------------------\n");
+
+	printk("%s/%d is trying to lock:\n", curr->comm, task_pid_nr(curr));
+	print_lock(lock);
+
+	printk("\nbut this task is not holding:\n");
+	printk("%s\n", nest->name);
+
+	printk("\nstack backtrace:\n");
+	dump_stack();
+
+	printk("\nother info that might help us debug this:\n");
+	lockdep_print_held_locks(curr);
+
+	printk("\nstack backtrace:\n");
+	dump_stack();
+
+	return 0;
+}
+
+static int __lock_is_held(struct lockdep_map *lock);
+
 /*
  * This gets called for every mutex_lock*()/spin_lock*() operation.
  * We maintain the dependency maps and validate the locking attempt:
@@ -3139,6 +3176,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	}
 	chain_key = iterate_chain_key(chain_key, id);
 
+	if (nest_lock && !__lock_is_held(nest_lock))
+		return print_lock_nested_lock_not_held(curr, hlock,
+						       nest_lock, ip);
+
 	if (!validate_chain(curr, lock, hlock, chain_head, chain_key))
 		return 0;