what is spinlock depth

Hi all, I am reading spinlock code and looking at struct raw_spinlock. I found struct lockdep_map as member. which is defined as struct lockdep_map { struct lock_class_key *key; struct lock_class *class_cache[NR_LOCKDEP_CACHING_CLASSES]; const char *name; #ifdef CONFIG_LOCK_STAT int cpu; unsigned long ip; #endif }; Can anyone help me explain, how this struct helps the spinlock. Why do we need lockdepth. Thanks for help in advance. -- Regards, Sri.