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.