What is the role of LIST_POISON1 and LIST_POISON2?

[Navy Cheng]

Hi,

When I read the code of list_del(), I find LIST_POISON1 and LIST_POISON2:

    static inline void list_del(struct list_head *entry)
    {
	__list_del(entry->prev, entry->next);
	entry->next = LIST_POISON1;
	entry->prev = LIST_POISON2;
    }

Why not set entry->next and entry->prev to NULL ?

Thank you!