What is the role of LIST_POISON1 and LIST_POISON2?

[navych@126.com (Navy Cheng)]

On Fri, Mar 04, 2016 at 02:07:26AM -0500, Valdis.Kletnieks at vt.edu wrote:
> On Fri, 04 Mar 2016 13:02:02 +0800, Navy Cheng said:
> > 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 ?
> 
> To more easily detect different classes of list corruption, use-after-free, and
> other programming errors.  If ->next and ->prev are NULL, it may be the result
> of following a bad pointer.  If  they're equal to POISON 1 and 2, you're almost
> certainly looking at a once-valid pointer that is a use-after-free situation.
> It's easy to end up pointing at a zeroed page.  The chances of pointing at
> some random data that happens to be POISON 1/2 is much lower.
> 
> See the code in lib/list_debug.c
> 

Thank you, but I don't quite understand. Could you give an example or tell me
some books and documnets about this?