Re: [PATCH] rbtree: undo augmented damage -v2

[Suresh Siddha <suresh.b.siddha@intel.com>]

On Tue, 2010-06-01 at 11:42 -0700, Peter Zijlstra wrote:
> On Tue, 2010-06-01 at 11:09 -0700, Venkatesh Pallipadi wrote:
> > On Tue, Jun 1, 2010 at 10:42 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > > On Tue, 2010-06-01 at 10:34 -0700, Venkatesh Pallipadi wrote:
> > >>
> > >> Yes. This will cover all the cases on insert. But on erase, there is
> > >> still a case where a rotate of sibling node is done during the
> > >> re-coloration process. There we have a child change on sibling's
> > >> child. I am not able to think of any easy way to handle that case.
> > >
> > > Let me go draw some figures with pen and paper to match up the erase
> > > path with the rb_augment_erase_begin() code, because I can't quite spot
> > > the case we're missing.
> > >
> > > If you have it handy, ascii art might help..
> > 
> > It is this case
> > 
> >     P
> >    / \
> >   N   S
> >      / \
> >     SL SR
> > 
> > changing to
> > 
> >     P
> >    / \
> >   N  SL
> >       \
> >        S
> >         \
> >         SR
> 
> Right, but see: http://en.wikipedia.org/wiki/Red-black_tree
> That is delete_case5, however then we fall into delete_case6 and perform
> a left rotation.
> 
> So suppose we start with the tree:
> 
>     P                  P               P                SL
>   /   \               / \             / \               / \
>  D     S      -->    N  S      -->   N   SL     -->    P   S
>   \   / \              / \                \           /     \
>    N SL  SR          SL* SR                S*        N      SR
>                                             \
>                                              SR
> 
> and then remove D, delete case 5 and finally delete case 6, * marks red.
> 
> rb_augment_erase_begin(D) will return N, and then rb_augment_path(N)
> will re-augment: N, P, SL and S.


      P                       SL
    /  \                     /  \
   N    S       --->        N    S
  /   /  \                 /      \
 C   SL  SR               C        SR

If P needs to be removed, we need to re-augment S also in this case,
right? It looks like we are not handling this case.

thanks,
suresh