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

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Message-ID: <AANLkTin5ruXABD7lGs_7QcTdge1zwjU6tkbhp4IcM59S@mail.gmail.com>
Date:	Tue, 1 Jun 2010 13:31:30 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	suresh.b.siddha@...el.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org,
	Fabio Checconi <fabio@...dalf.sssup.it>
Subject: Re: [PATCH] rbtree: undo augmented damage -v2

On Tue, Jun 1, 2010 at 11:42 AM, Peter Zijlstra <peterz@...radead.org> 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@...radead.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.
>

Yes. I had missed that rotate_right of parent following the
rotate_left of sibling (or vice-versa). Thanks for fixing this. The
latest patch looks good.

Acked-by: Venkatesh Pallipadi <venki@...gle.com>
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