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

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Message-ID: <AANLkTim9MqwWx4v51joJ1m_wjfn_bggEFPsjxsR6TBgY@mail.gmail.com>
Date:	Wed, 2 Jun 2010 18:15:59 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	Fabio Checconi <fabio@...dalf.sssup.it>
Subject: Re: [PATCH] rbtree: undo augmented damage -v2

On Wed, Jun 2, 2010 at 2:11 PM, Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> 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@...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.
>
>
>      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.
>

rb_augment_erase_begin() should take care of that. In this case, it
will return S as the deepest node and we start the walk-back-to-root
from there.

Thanks,
Venki
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