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From: Peter Zijlstra <peterz@infradead.org>
To: "Li,Rongqing" <lirongqing@baidu.com>
Cc: Michel Lespinasse <michel@lespinasse.org>,
	"dbueso@suse.de" <dbueso@suse.de>,
	"mingo@kernel.org" <mingo@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: 答复: 答复: [PATCH] rbtree: stop iteration early in rb_find_first
Date: Wed, 25 Aug 2021 21:20:56 +0200	[thread overview]
Message-ID: <20210825192056.GF17784@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <000ff2757fc24d91ade7cab0195ab9b4@baidu.com>

On Wed, Aug 25, 2021 at 06:26:59PM +0000, Li,Rongqing wrote:
> 
>                        10
>                      /
>                     5
>                        \
>                       10
> 
> the above case should not exist. like below, when second 10 is inserted, it should be inserted to right leaf
>                        10
>                      /
>                     5
> 
> as a result, it should be
> 
>                        10
>                      /     \
>                     5      10
> 
> since 10 is not less 10, so new 10 is inserted to right.

You're right that rb_add() does a tail-add for elements it considers
equal -- there is actually code in the tree that relies on this.

But that doesn't mean rb_find_first() should go right, it *must*not*,
because then it wouldn't find the 'first' aka 'leftmost' instance of the
equal elements.

Also, you're only considering building the tree in-order with rb_add(),
trees get modified all the time and the pattern Michel gave is perfectly
valid (also see rb_prev()).

Sure the snippet is not a balanced tree, but you can construct the
pattern as part of a larger tree just fine, just add some elements:


	 10(b)
	/  \
       5   10(c)
        \
	10(a)

is a tree that is balanced (remember, RB trees only require the left and
right depths to no more than double -- as opposed to AVL trees, which
have a tighter constraint). This tree has order: 5, 10(a), 10(b), 10(c).
Also note that the tree rotations are stable -- they must be since they
do not refence the order function.

As such, if we rb_find_first() for 10, we must find 10(a), the leftmost
10 in the tree.


  reply	other threads:[~2021-08-25 19:21 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-25  9:59 Li RongQing
2021-08-25 11:39 ` Peter Zijlstra
2021-08-25 11:53   ` Michel Lespinasse
2021-08-25 11:58     ` Michel Lespinasse
2021-08-25 12:31       ` Peter Zijlstra
2021-08-25 13:21       ` Peter Zijlstra
2021-08-25 16:08         ` 答复: " Li,Rongqing
     [not found]         ` <90ea3457ddc7485fbc8db5f7ca5b07ab@baidu.com>
2021-08-25 17:18           ` Peter Zijlstra
2021-08-25 18:26             ` 答复: " Li,Rongqing
2021-08-25 19:20               ` Peter Zijlstra [this message]
2021-08-26  5:03                 ` 答复: " Li,Rongqing
2021-08-25 12:29     ` Peter Zijlstra
2021-08-25 12:32       ` Peter Zijlstra

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