linux-kernel - Re: [PATCH] rbtree: stop iteration early in rb_find

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Date:   Wed, 25 Aug 2021 04:58:59 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Michel Lespinasse <michel@...pinasse.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Li RongQing <lirongqing@...du.com>, dbueso@...e.de,
        mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rbtree: stop iteration early in rb_find_first

On Wed, Aug 25, 2021 at 04:53:32AM -0700, Michel Lespinasse wrote:
> On Wed, Aug 25, 2021 at 01:39:26PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 25, 2021 at 05:59:48PM +0800, Li RongQing wrote:
> > > stop iteration if match is not NULL and result of cmp is
> > > not zero, this means the matched node has been found, and
> > > the node with same key has been passed
> > > 
> > > Signed-off-by: Li RongQing <lirongqing@...du.com>
> > > ---
> > >  include/linux/rbtree.h | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h
> > > index d31ecaf4fdd3..2689771df9bb 100644
> > > --- a/include/linux/rbtree.h
> > > +++ b/include/linux/rbtree.h
> > > @@ -324,6 +324,9 @@ rb_find_first(const void *key, const struct rb_root *tree,
> > >  		} else if (c > 0) {
> > >  			node = node->rb_right;
> > >  		}
> > > +
> > > +		if (match && c)
> > > +			break;
> > >  	}
> > >  
> > >  	return match;
> > 
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> NAK. This looked slightly wrong before, and is more wrong after.
> 
> Before:
> there was this weird condition  if (c <= 0) {} else if (c > 0) {} ,
> making you wonder what the third possibility may be. Easy fix would be
> to remove the second condition.
> 
> After:
> say the key is equal the root, so the code sets match=root and goes left.
> Then it stops searching because match is set and c<0.
> This doesn't work, the code needs to keep searching for the leftmost match.

Actually, my explanation is wrong too :) but so is the code.
A failing example would searching 10 in the following tree


                        10
		       /
		      5
		       \
		        10

The search would stop after visiting node 5, and miss the leaf which
is the expected node to be returned.