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

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Date:   Wed, 25 Aug 2021 04:53:32 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Li RongQing <lirongqing@...du.com>, dbueso@...e.de,
        mingo@...nel.org, michel@...pinasse.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rbtree: stop iteration early in rb_find_first

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.

--
Michel "walken" Lespinasse