linux-kernel - Re: list sort

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Message-ID: <1311779059.2801.27.camel@menhir>
Date:	Wed, 27 Jul 2011 16:04:19 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>,
	Don Mullis <don.mullis@...il.com>,
	Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org
Subject: Re: list sort

Hi,

On Wed, 2011-07-27 at 16:45 +0200, Clemens Ladisch wrote:
> Steven Whitehouse wrote:
> > There seems to be an issue with list_sort trying to compare an object to
> > itself. [...]
> > While it appears generally harmless, since the list does appear to be
> > correctly sorted, it is doing extra work needlessly.
> 
> There are sort algorithms where this happens naturally, and adding
> an additional check to avoid calling the comparison function when the
> pointers are equal would be more work overall than just calling cmp().
> 
> This does not happen with list_sort()'s merge sort, but I've noticed
> the following hack in merge_and_restore_back_links():
> 
> 	do {
> 		/*
> 		 * In worst cases this loop may run many iterations.
> 		 * Continue callbacks to the client even though no
> 		 * element comparison is needed, so the client's cmp()
> 		 * routine can invoke cond_resched() periodically.
> 		 */
> 		(*cmp)(priv, tail->next, tail->next);
> 
> 		tail->next->prev = tail;
> 		tail = tail->next;
> 	} while (tail->next);
> 
> When there is no cond_resched() call, this is indeed needless work.
> 
> However, if the list is so short that cond_resched() is not needed,
> the additional comparisons should not hurt anyway.
> 
> 
> Regards,
> Clemens

Ah, I see now... that makes some kind of sense I think. I didn't expect
the cmp routine to be dual purpose in that way,

Steve.


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