linux-kernel - Re: [PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH

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Date:   Sun, 10 Mar 2019 22:54:41 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     George Spelvin <lkml@....org>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Abramov <st5pub@...dex.ru>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Daniel Wagner <daniel.wagner@...mens.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Don Mullis <don.mullis@...il.com>,
        Dave Chinner <dchinner@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH 4/5] lib/list_sort: Simplify and remove
 MAX_LIST_LENGTH_BITS

On 05/03/2019 04.06, George Spelvin wrote:

>   * The comparison function @cmp must return a negative value if @a
>   * should sort before @b, and a positive value if @a should sort after
>   * @b. If @a and @b are equivalent, and their original relative
>   * ordering is to be preserved, @cmp must return 0.
> + *
> + * (Actually, it is always called with @a being the element which was
> + * originally first, so it is not necessary to to distinguish the @a < @b
> + * and @a == @b cases; the return value may be a simple boolean.  But if
> + * you ever *use* this freedom, be sure to update this comment to document
> + * that code now depends on preserving this property!)
>   */

This was and still is used at least by the block layer, and likely
others as well. While 3110fc79606fb introduced a bunch of if() return -1
else if () ... stuff, it still ends with a 0/1 result. Before
3110fc79606fb, it was even more obvious that this property was used. So
I agree that it is worth documenting this feature, both for users of
list_sort, but even more so for future refactorers of it - but you
probably want to change the wording somewhat.

Grepping around shows that this could probably be used in more places,
gaining a cycle or two per cmp callback, e.g. xfs_buf_cmp. But that's of
course outside the scope of this series.

Rasmus