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

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Date:   Sun, 10 Mar 2019 22:29:24 GMT
From:   George Spelvin <lkml@....org>
To:     linux-kernel@...r.kernel.org, linux@...musvillemoes.dk,
        lkml@....org
Cc:     akpm@...ux-foundation.org, andriy.shevchenko@...ux.intel.com,
        daniel.wagner@...mens.com, dchinner@...hat.com,
        don.mullis@...il.com, geert@...ux-m68k.org, st5pub@...dex.ru
Subject: Re: [PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH_BITS

Rasmus Villemoes wrote:
> On 05/03/2019 04.06, George Spelvin wrote:
>> + * (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.

Ah, thank you!  I actually read through every list_sort caller in
the kernel to see if I could find anywhere that used it and couldn't,
but I didn't study this code carefully enough to see that it does
in the last step.

Since someone *does* use this, I'll change the comment signiicantly.

> 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.

The one that misled me at first was _xfs_buf_obj_cmp, which returns 0/1,
but that's not used by list_sort().  xfs_buf_cmp returns -1/0/+1.

As you might see from the comment around the cmp_func typedef,
there are other things that could be cleaned up if we did a pass
over all the call sites.

(I'm almost tempted to tell the compiler than cmp_func is const,
since it's supposed to be independent of the pointer frobbing that
list_sort does, but then I remember Henry Spencer's maxim about
lying to the compiler.)