linux-kernel - [PATCH] Make div64_u64() precise on 32bit platforms

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Message-Id: <201008090930.56671.behlendorf1@llnl.gov>
Date:	Mon, 9 Aug 2010 09:30:52 -0700
From:	Brian Behlendorf <behlendorf1@...l.gov>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ben Woodard <bwoodard@...l.gov>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mark Grondona <mgrondona@...l.gov>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH] Make div64_u64() precise on 32bit platforms


> On Mon, 2 Aug 2010 18:09:51 +0200
>
> Oleg Nesterov <oleg@...hat.com> wrote:
> > We have a bugreport which blames div64_u64() on 32bit platforms.
> >
> > However, the code obviously doesn't even try to pretend it can do
> > the 64bit division precisely. If there is something in the high
> > word of divisor, div64_u64() just shifts both arguments and throws
> > out the low bits.
>
> Well that was a bit lazy of us - I wonder how hard it is to fix.
>
> At present people will test their code on 64-bit only to find out later
> that it doesn't work correctly on 32-bit.  Bad.  Perhaps we should
> similarly break the 64-bit version :)

Here's an even crazier idea, let's just fix the 32-bit version.  :)

The attached patch fully implements div64_u64() such that it will return 
precisely the right quotient even when the divisor exceeds 32-bits.  The 
patch also adds a div64_s64() function to fully support signed 64-bit 
division.

Because this fix is non-obvious I have also included a unsigned and signed 
regression test to verify the correctness of the patch.  Using a vanilla 
2.6.35 kernel the unsigned regression tests fails on 32-bit platforms.  With 
the proposed patch applied both the unsigned and signed tests pass.

-- 
Thanks,
Brian

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View attachment "0001-Fix-div64_u64-for-32bit-platforms.patch" of type "text/plain" (3927 bytes)

View attachment "div64_u64_test.c" of type "text/x-csrc" (1555 bytes)

View attachment "div64_s64_test.c" of type "text/x-csrc" (1533 bytes)

View attachment "README" of type "text/plain" (2085 bytes)