linux-kernel - Re: [PATCH] include/log2.h: Fix rounddown_pow_of

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Message-ID: <20111213060131.GA3575@dtor-ws.eng.vmware.com>
Date:	Mon, 12 Dec 2011 22:01:31 -0800
From:	Dmitry Torokhov <dtor@...are.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	pv-drivers@...are.com, Andrei Warkentin <andreiw@...are.com>,
	stable@...nel.org, Jesper Juhl <jj@...osbits.net>,
	Rolf Eike Beer <eike-kernel@...tec.de>
Subject: Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

On Mon, Dec 12, 2011 at 03:50:11PM -0800, Linus Torvalds wrote:
> On Mon, Dec 12, 2011 at 10:02 AM, Dmitry Torokhov <dtor@...are.com> wrote:
> > From: Andrei Warkentin <andreiw@...are.com>
> >
> > 1 is a power of two, therefore rounddown_pow_of_two(1) should return 1.
> > It does in case the argument is a variable but in case it's a constant
> > it behaves incorrectly and returns 0. Probably nobody ever did it so
> > this was never noticed, however drivers/net/vmxnet3 with latest GCC does
> > and breaks on unicpu systems.
> >
> > This is similar to Rolf's patch to roundup_pow_of_two(1).
> 
> Umm. I already applied this patch, but then I started looking at it
> more, and asked myself:
> 
>  - Why is that "n == 1" test there AT ALL?
> 
> Afaik, that whole test is just plain stupid. It seems to have been
> copied from the "roundup()" case (where it exists due to the "-1/+1"
> hackery that breaks ilog2()) without any thought about the actual math
> of the function at all.
> 
> I think the *real* fix is to just remove that incorrect line, no?

Yes, you are right, special-casing for 1 is not necessary in rounddown
case.

Thanks,
Dmitry

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