From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753752Ab1LLXue (ORCPT ); Mon, 12 Dec 2011 18:50:34 -0500 Received: from mail-bw0-f46.google.com ([209.85.214.46]:51385 "EHLO mail-bw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753391Ab1LLXuc (ORCPT ); Mon, 12 Dec 2011 18:50:32 -0500 MIME-Version: 1.0 In-Reply-To: <1323712953-13636-1-git-send-email-dtor@vmware.com> References: <1323712953-13636-1-git-send-email-dtor@vmware.com> From: Linus Torvalds Date: Mon, 12 Dec 2011 15:50:11 -0800 X-Google-Sender-Auth: axij8WEJHTsO3AFdRjO9Iz6J3lg Message-ID: Subject: Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1) To: Dmitry Torokhov Cc: linux-kernel@vger.kernel.org, Andrew Morton , pv-drivers@vmware.com, Andrei Warkentin , stable@kernel.org, Jesper Juhl , Rolf Eike Beer Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 12, 2011 at 10:02 AM, Dmitry Torokhov wrote: > From: Andrei Warkentin > > 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? It's a bit sad that we apparently have several reviewers for this trivial patch, and nobody reacted to the math just not making any sense. Linus