From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 732D0C282C8 for ; Mon, 28 Jan 2019 17:35:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3F9DF2147A for ; Mon, 28 Jan 2019 17:35:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730147AbfA1RfO (ORCPT ); Mon, 28 Jan 2019 12:35:14 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40712 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729228AbfA1RfM (ORCPT ); Mon, 28 Jan 2019 12:35:12 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 272E0A4287; Mon, 28 Jan 2019 17:35:12 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.163]) by smtp.corp.redhat.com (Postfix) with SMTP id EBE0116BF6; Mon, 28 Jan 2019 17:35:10 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 28 Jan 2019 18:35:10 +0100 (CET) Date: Mon, 28 Jan 2019 18:35:09 +0100 From: Oleg Nesterov To: Stanislaw Gruszka Cc: linux-kernel@vger.kernel.org, Andrew Morton , Brian Behlendorf , Siarhei Volkau Subject: Re: [PATCH] lib/div64: off by one in shift Message-ID: <20190128173508.GD22143@redhat.com> References: <1548686944-11891-1-git-send-email-sgruszka@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1548686944-11891-1-git-send-email-sgruszka@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Mon, 28 Jan 2019 17:35:12 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/28, Stanislaw Gruszka wrote: > > fls counts bits starting from 1 to 32 (returns 0 for zero argument). > If we add 1 we shift right one bit more and loose precision I forgot everything about this code, but I think this patch must be correct, divisor >> n; should have MSB == 1 or we loose the precision... Heh, I managed to find the initial version of this code, see https://lore.kernel.org/lkml/20101014121159.GA407@redhat.com/ and note that it uses __fls(), not fls()! I didn't notice the final version replaced __fls() with fls() which is __fls() + 1 if arg != 0. Acked-by: Oleg Nesterov