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=-4.7 required=3.0 tests=DATE_IN_PAST_96_XX, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=no 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 6B83FC2D0E5 for ; Sat, 28 Mar 2020 17:04:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4C60D20714 for ; Sat, 28 Mar 2020 17:04:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727118AbgC1REl (ORCPT ); Sat, 28 Mar 2020 13:04:41 -0400 Received: from mx.sdf.org ([205.166.94.20]:63281 "EHLO mx.sdf.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725807AbgC1REk (ORCPT ); Sat, 28 Mar 2020 13:04:40 -0400 Received: from sdf.org (IDENT:lkml@sdf.lonestar.org [205.166.94.16]) by mx.sdf.org (8.15.2/8.14.5) with ESMTPS id 02SH47cb023288 (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256 bits) verified NO); Sat, 28 Mar 2020 17:04:08 GMT Received: (from lkml@localhost) by sdf.org (8.15.2/8.12.8/Submit) id 02SH4767016334; Sat, 28 Mar 2020 17:04:07 GMT Message-Id: <202003281704.02SH4767016334@sdf.org> From: George Spelvin Date: Wed, 21 Aug 2019 20:30:18 -0400 Subject: [RFC PATCH v1 18/50] net/ipv6/addrconf.c: Use prandom_u32_max for rfc3315 backoff time computation MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit To: linux-kernel@vger.kernel.org, lkml@sdf.org Cc: Maciej Zenczykowski , "David S. Miller" , Alexey Kuznetsov , Hideaki YOSHIFUJI , netdev@vger.kernel.org Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org There's no need for 64-bit intermediate values and do_div. (Actually, the algorithm isn't changing much, except that the old code used a scaling factor of 1 million. prandom_u32_max uses a factor of 2^32, making the final division more efficient.) One thing that concerns me a bit is that the data types are all signed. The old code cast the inputs to unsigned and produced strange overflowed results if they were negative, so presumably that never happens in practice. The new code works the same for positive inputs, but produces different strange overflowed results if fed negative inputs. Signed-off-by: George Spelvin Cc: Maciej Żenczykowski Cc: "David S. Miller" Cc: Alexey Kuznetsov Cc: Hideaki YOSHIFUJI Cc: netdev@vger.kernel.org --- net/ipv6/addrconf.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-) diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c index ec3f472bc5a8f..5172f1f874363 100644 --- a/net/ipv6/addrconf.c +++ b/net/ipv6/addrconf.c @@ -103,20 +103,19 @@ static inline u32 cstamp_delta(unsigned long cstamp) static inline s32 rfc3315_s14_backoff_init(s32 irt) { /* multiply 'initial retransmission time' by 0.9 .. 1.1 */ - u64 tmp = (900000 + prandom_u32() % 200001) * (u64)irt; - do_div(tmp, 1000000); - return (s32)tmp; + s32 range = irt / 5; + return irt - (s32)(range/2) + (s32)prandom_u32_max(range); } static inline s32 rfc3315_s14_backoff_update(s32 rt, s32 mrt) { /* multiply 'retransmission timeout' by 1.9 .. 2.1 */ - u64 tmp = (1900000 + prandom_u32() % 200001) * (u64)rt; - do_div(tmp, 1000000); - if ((s32)tmp > mrt) { + s32 range = rt / 5; + s32 tmp = 2*rt - (s32)(range/2) + (s32)prandom_u32_max(range); + if (tmp > mrt) { /* multiply 'maximum retransmission time' by 0.9 .. 1.1 */ - tmp = (900000 + prandom_u32() % 200001) * (u64)mrt; - do_div(tmp, 1000000); + range = mrt / 5; + tmp = mrt - (s32)(range/2) + (s32)prandom_u32_max(range); } return (s32)tmp; } -- 2.26.0