From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756295AbYCKLqs (ORCPT ); Tue, 11 Mar 2008 07:46:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753143AbYCKLql (ORCPT ); Tue, 11 Mar 2008 07:46:41 -0400 Received: from one.firstfloor.org ([213.235.205.2]:42040 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750992AbYCKLqk (ORCPT ); Tue, 11 Mar 2008 07:46:40 -0400 Date: Tue, 11 Mar 2008 12:48:32 +0100 From: Andi Kleen To: Thomas Gleixner Cc: Andi Kleen , Ingo Molnar , linux-kernel@vger.kernel.org, akpm@osdl.org Subject: Re: [PATCH REPOST for 2.6.25] Use an own random generator for pageattr-test.c Message-ID: <20080311114832.GE18917@one.firstfloor.org> References: <20080311013014.GA28682@basil.nowhere.org> <20080311104531.GB18917@one.firstfloor.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 11, 2008 at 12:41:13PM +0100, Thomas Gleixner wrote: > > > On Tue, 11 Mar 2008, Andi Kleen wrote: > > > On Tue, Mar 11, 2008 at 09:25:21AM +0100, Thomas Gleixner wrote: > > > On Tue, 11 Mar 2008, Andi Kleen wrote: > > > > Use an own random generator for pageattr-test.c > > > > > > > > [Repost. Please ack/nack. This is a bug fix and imho a .25 late merge > > > > candidate because it fixes a subtle bug] > > > > > > Care to point out which "subtle bug" is fixed ? > > > > > > You replace a random generator by another to get repeateable > > > sequences. The non repeatability of the cpa test patterns is hardly a > > > "subtle bug". > > > > The subtle bug(s) are first that it is not repeatable (it really should), > > As I said before. It's hardly a bug. In fact it is questionable > whether fully reproducible test patterns are desired. Ok then you won't be able to repeat the test ever. I consider this bad practice in test code because it makes it impossible to stabilize bugs and when I wrote it I tried to avoid by using the srandom32(). But I originally fell into the trap of assuming it had the same semantics of stdlib srandom() which it didn't. This patch was my attempt to fix that mistake. > > > then that it only initializes the CPU where the code first runs > > (since srandom32 is per CPU) and later might change CPUs and then that it > > adds totally unnecessary state bits to CPU #0 (or whatever runs first). > > Can you please elaborate why changing the seed of the random generator > is a bug ? Networking reseeds the random generator itself, so what ? It adds a non random seed which does not add any randomness only to CPU #0. Strictly it doesn't hurt very much, but it's also not useful for anything. -Andi