linux-kernel - Re: [PATCH REPOST for 2.6.25] Use an own random generator for pageattr-test.c

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Message-ID: <20080311114832.GE18917@one.firstfloor.org>
Date:	Tue, 11 Mar 2008 12:48:32 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, akpm@...l.org
Subject: Re: [PATCH REPOST for 2.6.25] Use an own random generator for pageattr-test.c

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

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