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Date: Fri, 1 Aug 2008 14:09:49 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Alexey Dobriyan <adobriyan@gmail.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>,
       "Rafael J. Wysocki" <rjw@sisk.pl>,
       Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Adrian Bunk <bunk@kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Natalie Protasevich <protasnb@gmail.com>,
       Kernel Testers List <kernel-testers@vger.kernel.org>
Subject: Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25
Message-ID: <20080801210949.GJ14851@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <4yrA-dRoNlD.A.jcB.ny-cIB@albercik> <200807101725.36175.nickpiggin@yahoo.com.au> <20080710110213.GA6688@martell.zuzino.mipt.ru> <alpine.LFD.1.10.0807101020270.11076@woody.linux-foundation.org> <20080710173459.GA11648@elte.hu> <20080710180620.GA30844@elte.hu>
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On Thu, Jul 10, 2008 at 08:06:20PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> > cool! :)
> > 
> > (hm, could anyone please resend Nick's original mail? The original one 
> > is not in my lkml folder nor on lkml.org - only the quoted one.)
> 
> ok, got the mail now now:
> 
> | | Annoyed this wasn't a crazy obscure error in the algorithm I could 
> | | fix :) [...]
> 
> Paul recently ran a formal proof against all sorts of RCU details (and 
> found and fixed a few obscure races that way that no-one ever 
> triggered), so i'd be quite surprised if we found anything in the core 
> algorithm :-)
> 
> | | [...] I spent all day debugging it and had to make a special test 
> | | case (rcutorture didn't seem to trigger it), and a big RCU state 
> | | logging infrastructure to log millions of RCU state transitions and 
> | | events. Oh well.
> 
> nice debugging!

Indeed!!!

> Acked-by: Ingo Molnar <mingo@elte.hu>
> 
> i'm wondering why rcutorture didnt trigger it. I do run !HOTPLUG + 
> RCU_PREEMPT kernels and never saw this. Nor did Paul. That aspect is 
> weird.

Turns out that my environment was silently re-enabling HOTPLUG_CPU, so I
only -thought- I was testing !CPU_HOTPLUG.  Once I forced it to really
disable HOTPLUG_CPU (by manually also specifying CONFIG_SUSPEND=n and
CONFIG_HIBERNATION=n), then rcutorture complained within 10 seconds.

Sigh!!!

						Thanx, Paul