linux-kernel - Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25

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Date:	Fri, 1 Aug 2008 14:09:53 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25

On Thu, Jul 10, 2008 at 01:41:57PM -0700, Paul E. McKenney wrote:
> On Thu, Jul 10, 2008 at 08:06:20PM +0200, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@...e.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 :-)

Yeah, it was instead the simple stuff that I messed up...  :-/

> > | | [...] 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@...e.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!!!

And Nick's patch gets rid of the rcutorture failures for me as well,
now that I can reproduce them.  ;-)

							Thanx, Paul
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