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

[Paul E. McKenney]

On Thu, Jul 10, 2008 at 08:06:20PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@xxxxxxx> 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@xxxxxxx>
> 
> 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
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