linux-kernel - Re: CONFIG_PREEMPT causes corruption of application's FPU stack

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Date:	Mon, 2 Jun 2008 14:37:56 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Jürgen Mell <j.mell@...nline.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, suresh.b.siddha@...el.com,
	arjan@...ux.intel.com, mingo@...e.hu, hpa@...or.com,
	tglx@...utronix.de
Subject: Re: CONFIG_PREEMPT causes corruption of application's FPU stack

On Sun, Jun 01, 2008 at 06:47:29PM +0200, Jürgen Mell wrote:
> On Sonntag, 1. Juni 2008, Andi Kleen wrote:
> > j.mell@...nline.de writes:
> > > or it is restored more than
> > > once. Please keep in mind, that I am always running two Einstein
> > > processes simultaneously on my two cores!
> > > I am willing to do further testing of this problem if someone can give
> > > me a hint how to continue.
> >
> > My bet would have been actually on
> > aa283f49276e7d840a40fb01eee6de97eaa7e012 because it does some nasty
> > things (enable interrupts in the middle of __switch_to).
> >
> > I looked through the old patchkit and couldn't find any specific
> > PREEMPT problems. All code it changes should run with preempt_off
> >
> > You could verify with sticking WARN_ON_ONCE(preemptible()) into
> > all the places acc207616a91a413a50fdd8847a747c4a7324167
> > changes (__unlazy_fpu, math_state_restore) and see if that triggers
> > anywhere.
> 
> No, that did not trigger. I put the WARN_ON_ONCE into process.c, traps.c 
> and also into the __unlazy_fpu macro in i387.h but I got no messages 
> anywhere (dmesg, /var/log/messages, /var/log/warn) when the trap #8 
> occurred. 
> Meanwhile I am also running the tests on another machine to make sure it is 
> not a hardware-related problem.
> 
> Any new ideas are welcome!
> 
> Meanwhile I will go back to 2.6.20 and revert
> aa283f49276e7d840a40fb01eee6de97eaa7e012. Maybe I got on a wrong track...

2.6.20 doesn't have the commit 'aa283f49276e7d840a40fb01eee6de97eaa7e012'

As you are seeing this corruption problem starting from 2.6.20,
atleast recent(in 2.6.26 series) fpu changes don't play a role in this.

I will try to reproduce your issue.

thanks,
suresh
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