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

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Date:	Sun, 1 Jun 2008 18:47:29 +0200
From:	Jürgen Mell <j.mell@...nline.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	suresh.b.siddha@...el.com, arjan@...ux.intel.com
Subject: Re: CONFIG_PREEMPT causes corruption of application's FPU stack

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...

Bye, 
         Jürgen
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