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    SubjectRe: 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@t-online.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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