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    SubjectRe: 2.6.26-git: NULL pointer deref in __switch_to
    Suresh Siddha wrote:
    > Simon, Patrick, I am able to reproduce the oops in __switch_to()
    > with lguest. My debug showed that there is atleast one lguest specific
    > issue (which should be present in 2.6.25 and before aswell) and it got
    > exposed with a kernel oops with the recent fpu dynamic allocation patches.
    > In addition to the previous possible scenario (with fpu_counter), in the
    > presence of lguest, it is possible that the cpu's TS bit it still set and the
    > lguest launcher task's thread_info has TS_USEDFPU still set.
    > This is because of the way the lguest launcher handling the guest's TS bit.
    > (look at lguest_set_ts() in lguest_arch_run_guest()). This can result
    > in a DNA fault while doing unlazy_fpu() in __switch_to(). This will
    > end up causing a DNA fault in the context of new process thats
    > getting context switched in (as opossed to handling DNA fault in the context
    > of lguest launcher/helper process).
    > This is wrong in both pre and post 2.6.25 kernels. In the recent
    > 2.6.26-rc series, this is showing up as NULL pointer dereferences or
    > sleeping function called from atomic context(__switch_to()), as
    > we free and dynamically allocate the FPU context for the newly
    > created threads. Older kernels might show some FPU corruption for processes
    > running inside of lguest.
    > With the appended patch, my test system is running for more than 50 mins
    > now. So atleast some of your oops (hopefully all!) should get fixed.
    > Please give it a try. I will spend more time with this fix tomorrow.
    > Apart from the last hunk(MSR_IA32_SYSENTER_CS changes), I believe
    > the below patch is needed for 2.6.25 aswell.

    The patch seems to have fixed the problem for me, thanks Suresh.

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