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    SubjectCONFIG_PREEMPT causes corruption of application's FPU stack
    Date
    I am running the Einstein@home application (version 4.35, 
    http://einstein.phys.uwm.edu).This application does lots of computations
    mostly with FPU and SSE instructions.
    After I started experimenting with real-time optimized kernels the
    application began to crash with floating point errors like in the
    following message:

    APP DEBUG: Application caught signal 8.

    FPU status word ffffa0e1, flags: ERR_SUMM STACK_FAULT PRECISION INVALID
    Obtained 6 stack frames for this thread.
    Use gdb command: 'info line *0xADDRESS' to print corresponding line
    numbers.
    einstein_S5R3_4.35_i686-pc-linux-gnu[0x8069e7e]
    einstein_S5R3_4.35_i686-pc-linux-gnu[0x818d436]
    einstein_S5R3_4.35_i686-pc-linux-gnu[0x805db8f]
    einstein_S5R3_4.35_i686-pc-linux-gnu[0x806b11c]
    /lib/libc.so.6(__libc_start_main+0xe0)[0xb7e14fe0]
    einstein_S5R3_4.35_i686-pc-linux-gnu(shmat+0x59)[0x804bda1]
    Stack trace of LAL functions in worker thread:
    GetSemiCohToplist at line 3177 of
    file /home/bema/einsteinathome/HierarchicalSearch/EaH_build_release_einstein_S5R3_4.35/extra_sources/lalapps-CVS/src/pulsar/hough/src2/HierarchicalSearch.c
    At lowest level status code = 0, description: NO LAL ERROR REGISTERED
    called boinc_finish

    I tracked this down to a single kernel configuration option. If
    CONFIG_PREEMPT is set to 'y' the application will start crashing. If
    CONFIG_PREEMPT is replaced by CONFIG_PREEMPT_VOLUNTARY, the application
    will run without errors.

    The problem is reproducible in so far as the error always occurs when
    CONFIG_PREEMPT is set, but the time to the first occurrence varies greatly
    from some minutes up to more than 10 CPU hours.

    I found this error first on an openSUSE kernel 2.6.22.17-0.1-rt. I verified
    the problem on the following kernel versions:

    openSUSE 2.6.22.17-0.1-default
    openSUSE 2.6.23.17-ccj64-rt
    kernel.org 2.6.26-rc1
    kernel.org 2.6.26-rc2-git5

    My CPU is an Intel Core2Duo 6420, running two of the Einstein applications
    in 32-bit mode. From a discussion on the Einstein message boards I know
    that other user of the application are also affected.

    Please let me know if you need any additional information to track this
    down.
    Jürgen
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