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    Subject[patch] x86_64: Fix boot hang due to nmi watchdog init code
    2.6.19  stopped booting (or booted based on build/config) on our x86_64
    systems due to a bug introduced in 2.6.19. check_nmi_watchdog schedules an
    IPI on all cpus to busy wait on a flag, but fails to set the busywait
    flag if NMI functionality is disabled. This causes the secondary cpus
    to spin in an endless loop, causing the kernel bootup to hang.
    Depending upon the build, the busywait flag got overwritten (stack variable)
    and caused the kernel to bootup on certain builds. Following patch fixes
    the bug by setting the busywait flag before returning from check_nmi_watchdog.
    I guess using a stack variable is not good here as the calling function could
    potentially return while the busy wait loop is still spinning on the flag.
    I would think this is a good candidate for 2.6.19 stable as well.

    Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
    Signed-off-by: Shai Fultheim <shai@scalex86.org>

    Index: linux-2.6.19/arch/x86_64/kernel/nmi.c
    ===================================================================
    --- linux-2.6.19.orig/arch/x86_64/kernel/nmi.c 2006-11-29 13:57:37.000000000 -0800
    +++ linux-2.6.19/arch/x86_64/kernel/nmi.c 2006-12-04 18:02:42.462737000 -0800
    @@ -210,9 +210,10 @@ static __init void nmi_cpu_busy(void *da
    }
    #endif

    +static int endflag = 0;
    +
    int __init check_nmi_watchdog (void)
    {
    - volatile int endflag = 0;
    int *counts;
    int cpu;

    @@ -253,6 +254,7 @@ int __init check_nmi_watchdog (void)
    if (!atomic_read(&nmi_active)) {
    kfree(counts);
    atomic_set(&nmi_active, -1);
    + endflag = 1;
    return -1;
    }
    endflag = 1;
    -
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