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    Subject[BUG] NULL pointer crash in early NMI handler

    I'm hitting this bug in latest Linus tree:

    [ 0.161089] Setting APIC routing to flat
    [ 0.171346] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=0 pin2=0
    [ 0.180001] BUG: unable to handle kernel NULL pointer dereference at
    (null)
    [ 0.180001] IP: [<ffffffff8063f8a6>] nmi_watchdog_tick+0xd0/0x27d

    [...]

    [ 0.180001] Call Trace:
    [ 0.180001] <NMI> <0> [<ffffffff8063e9b7>] do_nmi+0x12e/0x3af
    [ 0.180001] [<ffffffff8063e59a>] nmi+0x1a/0x2c
    [ 0.180001] [<ffffffff806415c2>] ? add_preempt_count+0xdc/0x18b
    [ 0.180001] <<EOE>> <0> [<ffffffff8040944c>] delay_tsc+0xa7/0x13b
    [ 0.180001] [<ffffffff804092df>] __delay+0xf/0x11
    [ 0.180001] [<ffffffff80409322>] __const_udelay+0x41/0x43
    [ 0.180001] [<ffffffff80f18539>] timer_irq_works+0x4e/0xb0
    [ 0.180001] [<ffffffff80f18ad4>] setup_IO_APIC+0x539/0xb26
    [ 0.180001] [<ffffffff8041b840>] ? debug_smp_processor_id+0x38/0x170
    [ 0.180001] [<ffffffff80226152>] ? setup_apic_nmi_watchdog+0xb8/0xdb
    [ 0.180001] [<ffffffff80f14231>] native_smp_prepare_cpus+0x606/0x6be
    [ 0.180001] [<ffffffff80f05a30>] kernel_init+0x56/0x1fc
    [ 0.180001] [<ffffffff8020d7fa>] child_rip+0xa/0x20
    [ 0.180001] [<ffffffff8020d1c0>] ? restore_args+0x0/0x30
    [ 0.180001] [<ffffffff80f059da>] ? kernel_init+0x0/0x1fc
    [ 0.180001] [<ffffffff8020d7f0>] ? child_rip+0x0/0x20


    Looking into exactly where it crashed, it seems to be when it accesses the
    CPU mask variable backtrace_mask.

    When the APIC routing is set to flat, it somehow starts triggering the NMI
    watchdog. This happens before we run "check_nmi_watchdog" which is what
    allocates the backtrace_mask cpu mask.

    Yes I have CONFIG_CPUMASK_OFFSTACK=y.

    When I disable it, the box boots up fine.

    -- Steve



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