This patch addresses crashes when hotplugging cpus while profiling.
I used the following script to test :

    #!/bin/bash
    startup()
    {
      opcontrol --init
      opcontrol --vmlinux=/boot/vmlinux
      opcontrol --reset
      opcontrol  --start
      opcontrol --dump
    }
    shutdown()
    {
      opcontrol --dump
      opcontrol -h
    }
    startup
    echo 0 > /sys/devices/system/cpu/cpu2/online
    echo 0 > /sys/devices/system/cpu/cpu1/online
    shutdown
    startup
    echo 1 > /sys/devices/system/cpu/cpu2/online
    echo 1 > /sys/devices/system/cpu/cpu1/online
    shutdown
    startup
    echo 0 > /sys/devices/system/cpu/cpu2/online
    shutdown
    startup
    echo 1 > /sys/devices/system/cpu/cpu2/online
    echo 0 > /sys/devices/system/cpu/cpu2/online
    shutdown
    echo 1 > /sys/devices/system/cpu/cpu2/online

Without the patch on my Power machine (ppc970mp) I get the following error:

    Vector: 300 (Data Access) at [c000000276143950]
    pc: d0000000000366e8: .add_event_entry+0x60/0xb0 [oprofile]
    lr: d000000000035e60: .sync_buffer+0x68/0x4ac [oprofile]

Without the patch on my x86 (Core 2 Duo) machine:

    mutex_lock +0x8/0x20
    sync_buffer +0x29/0x3e0
    wq_sync_buffer +0x0/0x70

Since I'm guessing hotplugging cpus and using oprofile is not a common 
occurrence, this patch is just a do-no-harm fix, instead
of a full solution with a hotplug callback, etc.

Thanks,
--chris