Comment 23 for bug 295091

@gbon -- thanks for that. The dmesg is most enlightening. It seems we have gotten into a spot of bother because we were unable to start the second CPU:

    [ 59.187433] CPU0: AMD Athlon(tm) 64 X2 Dual Core Processor 5600+ stepping 03
    [ 59.187585] SMP alternatives: switching to SMP code
    [ 59.187949] Booting processor 1/1 eip 3000
    [ 78.776093] Not responding.
    [ 78.776147] Inquiring remote APIC #1...
    [ 78.776193] ... APIC #1 ID: 1000000
    [ 78.776382] ... APIC #1 VERSION: 80050010
    [ 78.776571] ... APIC #1 SPIV: ff
    [ 78.776768] CPU #1 not responding - cannot use it.

This then leads us to only initialise the 0th CPU:

    [ 79.002377] APW: registering cpus:
    [ 79.002424] APW: cpu=0
    [ 79.002470] APW: register_cpu cpu=0 sysdev=c0483e68
    [ 79.002531] APW: sysdev_register error=0
    [ 99.002016] APW: registering cpus complete

So when we try and make links for the active CPUs listed in ACPI we find we never initialised the second one, and blow up trying to use its sysfs object:

    [ 121.440692] APW: device<f7c5f800> sysdev<c0483e68> pr->id<0>
    [ 121.440744] APW: after link
    [ 121.440798] APW: acpi_processor_start complete
    [ 141.439407] APW: device<f7c5fc00> sysdev<00000000> pr->id<1>
    [ 141.439455] APW: sysdev NULL
    [ 161.439192] APW: link AVOIDED
    [ 161.439238] APW: after link

Bad. So there are two bugs. First, the system should boot your second CPU, which I assume we can see normally in your older/newer kernels. Second, the system should not explode if the CPUs do not start correctly.