Re: latest -git: kernel BUG at arch/x86/kernel/microcode.c:142!

[Dmitry Adamushko]

2008/7/24 Vegard Nossum <vegard.nossum@xxxxxxxxx>:
> On Thu, Jul 24, 2008 at 12:48 PM, Vegard Nossum <vegard.nossum@xxxxxxxxx> wrote:
>> Hi,
>>
>> I just got this when doing CPU hotplug:
>>
>> ------------[ cut here ]------------
>> kernel BUG at arch/x86/kernel/microcode.c:142!
>> invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>>
>> Pid: 4140, comm: bash Not tainted (2.6.26-06371-g338b9bb-dirty #14)
>> EIP: 0060:[<c0117f1e>] EFLAGS: 00210202 CPU: 0
>> EIP is at __mc_sysdev_add+0x1ee/0x200
>> EAX: 00000000 EBX: c1f61028 ECX: 01798000 EDX: c081ac80
>> ESI: 00000001 EDI: 00000001 EBP: f5bcbe24 ESP: f5bcbdcc
>>  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
>> Process bash (pid: 4140, ti=f5bca000 task=f4066f90 task.ti=f5bca000)
>> Stack: 00000000 f5bcbe24 c028300b 00000001 000000d0 c06d8dc3 f73f77d0 00000000
>>       00000000 00000014 00000000 00000000 c0829254 f4f0fa00 f6e950f0 00200282
>>       f6d5180c 00000002 00000003 00000002 00000001 c1f61028 f5bcbe2c c0117f3a
>> Call Trace:
>>  [<c028300b>] ? kobject_uevent_env+0xdb/0x380
>>  [<c0117f3a>] ? mc_sysdev_add+0xa/0x10
>>  [<c05875fa>] ? mc_cpu_callback+0x1ea/0x240
>>  [<c014db67>] ? notifier_call_chain+0x37/0x70
>>  [<c014dbd9>] ? __raw_notifier_call_chain+0x19/0x20
>>  [<c014dbfa>] ? raw_notifier_call_chain+0x1a/0x20
>>  [<c0589477>] ? _cpu_up+0xa7/0x100
>>  [<c0589519>] ? cpu_up+0x49/0x80
>>  [<c056a3d8>] ? store_online+0x58/0x80
>>  [<c056a380>] ? store_online+0x0/0x80
>>  [<c02ff57c>] ? sysdev_store+0x2c/0x40
>>  [<c01de412>] ? sysfs_write_file+0xa2/0x100
>>  [<c01a0386>] ? vfs_write+0x96/0x130
>>  [<c01de370>] ? sysfs_write_file+0x0/0x100
>>  [<c01a08cd>] ? sys_write+0x3d/0x70
>>  [<c0103f5b>] ? sysenter_do_call+0x12/0x3f
>>  =======================
>> Code: 4d d8 c7 01 00 00 00 00 b8 00 1a 6f c0 e8 fb 46 47 00 8d 55 f0
>> 64 a1 00 90 7c c0 e8 0d 75 01 00 8b 45 d4 83 c4 4c 5b 5e 5f 5d c3 <0f>
>> 0b eb fe 8d b4 26 00 00 00 00 8d bc 27 00 00 00 00 55 31 d2
>> EIP: [<c0117f1e>] __mc_sysdev_add+0x1ee/0x200 SS:ESP 0068:f5bcbdcc
>> ---[ end trace 8c86c730d90bf362 ]---
>>
>> It's this one:
>>
>>        /* We should bind the task to the CPU */
>>        BUG_ON(raw_smp_processor_id() != cpu_num);
>>
>> Maybe related to recently merged per-cpu changes? (Yesterday's tests ran fine.)
>>
>> It seems 100% reproducible, so I'll start bisecting it.
>
> Ahha, after many hours of hitting various unrelated crashes,
> miscompiles, etc. I finally arrive at this commit:
>
> commit e761b7725234276a802322549cee5255305a0930
> Author: Max Krasnyansky <maxk@xxxxxxxxxxxx>
> Date:   Tue Jul 15 04:43:49 2008 -0700

Yeah, there seems to be a funny situation here :-) I'd expect it to be
100% reproduceable with CONFIG_MICROCODE=y.

cpu_up() -> raw_notifier_call_chain(CPU_ONLINE, ...) ->

(microcode's part)

mc_cpu_callback() -> mc_sysdev_add() -> microcode_init_cpu()

and here we have:

        set_cpus_allowed_ptr(current, &cpumask_of_cpu(cpu));
        mutex_lock(&microcode_mutex);
        collect_cpu_info(cpu);

this code expects that after set_cpus_allowed_ptr() has been
completed, it will continue running on "cpu"

that's why BUG_ON(raw_smp_processor_id() != cpu_num);

the funny thing is that (1) it doesn't check for an error (otherwise
it would see an error)
and (2) cpu_active_map does _not_ yet have a bit for 'cpu' at this moment.

so migrate_task() will forward a migration request to migration_thread
(because 'current' is on-the-queue/running at this point and we can't
migrate it immediatelly -- current gets blocked inside migrate_task()
waiting for request's completion)

it all will end up in migration_thread() -> __migrate_task()
which does a test for cpu_active(dest_cpu) and bails out.

summary, with cpu_active_map as it's being used now this microcode's
scheme (the fact that it expects to be migrated onto 'cpu' while its
cpu_up(cpu) is not completely finished) doesn't work.

note, I've only taken a quick look so I don't make any judgements,
(good-bad)design-wise. But it's quite a funny use-case of
cpu-hotplug-notifications and CPU_ONLINE in particular :-)


-- 
Best regards,
Dmitry Adamushko
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