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SubjectRe: [PATCH v2] x86/mce: Fix endless loop when run task works after #MC
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On 2021/7/7 11:39, Ding Hui wrote:
> On 2021/7/7 0:44, Luck, Tony wrote:
>> On Tue, Jul 06, 2021 at 08:16:06PM +0800, Ding Hui wrote:
>>> Recently we encounter multi #MC on the same task when it's
>>> task_work_run() has not been called, current->mce_kill_me was
>>> added to task_works list more than once, that make a circular
>>> linked task_works, so task_work_run() will do a endless loop.
>>
>> I saw the same and posted a similar fix a while back:
>>
>> https://www.spinics.net/lists/linux-mm/msg251006.html
>>
>> It didn't get merged because some validation tests began failing
>> around the same time.  I'm now pretty sure I understand what happened
>> with those other tests.
>>
>> I'll post my updated version (second patch in a three part series)
>> later today.
>>
>
> Thanks for your fixes.
>
> After digging my original problem, maybe I find out why I met #MC flood.
>
> My test case:
> 1. run qemu-kvm guest VM, OS is memtest86+.iso
> 2. inject SRAR UE to VM memory and wait #MC
> When VM trigger #MC, I expect that qemu will receive SIGBUS signal ASAP,
> and with the modifed qemu, I will kill VM.
>
> In this case, do_machine_check() maybe called by kvm_machine_check() in
> vmx.c.
>
> Before [1], memory_failure() is called in do_machine_check(), so
> TIF_SIGPENDING is set on due to SIGBUS signal, vcpu_run() checked the
> pending singal, so return to qemu to handle SIGBUS.
>
> After [1], do_machine_check() only add task work but not send SIGBUS
> directly, vcpu_run() will not break the for-loop because
> vcpu_enter_guest() return 1 and not set TIF_SIGPENDING on, task works
> never executed until sth else happen. So the kvm enter guest repeatedly
> and the #MC is triggered repeatedly.
>

Sorry for my incorrect description.

I figure out that my test kernel is not the lastest, it's without [2]
commit 72c3c0fe54a3 ("x86/kvm: Use generic xfer to guest work
function"), so vcpu_run() only care about signal_pending but not
TIF_NOTIFY_RESUME which set on in task_work_add().

After [2], #MC flood should not exist.

Also thank Thomas Gleixner.

> Can you consider to fix cases like this?
>
> And do you mind to give me some advice for my temporary workaround about
> this #MC flood:
> I want to check the context of do_machine_check() is exception or kvm,
> and fallback to call kill_me_xxx directly when in kvm context. (I
> already tested simply and met my expection)
>

So ignore my ask, please.

> [1]: commit 5567d11c21a1 ("x86/mce: Send #MC singal from task work")


--
Thanks,
- Ding Hui

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