Re: [PATCH v2] x86/mce: Fix endless loop when run task works after #MC

[Ding Hui]

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.

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)

[1]: commit 5567d11c21a1 ("x86/mce: Send #MC singal from task work")
--
Thanks,
- Ding Hui