Re: 答复: [PATCH v1 2/2] system/cpus: Fix resume_all_vcpus() under vCPU hotplug condition

[David Hildenbrand <david@redhat.com>]

On 19.03.24 06:11, zhukeqian wrote:
> Hi David,
> 
> On 17.03.24 09:37, Keqian Zhu via wrote:
>>> For vCPU being hotplugged, qemu_init_vcpu() is called. In this
>>> function, we set vcpu state as stopped, and then wait vcpu thread to
>>> be created.
>>>
>>> As the vcpu state is stopped, it will inform us it has been created
>>> and then wait on halt_cond. After we has realized vcpu object, we will
>>> resume the vcpu thread.
>>>
>>> However, during we wait vcpu thread to be created, the bql is
>>> unlocked, and other thread is allowed to call resume_all_vcpus(),
>>> which will resume the un-realized vcpu.
>>>
>>> This fixes the issue by filter out un-realized vcpu during
>>> resume_all_vcpus().
>>
>> Similar question: is there a reproducer?
>>
>> How could we currently hotplug a VCPU, and while it is being created, see pause_all_vcpus()/resume_all_vcpus() getting claled.
>>
> I described the reason for this at patch 1.
> 
>> If I am not getting this wrong, there seems to be some other mechanism missing that makes sure that this cannot happen. Dropping the BQL half-way through creating a VCPU might be the problem.
>>
> When we add retry mechanism in pause_all_vcpus(), we can solve this problem. With the sematic unchanged for user, which means:
> With bql, we can make sure all vcpus are paused after pause_all_vcpus() finish,  and all vcpus are resumed after resume_all_vcpus() finish.

Okay, got it. As just replied to #1, please see if you can avoid messing 
with pause_all_vcpus() by inhibiting KVM IOCTLs like KVM does. That 
would be preferable.

-- 
Cheers,

David / dhildenb