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

[zhukeqian via <qemu-devel@nongnu.org>]

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.

Thanks,
Keqian

>
>
--
Cheers,

David / dhildenb