Re: KVM: kvm_vcpu_block task state race

[Avi Kivity <avi@qumranet.com>]

Marcelo Tosatti wrote:
> On Fri, May 09, 2008 at 10:40:47AM +0300, Avi Kivity wrote:
>
>   
>>> Unfortunately it can't use wait_event_interruptible() due to
>>> vcpu_put/vcpu_load.
>>>
>>>  
>>>       
>> schedule() will call vcpu_put()/vcpu_load() for us through preempt 
>> notifiers.  I feel a little uneasy about it, but no concreate reason why 
>> not to rely on it.
>>     
>
> The preempt notifiers hook call kvm_arch_vcpu_load / kvm_arch_vcpu_put,
> which won't unlock the vcpu mutex, right?
>
>   

Yes.

> I worry about a possible deadlock where some other operation that
> requires the vcpu mutex happens but the vcpu thread itself is in hlt.
>   

Suppose the guest executed a busy-spin waiting for an interrupt instead 
of a hlt?  We need to be able to handle that too.

The best practice is to issue all vcpu ioctls from the thread that 
created the vcpu; this becomes mandatory if we ever switch to a syscall 
interface and remove the mutex.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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