Re: [PATCH 2/2] KVM: Convert read-only users of vm_list to RCU

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-10 15:26, Avi Kivity wrote:
> On 02/10/2011 03:47 PM, Jan Kiszka wrote:
>>>>
>>>>  Accept for mmu_shrink, which is write but not delete, thus works without
>>>>  that slow synchronize_rcu.
>>>
>>>  I don't really see how you can implement list_move_rcu(), it has to be
>>>  atomic or other users will see a partial vm_list.
>>
>> Right, even if we synchronized that step cleanly, rcu-protected users
>> could miss the moving vm during concurrent list walks.
>>
>> What about using a separate mutex for protecting vm_list instead?
>> Unless I missed some detail, mmu_shrink should allow blocking.
> 
> What else does kvm_lock protect?

Someone tried to write a locking.txt and stated that it's also
protecting enabling/disabling hardware virtualization. But that guy may
have overlooked something.

> 
> I think we could simply reduce the amount of time we hold kvm_lock.  
> Pick a vm, ref it, list_move_tail(), unlock, then do the actual 
> shrinking.  Of course taking a ref must be done carefully, we might 
> already be in kvm_destroy_vm() at that time.
> 

Plain mutex held across the whole mmu_shrink loop is still simpler and
should be sufficient - unless we also have to deal with scalability
issues if that handler is able to run concurrently. But based on how we
were using kvm_lock so far...

Jan

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