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

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

On 2011-02-10 15:47, Avi Kivity wrote:
> On 02/10/2011 04:34 PM, Jan Kiszka wrote:
>> 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.
> 
> Right.  I guess splitting that lock makes sense.
> 
>>>
>>>  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...
> 
> I don't think a mutex would work for kvmclock_cpufreq_notifier().  At 
> the very least, we'd need a preempt_disable() there.  At the worst, the 
> notifier won't like sleeping.

Damn, there was that other user. Yes, this means we need to break the
lock in mmu_shrink.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux