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SubjectRe: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor
On 07/17/2013 06:55 PM, Gleb Natapov wrote:
> On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:
>> On 07/17/2013 06:15 PM, Gleb Natapov wrote:
>>> On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:
>>>>>> Instead of halt we started with a sleep hypercall in those
>>>>>> versions. Changed to halt() once Avi suggested to reuse existing sleep.
>>>>>>
>>>>>> If we use older hypercall with few changes like below:
>>>>>>
>>>>>> kvm_pv_wait_for_kick_op(flags, vcpu, w->lock )
>>>>>> {
>>>>>> // a0 reserved for flags
>>>>>> if (!w->lock)
>>>>>> return;
>>>>>> DEFINE_WAIT
>>>>>> ...
>>>>>> end_wait
>>>>>> }
>>>>>>
>>>>> How would this help if NMI takes lock in critical section. The thing
>>>>> that may happen is that lock_waiting->want may have NMI lock value, but
>>>>> lock_waiting->lock will point to non NMI lock. Setting of want and lock
>>>>> have to be atomic.
>>>>
>>>> True. so we are here
>>>>
>>>> non NMI lock(a)
>>>> w->lock = NULL;
>>>> smp_wmb();
>>>> w->want = want;
>>>> NMI
>>>> <---------------------
>>>> NMI lock(b)
>>>> w->lock = NULL;
>>>> smp_wmb();
>>>> w->want = want;
>>>> smp_wmb();
>>>> w->lock = lock;
>>>> ---------------------->
>>>> smp_wmb();
>>>> w->lock = lock;
>>>>
>>>> so how about fixing like this?
>>>>
>>>> again:
>>>> w->lock = NULL;
>>>> smp_wmb();
>>>> w->want = want;
>>>> smp_wmb();
>>>> w->lock = lock;
>>>>
>>>> if (!lock || w->want != want) goto again;
>>>>
>>> NMI can happen after the if() but before halt and the same situation
>>> we are trying to prevent with IRQs will occur.
>>
>> True, we can not fix that. I thought to fix the inconsistency of
>> lock,want pair.
>> But NMI could happen after the first OR condition also.
>> /me thinks again
>>
> lock_spinning() can check that it is called in nmi context and bail out.

Good point.
I think we can check for even irq context and bailout so that in irq
context we continue spinning instead of slowpath. no ?

> How often this will happens anyway.
>

I know NMIs occur frequently with watchdogs. or used by sysrq-trigger
etc.. But I am not an expert how frequent it is otherwise. But even
then if they do not use spinlock, we have no problem as already pointed.

I can measure with debugfs counter how often it happens.



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