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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 06:15 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

But if NMI handler do not
> take locks we shouldn't worry.

Okay. Thanks for the reviews.
'll spin the next version with all the suggested changes.



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