Re: [Xen-devel] [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

[Gleb Natapov <gleb@xxxxxxxxxx>]

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.
How often this will happens anyway.

--
                        Gleb.

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