Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support forlinux guests running on KVM hypervisor

[Raghavendra K T]

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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