Messages in this thread | | | Date | Wed, 17 Jul 2013 19:43:01 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor |
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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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