Messages in this thread | | | Date | Wed, 17 Jul 2013 18:25:05 +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 06:15 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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