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Date: Wed, 17 Jul 2013 19:44:35 +0530 From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> To: Gleb Natapov <gleb@...hat.com>, peterz@...radead.org CC: mingo@...hat.com, jeremy@...p.org, x86@...nel.org, konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com, linux-doc@...r.kernel.org, habanero@...ux.vnet.ibm.com, xen-devel@...ts.xensource.com, mtosatti@...hat.com, stefano.stabellini@...citrix.com, andi@...stfloor.org, ouyang@...pitt.edu, agraf@...e.de, chegu_vinod@...com, torvalds@...ux-foundation.org, avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, riel@...hat.com, drjones@...hat.com, virtualization@...ts.linux-foundation.org, srivatsa.vaddagiri@...il.com Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor On 07/17/2013 07:43 PM, Raghavendra K T wrote: > 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. Forgot to ask if Peter has any points on NMI frequency. 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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