Messages in this thread | | | Date | Wed, 7 Dec 2011 10:33:30 -0200 | From | Marcelo Tosatti <> | Subject | Re: [PATCH RFC V3 2/4] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks |
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On Wed, Dec 07, 2011 at 05:24:59PM +0530, Raghavendra K T wrote: > On 12/07/2011 04:18 PM, Marcelo Tosatti wrote: > >On Wed, Nov 30, 2011 at 02:29:59PM +0530, Raghavendra K T wrote: > >> > >>+/* > >>+ * kvm_pv_kick_cpu_op: Kick a vcpu. > >>+ * > >>+ * @cpu - vcpu to be kicked. > >>+ */ > >>+static void kvm_pv_kick_cpu_op(struct kvm *kvm, int cpu) > >>+{ > >>+ struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, cpu); > >>+ struct kvm_mp_state mp_state; > >>+ > >>+ mp_state.mp_state = KVM_MP_STATE_RUNNABLE; > > > >Since vcpu->mp_state is not protected by a lock, this is potentially racy. For example: > > > >CPU0 CPU1 > >kvm_pv_kick_cpu_op running vcpuN > >vcpuN->mp_state = KVM_MP_STATE_RUNNABLE; > > kvm_emulate_halt > > vcpuN->mp_state = KVM_MP_STATE_HALTED > > > >Is it harmless to lose a kick? > > > > Yes you are right. It was potentially racy and it was harmful too!. > I had observed that it was stalling the CPU before I introduced > kicked flag. > > But now, > > vcpu->kicked = 1 ==> kvm_make_request(KVM_REQ_UNHALT, vcpu); ==>
Ok, please use a more descriptive name, such as "pvlock_kicked" or something.
> > __vcpu_run() ==> kvm_check_request(KVM_REQ_UNHALT, vcpu) ==> > > vcpuN->mp_state = KVM_MP_STATE_RUNNABLE; so eventually we will end up > in RUNNABLE. > > Also Avi pointed that, logically kvm_arch_vcpu_ioctl_set_mpstate should > be called only in vcpu thread, so after further debugging, I noticed > that, setting vcpuN->mp_state = KVM_MP_STATE_RUNNABLE; is not > necessary. > I 'll remove that in the next patch. Thanks for pointing.
In fact you don't need kvm_arch_vcpu_ioctl_set_mpstate either, only the new "kicked" flag.
> > >
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