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Re: [Xen-devel] [PATCH RFC V3 2/4] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks



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); ==>

__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.


                        


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