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    SubjectRe: [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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