Messages in this thread | | | Date | Wed, 07 Dec 2011 17:24:59 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC V3 2/4] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks |
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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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