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

[Raghavendra K T <raghukt@linux.vnet.ibm.com>]

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.