Re: [PATCH 2/4] KVM: document memory barriers for kvm->vcpus/kvm->online_vcpus

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/07/2015 15:57, Christian Borntraeger wrote:
>>>>> +	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, in case
>>>>> +	 * the caller has read kvm->online_vcpus before (as is the case
>>>>> +	 * for kvm_for_each_vcpu, for example).
>>>>> +	 */
> is somewhat distracting because of "read" and "before". So something like
> 
>  /* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, to serialize the setting
> of kvm->vcpus and setting kvm->online_vcpus....
> 
> might be better.

What you are suggesting would go in kvm_vm_ioctl_create_vcpu, e.g.
rewriting:

	/* Pairs with smp_rmb() in kvm_get_vcpu.  */
	smp_wmb();

like this:

	/*
	 * Pairs with smp_rmb() in kvm_get_vcpu.  Write kvm->vcpus
	 * before kvm->online_vcpu's incremented value.
	 */
	smp_wmb();

Instead, kvm_get_cpu has:

	/*
	 * Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, in case
	 * the caller has read kvm->online_vcpus before (as is the case
	 * for kvm_for_each_vcpu, for example).
	 */
	smp_rmb();

which already describes which loads are serialized here.

Is this correct?

Paolo