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SubjectRe: [PATCH 2/4] KVM: document memory barriers for kvm->vcpus/kvm->online_vcpus
Am 30.07.2015 um 14:46 schrieb Paolo Bonzini:
>
>
> On 30/07/2015 13:40, 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).
>>>> + */
>>>> smp_rmb();
>> Hmmm, wouldnt something like smp_mb__after_atomic
>>
>>>> return kvm->vcpus[i];
>>>> }
>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>> index 8dc4828f623f..093b3d10b411 100644
>>>> --- a/virt/kvm/kvm_main.c
>>>> +++ b/virt/kvm/kvm_main.c
>>>> @@ -2206,6 +2206,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>>>> }
>>>>
>>>> kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
>> and smp_mb__before_atomic
>>
>> be the better function?
>>
>>
>>>> +
>>>> + /* Pairs with smp_rmb() in kvm_get_vcpu. */
>>>> smp_wmb();
>>>> atomic_inc(&kvm->online_vcpus);
>
> The difference between smp_rmb/smp_wmb on one side, and smp_mb on the
> other side, is that smp_mb protects previous stores from subsequent loads.
>
> smp_mb__before_atomic would be overkill for atomic_inc, where there's no
> race involving the read of online_vcpus (there's only one writer and
> it's protected by kvm->lock). We only care about ordering the
> kvm->vcpus store with the kvm->online_vcpus store.

Ok, makes sense, if we only want to order these two stores.

But then the comment

>>>> + /* 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.




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