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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
