From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [PATCH 2/4] KVM: document memory barriers for kvm->vcpus/kvm->online_vcpus Date: Thu, 30 Jul 2015 16:05:46 +0200 Message-ID: <55BA2F3A.8060501@de.ibm.com> References: <1438176538-26191-1-git-send-email-pbonzini@redhat.com> <1438176538-26191-3-git-send-email-pbonzini@redhat.com> <55BA0D1D.7090002@de.ibm.com> <55BA1C94.80200@redhat.com> <55BA2D2E.7080901@de.ibm.com> <55BA2E68.9030207@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: mtosatti@redhat.com, srutherford@google.com, rkrcmar@redhat.com To: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Return-path: In-Reply-To: <55BA2E68.9030207@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Am 30.07.2015 um 16:02 schrieb Paolo Bonzini: > > > 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? Right you are.