KVM: introduce kvm->created_vcpus

The race between creating the irqchip and the first VCPU is
currently fixed by checking the presence of an irqchip before
updating kvm->online_vcpus, and undoing the whole VCPU creation
if someone created the irqchip in the meanwhile.

Instead, introduce a new field in struct kvm that will count VCPUs
under a mutex, without the atomic access and memory ordering that we
need elsewhere to protect the vcpus array.  This also plugs the race
and is more easily applicable in all similar circumstances.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 6 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 02e98f3..15b757a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2346,9 +2346,20 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
 	if (id >= KVM_MAX_VCPU_ID)
 		return -EINVAL;
 
+	mutex_lock(&kvm->lock);
+	if (kvm->created_vcpus == KVM_MAX_VCPUS) {
+		mutex_unlock(&kvm->lock);
+		return -EINVAL;
+	}
+
+	kvm->created_vcpus++;
+	mutex_unlock(&kvm->lock);
+
 	vcpu = kvm_arch_vcpu_create(kvm, id);
-	if (IS_ERR(vcpu))
-		return PTR_ERR(vcpu);
+	if (IS_ERR(vcpu)) {
+		r = PTR_ERR(vcpu);
+		goto vcpu_decrement;
+	}
 
 	preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops);
 
@@ -2361,10 +2372,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
 		r = -EINVAL;
 		goto unlock_vcpu_destroy;
 	}
-	if (atomic_read(&kvm->online_vcpus) == KVM_MAX_VCPUS) {
-		r = -EINVAL;
-		goto unlock_vcpu_destroy;
-	}
 	if (kvm_get_vcpu_by_id(kvm, id)) {
 		r = -EEXIST;
 		goto unlock_vcpu_destroy;
@@ -2397,6 +2404,10 @@ unlock_vcpu_destroy:
 	mutex_unlock(&kvm->lock);
 vcpu_destroy:
 	kvm_arch_vcpu_destroy(vcpu);
+vcpu_decrement:
+	mutex_lock(&kvm->lock);
+	kvm->created_vcpus--;
+	mutex_unlock(&kvm->lock);
 	return r;
 }