[PATCH 11/11] KVM: fix vcpu freeing bug

[Avi Kivity <avi@qumranet.com>]

From: Ingo Molnar <mingo@elte.hu>

vcpu_load() can return NULL and it sometimes does in failure paths (for 
example when the userspace ABI version is too old) - causing a 
preemption count underflow in the ->vcpu_free() later on. So check for 
NULL.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Avi Kivity <avi@qumranet.com>

Index: linux-2.6/drivers/kvm/kvm_main.c
===================================================================
--- linux-2.6.orig/drivers/kvm/kvm_main.c
+++ linux-2.6/drivers/kvm/kvm_main.c
@@ -272,7 +272,9 @@ static void kvm_free_physmem(struct kvm 
 
 static void kvm_free_vcpu(struct kvm_vcpu *vcpu)
 {
-	vcpu_load(vcpu->kvm, vcpu_slot(vcpu));
+	if (!vcpu_load(vcpu->kvm, vcpu_slot(vcpu)))
+		return;
+
 	kvm_mmu_destroy(vcpu);
 	vcpu_put(vcpu);
 	kvm_arch_ops->vcpu_free(vcpu);