KVM: Fix kvm_vcpu_block() task state race

There's still a race in kvm_vcpu_block(), if a wake_up_interruptible()
call happens before the task state is set to TASK_INTERRUPTIBLE:

CPU0                            CPU1

kvm_vcpu_block

add_wait_queue

kvm_cpu_has_interrupt = 0
                                set interrupt
                                if (waitqueue_active())
                                        wake_up_interruptible()

kvm_cpu_has_pending_timer
kvm_arch_vcpu_runnable
signal_pending

set_current_state(TASK_INTERRUPTIBLE)
schedule()

Can be fixed by using prepare_to_wait() which sets the task state before
testing for the wait condition.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

virt/kvm/kvm_main.c

@@ -758,25 +758,26 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
  */
 void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 {
-	DECLARE_WAITQUEUE(wait, current);
-
-	add_wait_queue(&vcpu->wq, &wait);
-
-	/*
-	 * We will block until either an interrupt or a signal wakes us up
-	 */
-	while (!kvm_cpu_has_interrupt(vcpu)
-	       && !kvm_cpu_has_pending_timer(vcpu)
-	       && !signal_pending(current)
-	       && !kvm_arch_vcpu_runnable(vcpu)) {
-		set_current_state(TASK_INTERRUPTIBLE);
+	DEFINE_WAIT(wait);
+
+	for (;;) {
+		prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
+
+		if (kvm_cpu_has_interrupt(vcpu))
+			break;
+		if (kvm_cpu_has_pending_timer(vcpu))
+			break;
+		if (kvm_arch_vcpu_runnable(vcpu))
+			break;
+		if (signal_pending(current))
+			break;
+
 		vcpu_put(vcpu);
 		schedule();
 		vcpu_load(vcpu);
 	}
 
-	__set_current_state(TASK_RUNNING);
-	remove_wait_queue(&vcpu->wq, &wait);
+	finish_wait(&vcpu->wq, &wait);
 }
 
 void kvm_resched(struct kvm_vcpu *vcpu)