KVM: Grab pic lock in kvm_pic_clear_isr_ack

isr_ack is protected by kvm_pic->lock.
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/kvm/i8259.c

@@ -72,8 +72,10 @@ static void pic_clear_isr(struct kvm_kpic_state *s, int irq)
 void kvm_pic_clear_isr_ack(struct kvm *kvm)
 {
 	struct kvm_pic *s = pic_irqchip(kvm);
+	pic_lock(s);
 	s->pics[0].isr_ack = 0xff;
 	s->pics[1].isr_ack = 0xff;
+	pic_unlock(s);
 }
 
 /*