KVM: improve comment on rcu use in irqfd_deassign

The RCU use in kvm_irqfd_deassign is tricky: we have rcu_assign_pointer
but no synchronize_rcu: synchronize_rcu is done by kvm_irq_routing_update
which we share a spinlock with.

Fix up a comment in an attempt to make this clearer.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

virt/kvm/eventfd.c

@@ -313,8 +313,9 @@ kvm_irqfd_deassign(struct kvm *kvm, int fd, int gsi)
 		if (irqfd->eventfd == eventfd && irqfd->gsi == gsi) {
 			/*
 			 * This rcu_assign_pointer is needed for when
-			 * another thread calls kvm_irqfd_update before
-			 * we flush workqueue below.
+			 * another thread calls kvm_irq_routing_update before
+			 * we flush workqueue below (we synchronize with
+			 * kvm_irq_routing_update using irqfds.lock).
 			 * It is paired with synchronize_rcu done by caller
 			 * of that function.
 			 */