Index: sys/kern/subr_intr.c
===================================================================
--- sys/kern/subr_intr.c
+++ sys/kern/subr_intr.c
@@ -445,6 +445,10 @@
 	if (irq_sources[isrc->isrc_irq] != isrc)
 		return (EINVAL);
 
+	/* signal availability to isrc_alloc_irq(), if needed */
+	if (irq_next_free >= maxirqs)
+		irq_next_free = isrc->isrc_irq;
+
 	irq_sources[isrc->isrc_irq] = NULL;
 	isrc->isrc_irq = INTR_IRQ_INVALID;	/* just to be safe */
 	return (0);