Index: sys/kern/subr_intr.c
===================================================================
--- sys/kern/subr_intr.c
+++ sys/kern/subr_intr.c
@@ -401,33 +401,25 @@
 static inline int
 isrc_alloc_irq(struct intr_irqsrc *isrc)
 {
-	u_int maxirqs, irq;
+	u_int maxirqs;
 
 	mtx_assert(&isrc_table_lock, MA_OWNED);
 
 	maxirqs = intr_nirq;
-	if (irq_next_free >= maxirqs)
-		return (ENOSPC);
 
-	for (irq = irq_next_free; irq < maxirqs; irq++) {
-		if (irq_sources[irq] == NULL)
-			goto found;
-	}
-	for (irq = 0; irq < irq_next_free; irq++) {
-		if (irq_sources[irq] == NULL)
+	while (irq_next_free < maxirqs) {
+		if (irq_sources[irq_next_free] == NULL)
 			goto found;
+		++irq_next_free;
 	}
 
-	irq_next_free = maxirqs;
 	return (ENOSPC);
 
 found:
-	isrc->isrc_irq = irq;
-	irq_sources[irq] = isrc;
+	isrc->isrc_irq = irq_next_free;
+	irq_sources[irq_next_free] = isrc;
 
-	irq_next_free = irq + 1;
-	if (irq_next_free >= maxirqs)
-		irq_next_free = 0;
+	++irq_next_free;
 	return (0);
 }
 
@@ -445,6 +437,10 @@
 	if (irq_sources[isrc->isrc_irq] != isrc)
 		return (EINVAL);
 
+	/* Signal availability to isrc_alloc_irq(). */
+	if (isrc->isrc_irq < irq_next_free)
+		irq_next_free = isrc->isrc_irq;
+
 	irq_sources[isrc->isrc_irq] = NULL;
 	isrc->isrc_irq = INTR_IRQ_INVALID;	/* just to be safe */
 	return (0);