Index: sys/kern/subr_epoch.c
===================================================================
--- sys/kern/subr_epoch.c
+++ sys/kern/subr_epoch.c
@@ -322,23 +322,36 @@
 	thread_unlock(td);
 }
 
+/*
+ * NOTE: epoch_alloc() and epoch_free() should only be called from
+ * SYSINIT() and SYSUNINIT(), due to internal serialization
+ * requirements.
+ */
 epoch_t
 epoch_alloc(const char *name, int flags)
 {
 	epoch_t epoch;
+	int i;
 
 	if (__predict_false(!inited))
 		panic("%s called too early in boot", __func__);
 	epoch = malloc(sizeof(struct epoch), M_EPOCH, M_ZERO | M_WAITOK);
 	ck_epoch_init(&epoch->e_epoch);
 	epoch_ctor(epoch);
-	MPASS(epoch_count < MAX_EPOCHS - 2);
+
+	for (i = 0; i != epoch_count; i++) {
+		if (allepochs[i] == NULL)
+			break;
+	}
+	MPASS(i < MAX_EPOCHS - 2);
 	epoch->e_flags = flags;
-	epoch->e_idx = epoch_count;
+	epoch->e_idx = i;
 	epoch->e_name = name;
 	sx_init(&epoch->e_drain_sx, "epoch-drain-sx");
 	mtx_init(&epoch->e_drain_mtx, "epoch-drain-mtx", NULL, MTX_DEF);
-	allepochs[epoch_count++] = epoch;
+	allepochs[i++] = epoch;
+	if (i > epoch_count)
+		epoch_count = i;
 	return (epoch);
 }