diff --git a/sys/cam/cam_periph.c b/sys/cam/cam_periph.c
index d814c0ef6d89..cc069325748c 100644
--- a/sys/cam/cam_periph.c
+++ b/sys/cam/cam_periph.c
@@ -470,6 +470,12 @@ cam_periph_release(struct cam_periph *periph)
 	mtx_unlock(mtx);
 }
 
+/*
+ * hold/unhold act as mutual exclusion for sections of the code that
+ * need to sleep and want to make sure that other sections that
+ * will interfere are held off. This only protects exclusive sections
+ * from each other.
+ */
 int
 cam_periph_hold(struct cam_periph *periph, int priority)
 {
diff --git a/sys/cam/scsi/scsi_da.c b/sys/cam/scsi/scsi_da.c
index 701d141ff61e..2483c5e7633e 100644
--- a/sys/cam/scsi/scsi_da.c
+++ b/sys/cam/scsi/scsi_da.c
@@ -2697,9 +2697,10 @@ daregister(struct cam_periph *periph, void *arg)
 	TASK_INIT(&softc->sysctl_task, 0, dasysctlinit, periph);
 
 	/*
-	 * Take an exclusive refcount on the periph while dastart is called
-	 * to finish the probe.  The reference will be dropped in dadone at
-	 * the end of probe.
+	 * Take an exclusive section lock qon the periph while dastart is called
+	 * to finish the probe.  The lock will be dropped in dadone at the end
+	 * of probe. This locks out daopen and daclose from racing with the
+	 * probe.
 	 *
 	 * XXX if cam_periph_hold returns an error, we don't hold a refcount.
 	 */