Index: sys/cam/cam.h
===================================================================
--- sys/cam/cam.h
+++ sys/cam/cam.h
@@ -92,8 +92,9 @@
 	u_int32_t generation;
 	int       index;
 #define CAM_UNQUEUED_INDEX	-1
-#define CAM_ACTIVE_INDEX	-2	
-#define CAM_DONEQ_INDEX		-3	
+#define CAM_TRANSIENT_INDEX	-2
+#define CAM_ACTIVE_INDEX	-3
+#define CAM_DONEQ_INDEX		-4
 #define CAM_EXTRAQ_INDEX	INT_MAX
 } cam_pinfo;
 
Index: sys/cam/cam_queue.h
===================================================================
--- sys/cam/cam_queue.h
+++ sys/cam/cam_queue.h
@@ -136,7 +136,7 @@
  * camq_remove: Remove and arbitrary entry from the queue maintaining
  * queue order.
  */
-cam_pinfo	*camq_remove(struct camq *queue, int index);
+cam_pinfo	*camq_remove(struct camq *queue, int index, int new_index);
 #define CAMQ_HEAD 1	/* Head of queue index */
 
 /* Index the first element in the heap */
@@ -203,7 +203,8 @@
 	 */
 	if (queue->entries == queue->array_size &&
 	    camq_resize(&ccbq->queue, queue->array_size * 2) != CAM_REQ_CMP) {
-		old_ccb = (struct ccb_hdr *)camq_remove(queue, queue->entries);
+		old_ccb = (struct ccb_hdr *)camq_remove(queue, queue->entries,
+		    CAM_TRANSIENT_INDEX);
 		TAILQ_INSERT_HEAD(&ccbq->queue_extra_head, old_ccb,
 		    xpt_links.tqe);
 		old_ccb->pinfo.index = CAM_EXTRAQ_INDEX;
@@ -223,12 +224,12 @@
 	if (ccb->ccb_h.pinfo.index == CAM_EXTRAQ_INDEX) {
 		TAILQ_REMOVE(&ccbq->queue_extra_head, &ccb->ccb_h,
 		    xpt_links.tqe);
-		ccb->ccb_h.pinfo.index = CAM_UNQUEUED_INDEX;
+		ccb->ccb_h.pinfo.index = CAM_TRANSIENT_INDEX;
 		ccbq->queue_extra_entries--;
 		return;
 	}
 
-	camq_remove(queue, ccb->ccb_h.pinfo.index);
+	camq_remove(queue, ccb->ccb_h.pinfo.index, CAM_TRANSIENT_INDEX);
 
 	/*
 	 * If there are some CCBs on TAILQ, find the best one and move it
Index: sys/cam/cam_queue.c
===================================================================
--- sys/cam/cam_queue.c
+++ sys/cam/cam_queue.c
@@ -170,12 +170,19 @@
  * Heap(1, num_elements) property and an index such that 1 <= index <=
  * num_elements, remove that entry and restore the Heap(1, num_elements-1)
  * property.
+ *
+ * Care should be taken to avoid glitches in the value of index because in some
+ * contexts it may be inspected by a thread that does not possess a contending
+ * lock.  In particular, a temporary value of CAM_UNQUEUED_INDEX may be
+ * interpreted as "done" in some contexts.
  */
 cam_pinfo *
-camq_remove(struct camq *queue, int index)
+camq_remove(struct camq *queue, int index, int new_index)
 {
 	cam_pinfo *removed_entry;
 
+	KASSERT(new_index < 0, ("%s: bad new_index %d", __func__, new_index));
+
 	if (index == 0 || index > queue->entries)
 		return (NULL);
 	removed_entry = queue->queue_array[index];
@@ -184,7 +191,7 @@
 		queue->queue_array[index]->index = index;
 		heap_down(queue->queue_array, index, queue->entries - 1);
 	}
-	removed_entry->index = CAM_UNQUEUED_INDEX;
+	removed_entry->index = new_index;
 	queue->entries--;
 	return (removed_entry);
 }
Index: sys/cam/cam_xpt.c
===================================================================
--- sys/cam/cam_xpt.c
+++ sys/cam/cam_xpt.c
@@ -3257,7 +3257,8 @@
 		struct xpt_proto *proto;
 
 		device = (struct cam_ed *)camq_remove(&devq->send_queue,
-							   CAMQ_HEAD);
+						      CAMQ_HEAD,
+						      CAM_UNQUEUED_INDEX);
 		CAM_DEBUG_PRINT(CAM_DEBUG_XPT,
 				("running device %p\n", device));
 
@@ -4316,7 +4317,8 @@
 	freeze = (dev->ccbq.queue.qfrozen_cnt += count);
 	/* Remove frozen device from sendq. */
 	if (device_is_queued(dev))
-		camq_remove(&devq->send_queue, dev->devq_entry.index);
+		camq_remove(&devq->send_queue, dev->devq_entry.index,
+		    CAM_UNQUEUED_INDEX);
 	return (freeze);
 }