block: set device_list.tqe_prev to NULL on BDS removal

This fixes a regression introduced with commit 3f09bfbc7.  Multiple
bugs arise in conjunction with live snapshots and mirroring operations
(which include active layer commit).

After a live snapshot occurs, the active layer and the base layer both
have a non-NULL tqe_prev field in the device_list, although the base
node's tqe_prev field points to a NULL entry.  This non-NULL tqe_prev
field occurs after the bdrv_append() in the external snapshot calls
change_parent_backing_link().

In change_parent_backing_link(), when the previous active layer is
removed from device_list, the device_list.tqe_prev pointer is not
set to NULL.

The operating scheme in the block layer is to indicate that a BDS belongs
in the bdrv_states device_list iff the device_list.tqe_prev pointer
is non-NULL.

This patch does two things:

1.) Introduces a new block layer helper bdrv_device_remove() to remove a
    BDS from the device_list, and
2.) uses that new API, which also fixes the regression once used in
    change_parent_backing_link().

Signed-off-by: Jeff Cody <jcody@redhat.com>
Message-id: 0cd51e11c0666c04ddb7c05293fe94afeb551e89.1454376655.git.jcody@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -2242,21 +2242,25 @@ void bdrv_close_all(void)
     }
 }
 
+/* Note that bs->device_list.tqe_prev is initially null,
+ * and gets set to non-null by QTAILQ_INSERT_TAIL().  Establish
+ * the useful invariant "bs in bdrv_states iff bs->tqe_prev" by
+ * resetting it to null on remove.  */
+void bdrv_device_remove(BlockDriverState *bs)
+{
+    QTAILQ_REMOVE(&bdrv_states, bs, device_list);
+    bs->device_list.tqe_prev = NULL;
+}
+
 /* make a BlockDriverState anonymous by removing from bdrv_state and
  * graph_bdrv_state list.
    Also, NULL terminate the device_name to prevent double remove */
 void bdrv_make_anon(BlockDriverState *bs)
 {
-    /*
+    /* Take care to remove bs from bdrv_states only when it's actually
-     * Take care to remove bs from bdrv_states only when it's actually
+     * in it. */
-     * in it.  Note that bs->device_list.tqe_prev is initially null,
-     * and gets set to non-null by QTAILQ_INSERT_TAIL().  Establish
-     * the useful invariant "bs in bdrv_states iff bs->tqe_prev" by
-     * resetting it to null on remove.
-     */
     if (bs->device_list.tqe_prev) {
-        QTAILQ_REMOVE(&bdrv_states, bs, device_list);
+        bdrv_device_remove(bs);
-        bs->device_list.tqe_prev = NULL;
     }
     if (bs->node_name[0] != '\0') {
         QTAILQ_REMOVE(&graph_bdrv_states, bs, node_list);
@@ -2297,7 +2301,7 @@ static void change_parent_backing_link(BlockDriverState *from,
         if (!to->device_list.tqe_prev) {
             QTAILQ_INSERT_BEFORE(from, to, device_list);
         }
-        QTAILQ_REMOVE(&bdrv_states, from, device_list);
+        bdrv_device_remove(from);
     }
 }
 

blockdev.c

@@ -2408,8 +2408,7 @@ void qmp_x_blockdev_remove_medium(const char *device, Error **errp)
 
     /* This follows the convention established by bdrv_make_anon() */
     if (bs->device_list.tqe_prev) {
-        QTAILQ_REMOVE(&bdrv_states, bs, device_list);
+        bdrv_device_remove(bs);
-        bs->device_list.tqe_prev = NULL;
     }
 
     blk_remove_bs(blk);

include/block/block.h

@@ -199,6 +199,7 @@ int bdrv_create(BlockDriver *drv, const char* filename,
 int bdrv_create_file(const char *filename, QemuOpts *opts, Error **errp);
 BlockDriverState *bdrv_new_root(void);
 BlockDriverState *bdrv_new(void);
+void bdrv_device_remove(BlockDriverState *bs);
 void bdrv_make_anon(BlockDriverState *bs);
 void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top);
 void bdrv_replace_in_backing_chain(BlockDriverState *old,