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>

1 files changed, 1 insertions, 2 deletions

diff --git a/blockdev.c b/blockdev.c
index 35e1e5c58f..be4ca44ab2 100644
--- a/blockdev.c
+++ b/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);
-        bs->device_list.tqe_prev = NULL;
+        bdrv_device_remove(bs);
     }
 
     blk_remove_bs(blk);