[v2,1/2] block: set device_list.tqe_prev to NULL on BDS removal

Commit Message

Jeff Cody Feb. 2, 2016, 1:33 a.m. UTC

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 <[e-mail]>
---
 block.c               | 24 ++++++++++++++----------
 blockdev.c            |  3 +--
 include/block/block.h |  1 +
 3 files changed, 16 insertions(+), 12 deletions(-)

Comments

Max Reitz Feb. 2, 2016, 5:02 p.m. UTC | #1

On 02.02.2016 02:33, Jeff Cody wrote:
> 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 <[e-mail]>
> ---
>  block.c               | 24 ++++++++++++++----------
>  blockdev.c            |  3 +--
>  include/block/block.h |  1 +
>  3 files changed, 16 insertions(+), 12 deletions(-)

Reviewed-by: Max Reitz <[e-mail]>

Patch

diff --git a/block.c b/block.c
index 5709d3d..08cc130 100644
--- a/block.c
+++ b/block.c
@@ -2220,21 +2220,25 @@  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
-     * 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.
-     */
+    /* Take care to remove bs from bdrv_states only when it's actually
+     * in it. */
     if (bs->device_list.tqe_prev) {
-        QTAILQ_REMOVE(&bdrv_states, bs, device_list);
-        bs->device_list.tqe_prev = NULL;
+        bdrv_device_remove(bs);
     }
     if (bs->node_name[0] != '\0') {
         QTAILQ_REMOVE(&graph_bdrv_states, bs, node_list);
@@ -2275,7 +2279,7 @@  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);
     }
 }
 
diff --git a/blockdev.c b/blockdev.c
index 07cfe25..5f8e1b6 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2382,8 +2382,7 @@  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);
diff --git a/include/block/block.h b/include/block/block.h
index 25f36dc..8c53b93 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -198,6 +198,7 @@  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,