blockdev: acquire AioContext in change-backing-file

Add dataplane support to the change-backing-file QMP commands.  By
acquiring the AioContext we avoid race conditions with the dataplane
thread which may also be accessing the BlockDriverState.

Note that this command operates on both bs and a node in its chain
(image_bs).  The bdrv_chain_contains(bs, image_bs) check guarantees that
bs and image_bs are in the same AioContext.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -2584,6 +2584,7 @@ void qmp_change_backing_file(const char *device,
                              Error **errp)
 {
     BlockDriverState *bs = NULL;
+    AioContext *aio_context;
     BlockDriverState *image_bs = NULL;
     Error *local_err = NULL;
     bool ro;
@@ -2597,34 +2598,37 @@ void qmp_change_backing_file(const char *device,
         return;
     }
 
+    aio_context = bdrv_get_aio_context(bs);
+    aio_context_acquire(aio_context);
+
     image_bs = bdrv_lookup_bs(NULL, image_node_name, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
-        return;
+        goto out;
     }
 
     if (!image_bs) {
         error_setg(errp, "image file not found");
-        return;
+        goto out;
     }
 
     if (bdrv_find_base(image_bs) == image_bs) {
         error_setg(errp, "not allowing backing file change on an image "
                          "without a backing file");
-        return;
+        goto out;
     }
 
     /* even though we are not necessarily operating on bs, we need it to
      * determine if block ops are currently prohibited on the chain */
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_CHANGE, errp)) {
-        return;
+        goto out;
     }
 
     /* final sanity check */
     if (!bdrv_chain_contains(bs, image_bs)) {
         error_setg(errp, "'%s' and image file are not in the same chain",
                    device);
-        return;
+        goto out;
     }
 
     /* if not r/w, reopen to make r/w */
@@ -2635,7 +2639,7 @@ void qmp_change_backing_file(const char *device,
         bdrv_reopen(image_bs, open_flags | BDRV_O_RDWR, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
-            return;
+            goto out;
         }
     }
 
@@ -2655,6 +2659,9 @@ void qmp_change_backing_file(const char *device,
             error_propagate(errp, local_err); /* will preserve prior errp */
         }
     }
+
+out:
+    aio_context_release(aio_context);
 }
 
 void qmp_blockdev_add(BlockdevOptions *options, Error **errp)

hw/block/dataplane/virtio-blk.c

@@ -197,6 +197,7 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *conf,
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_RESIZE, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_DRIVE_DEL, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_BACKUP_SOURCE, s->blocker);
+    blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_CHANGE, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_COMMIT, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_EJECT, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE,