1 files changed, 13 insertions, 1 deletions
diff --git a/include/block/block.h b/include/block/block.h
index e474f2541b..a91f37bedf 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -70,8 +70,20 @@ typedef enum {
      * content. */
     BDRV_REQ_WRITE_UNCHANGED    = 0x40,
 
+    /*
+     * BDRV_REQ_SERIALISING forces request serialisation for writes.
+     * It is used to ensure that writes to the backing file of a backup process
+     * target cannot race with a read of the backup target that defers to the
+     * backing file.
+     *
+     * Note, that BDRV_REQ_SERIALISING is _not_ opposite in meaning to
+     * BDRV_REQ_NO_SERIALISING. A more descriptive name for the latter might be
+     * _DO_NOT_WAIT_FOR_SERIALISING, except that is too long.
+     */
+    BDRV_REQ_SERIALISING        = 0x80,
+
     /* Mask of valid flags */
-    BDRV_REQ_MASK               = 0x7f,
+    BDRV_REQ_MASK               = 0xff,
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {