block/io: fix copy_range

Here two things are fixed:

1. Architecture

On each recursion step, we go to the child of src or dst, only for one
of them. So, it's wrong to create tracked requests for both on each
step. It leads to tracked requests duplication.

2. Wait for serializing requests on write path independently of
   BDRV_REQ_NO_SERIALISING

Before commit 9ded4a01149 "backup: Use copy offloading",
BDRV_REQ_NO_SERIALISING was used for only one case: read in
copy-on-write operation during backup. Also, the flag was handled only
on read path (in bdrv_co_preadv and bdrv_aligned_preadv).

After 9ded4a01149, flag is used for not waiting serializing operations
on backup target (in same case of copy-on-write operation). This
behavior change is unsubstantiated and potentially dangerous, let's
drop it and add additional asserts and documentation.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/block/block.h b/include/block/block.h
index 706ef009ad..f7ddff45b6 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -50,6 +50,18 @@ typedef enum {
      * opened with BDRV_O_UNMAP.
      */
     BDRV_REQ_MAY_UNMAP          = 0x4,
+
+    /*
+     * The BDRV_REQ_NO_SERIALISING flag is only valid for reads and means that
+     * we don't want wait_serialising_requests() during the read operation.
+     *
+     * This flag is used for backup copy-on-write operations, when we need to
+     * read old data before write (write notifier triggered). It is okay since
+     * we already waited for other serializing requests in the initiating write
+     * (see bdrv_aligned_pwritev), and it is necessary if the initiating write
+     * is already serializing (without the flag, the read would deadlock
+     * waiting for the serialising write to complete).
+     */
     BDRV_REQ_NO_SERIALISING     = 0x8,
     BDRV_REQ_FUA                = 0x10,
     BDRV_REQ_WRITE_COMPRESSED   = 0x20,