block: Open backing image in force share mode for size probe

Management tools create overlays of running guests with qemu-img:

  $ qemu-img create -b /image/in/use.qcow2 -f qcow2 /overlay/image.qcow2

but this doesn't work anymore due to image locking:

    qemu-img: /overlay/image.qcow2: Failed to get shared "write" lock
    Is another process using the image?
    Could not open backing image to determine size.
Use the force share option to allow this use case again.

Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit cc954f01e3c004aad081aa36736a17e842b80211)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

diff --git a/block.c b/block.c
index 9a1a0d1e73c44209b5d8993c1e9aa5bee456a1ad..f236431da1aeb3cfe5c2ce18fa8419b899bb8673 100644 (file)
--- a/block.c
+++ b/block.c
@@ -4593,10 +4593,11 @@ void bdrv_img_create(const char *filename, const char *fmt,
         back_flags = flags;
         back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
 
+        backing_options = qdict_new();
         if (backing_fmt) {
-            backing_options = qdict_new();
             qdict_put_str(backing_options, "driver", backing_fmt);
         }
+        qdict_put_bool(backing_options, BDRV_OPT_FORCE_SHARE, true);
 
         bs = bdrv_open(full_backing, NULL, backing_options, back_flags,
                        &local_err);