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[Qemu-block] [PULL v3 13/35] block: Open backing image in force share mo

From: Kevin Wolf
Subject: [Qemu-block] [PULL v3 13/35] block: Open backing image in force share mode for size probe
Date: Fri, 22 Dec 2017 16:18:24 +0100

From: Fam Zheng <address@hidden>

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: address@hidden
Signed-off-by: Fam Zheng <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
Signed-off-by: Kevin Wolf <address@hidden>
 block.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index db0be7ef36..dd90dca896 100644
--- a/block.c
+++ b/block.c
@@ -4605,10 +4605,11 @@ void bdrv_img_create(const char *filename, const char 
         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,

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